Chapter 34
SILICON, CARBON, BORON.
SILICON AND SILICATES.
In assaying, more especially products direct from the mine, there is always found, when the rock is siliceous, a quantity of white sandy-looking substance, insoluble in acids, which is sometimes accompanied by a light gelatinous material very difficult to filter. This is variously described as "insoluble," "sand," "insoluble silicates," "gangue," or "rocky matter." It may be pure quartz; but oftener it is mixed with silicates from the rock containing the mineral. Some silicates, but not many, are completely decomposed by boiling with hydrochloric acid or aqua regia; and others are partly so, they yield a gelatinous precipitate of silica which greatly interferes with the filtering. It is a common practice with assayers to carry the first attack of the sample with acids to dryness, and to take up with a fresh portion of acid. By this means the separated silica becomes granular and insoluble, and capable of being filtered off and washed with comparative ease.
This residue may be ignited and weighed; and be reported as so much per cent. of "silica and silicates insoluble in acids." Unless specially wanted, a determination of its constituents need not be made. When required, the analysis is best made on the ignited residue, and separately reported as "analysis of the insoluble portion."
Silicon only occurs in nature in the oxidised state; but the oxide generally known as silica (SiO_{2}) is common, being represented by the abundant minerals--quartz, flint, &c. Silica, combined with alumina, lime, oxide of iron, magnesia and the alkalies, forms a large number of rock-forming minerals. Most rock masses, other than limestones, contain over 50 per cent. of silica. The following are analyses of some of the commoner silicates; but it must be noted that these minerals often show great variation in composition. This is more especially true of chlorite, schorl, hornblende and augite.
[Table has been split into two because of its width--Transcriber]
+--------+------------+------------+-------+---------------- | | | Ferric |Ferrous| | Silica | Alumina | Oxide, | Oxide,| Fluorine, |SiO_{2}.|Al_{2}O_{3}.|Fe_{2}O_{3}.| FeO. | Water &c. ------------------+--------+------------+------------+-------+---------------- Potash-felspar | 65.2 | 18.2 | 0.2 | -- | Soda-felspar | 67.0 | 19.2 | -- | 0.3 | Lime-felspar | 43.3 | 35.4 | -- | 1.3 | Potash-mica | 45.7 | 33.7 | 3.1 | -- |F (0.8) | | | | | H_{2}O (4.9) Magnesia-mica | 39.1 | 15.4 | 7.1 | -- |F (0.7) Hornblende | 40.6 | 14.3 | 5.8 | 7.2 | Augite | 50.0 | 3.7 | 2.4 | 6.6 |MnO (0.1) Almandine (Garnet)| 39.7 | 19.7 | -- | 39.7 |MnO (1.8) Chlorite (Peach) | 32.1 | 18.5 | -- | -- |H_{2}O (12.1) Schorl | 37.0 | 33.1 | 9.3 | 6.2 |B_{2}O_{3} (7.7) | | | | | F (1.5) China-clay | 46.7 | 39.6 | -- | -- |H_{2}O (13.4) Talc | 61.7 | -- | -- | 1.7 |H_{2}O (3.8) Serpentine | 42.9 | -- | -- | 3.8 |H_{2}O (12.6) Olivine | 39.3 | -- | -- | 14.8 | ------------------+--------+------------+------------+-------+----------------
+-----+--------+-------+--------+------------------------ | | | | | |Lime,|Magnesia|Potash | Soda, | Fluorine, |CaO. | MgO. |K_{2}O.|Na_{2}O.| Water, &c. ------------------+-----+--------+-------+--------+------------------------ Potash-felspar | -- | -- | 14.7 | 1.5 | Soda-felspar | 1.2 | 1.8 | 2.2 | 7.2 | Lime-felspar |17.4 | 0.35 | 0.5 | 0.9 | Potash-mica | -- | 1.1 | 7.5 | 2.8 |F (0.8) H_{2}O (4.9) | | | | | Magnesia-mica | -- | 23.6 | 7.5 | 2.6 |F (0.7) Hornblende |12.5 | 14.0 | 1.5 | 1.6 | Augite |22.8 | 13.5 | -- | -- |MnO (0.1) Almandine (Garnet)| -- | -- | -- | -- |MnO (1.8) Chlorite (Peach) | -- | 36.7 | -- | -- |H_{2}O (12.1) Schorl | 0.5 | 2.6 | 0.7 | 1.4 |B_{2}O_{3} (7.7) F (1.5) | | | | | China-clay | -- | -- | -- | -- |H_{2}O (13.4) Talc | -- | 31.7 | -- | -- |H_{2}O (3.8) Serpentine | -- | 40.5 | -- | -- |H_{2}O (12.6) Olivine | -- | 45.8 | -- | -- | ------------------+-----+--------+-------+--------+------------------------
Silicon, from a chemical point of view, is an interesting body. It combines with iron to form a silicide; and is present in this condition in cast iron. Only in the case of the analysis of this and similar substances is the assayer called on to report the percentage of _silicon_. Silicon is readily converted into silica by the action of oxidizing agents. Silica forms only one series of salts--the silicates--which have in many cases a complex constitution; thus there are a large number of double silicates, which vary among themselves, not only in the relation of base to acid (which is the essential difference), but also in the ratio of the bases between themselves (which varies with almost every specimen).
Silica is detected by heating the substance with a fluoride and sulphuric acid in a platinum-crucible. On holding a rod, moistened with a drop of water, over the escaping fumes, the white crust of silica formed on the drop of water shows its presence. The insolubility of a fragment of the mineral in a bead of microcosmic salt, is also a very good test; the fragment, on prolonged heating, does not lose its angular form.
There is no dry assay for this substance, nor volumetric method; when the determination is required, it is carried out gravimetrically and, generally, by the following plan.
If the sample contains oxides, sulphides, &c., in any quantity, these are first dissolved out by treatment with acid, evaporated to dryness, taken up with hydrochloric acid, and filtered. The dried residue is treated in the same way as the silicates. Some silicates are completely decomposed by such treatment; but it saves time (unless one is sure that no undecomposable silicate is present) to treat these in the same way as the others. On the other hand, there are some silicates which are only attacked with difficulty even by fusion with alkaline carbonates; consequently, it is always well to have the substance reduced to the finest state of division by careful powdering, as this greatly assists the subsequent action. With very hard silicates, the grinding away of the mortar in this operation will be perceptible; the foreign matter thus introduced must not be ignored. Previously igniting the substance sometimes assists the powdering; but it is best to use a steel mortar. The particles of steel can be removed by a magnet, or, where the nature of the substance will allow it, by boiling with a little dilute hydrochloric acid.
The dried and powdered material is intimately mixed with four times its weight of "fusion mixture" in a platinum-crucible or dish. It is then moderately heated over a Bunsen burner, and afterwards more strongly fused over a blast, or enclosed in a clay crucible in the wind-furnace. The action is continued until the fused mass is perfectly tranquil. With very refractory substances, the action must be long continued at a high temperature. When sufficiently cold, the crucible is examined to see that no particles of foreign matter are adhering to its outer surface. It is then transferred to a five- or six-inch evaporating-dish, where its contents are acted upon with warm water for some time. The "melt" will slowly dissolve, but the solution should be hastened by keeping the liquid moderately acid with hydrochloric acid. When the "melt" has dissolved, clean and remove the platinum-dish, and evaporate the solution to a paste. Continue the evaporation to dryness on a water-bath (not on the hot plate), and whilst drying stir with a glass rod, feeling at the bottom of the dish for any unfused particles, which, if present, can be detected by their grittiness. If there is much grit, it will be necessary to repeat the assay; but with a small quantity it will only be necessary to refuse the grit and silica after ignition.
During solution of the "melt" and evaporation (which may be carried on together), a clear solution will not be obtained, a flocculent silica will separate out, and towards the end of the evaporation the mass will get gelatinous. The drying of the jelly must be finished on the water-bath; first, because at this temperature the silica is rendered insoluble in hydrochloric acid, whilst the solubility of the alumina, iron, &c., is unaffected, which would not be the case at a much higher temperature; and second, because the gelatinous residue requires very cautious drying to prevent loss from spirting.
When dry, the substance is moistened, and heated with strong hydrochloric acid, and the sides of the dish are washed down with water. The silica is washed by decantation two or three times with hydrochloric acid and hot water, before being thrown on to the filter. The filtrate is again evaporated to dryness, taken up with a little hydrochloric acid and water and again filtered. The residue on the filter is silica. The two lots of silica are washed free from chlorides with hot water, dried on an air-bath, transferred to a platinum-crucible, ignited gently at first, at last strongly over the blast or in a muffle, cooled in a desiccator, and weighed.
The white powdery precipitate is silica (SiO_{2}), and its weight, multiplied by 100, and divided by the weight of ore taken, gives the percentage of silica in the sample. Where the percentage of silicon is wanted, which is very rarely the case, it is got by multiplying this result by 0.4667. It is always necessary to examine the purity of the body weighed as silica. This is done by re-fusing the material weighed, and re-determining the silica in it; or, better, by mixing a weighed portion in a platinum-dish with a little strong sulphuric acid, covering with hydrofluoric acid, and evaporating. In the latter case, the silica will be converted into fluoride, which will be driven off, and the impurities will be left behind as sulphates of barium, phosphate and oxide of tin, titanium, &c. This must be weighed and deducted from the weight of the silica. In a complete examination of a silicate it should be treated with the precipitate containing alumina, ferric oxide, &c.
EXAMINATION OF SILICATES.
The student interested in the analysis of rocks and rock-forming minerals is advised to consult a valuable paper by Dr. W.F. Hillebrand in the _Bulletin of the United States Geological Survey_, _No._ 148, to which I am very largely indebted in the revision of the following pages.
~Moisture.~--Five grams of the powdered sample is dried between watch-glasses in the water-oven for two hours, or till its weight is constant; and the loss is reported as water lost at 100° C. The rest of the determinations are made on this dried mineral.
~Combined Water, &c.~--Weigh up 1 gram of the substance, and ignite over the blowpipe for some time in a platinum-crucible, cool in a desiccator, and weigh. Record the loss as "loss on ignition," not as "combined water."
~Silica.~--The ignition should have been performed in an oxidising atmosphere in a muffle or over a slanting blowpipe flame; this will ensure the oxidation of any pyrites or other sulphide present, which if unoxidised would injure the crucible in the next operation. The ignited residue is mixed with 6 or 7 grams of anhydrous sodium carbonate. This reagent should be the purest obtainable, but its purity should be checked, or rather its impurities should be determined by running a "check" or "blank" assay with 10 grams of it through the stages of the analysis; the impurities will be chiefly silica, alumina and lime, and altogether they ought not to exceed 1 milligram. The crucible with the mixture is heated at first gently over a Bunsen and afterwards more strongly in an oxidising atmosphere in a muffle or over the blowpipe. The fused mass is allowed to cool in the crucible, and is then dissolved out in a basin with water and a small excess of hydrochloric acid. After the removal and cleaning of the crucible, the liquor is evaporated almost to dryness. Dr. Hillebrand advises stopping short of complete dryness. The residue is taken up with a little hydrochloric acid and water and filtered and washed. The liquor, including the washings, is again evaporated and taken up with water and a little acid. Usually about 1 per cent. of silica will be thus recovered. It is to be filtered off and washed and added to the main silica. The filtrate is reserved. The silica, thoroughly washed, is dried and ignited at a high temperature for twenty or thirty minutes. It is then weighed in a platinum crucible. After weighing it is treated with hydrofluoric acid and a little sulphuric, carefully evaporated and ignited strongly. The residue, which in extreme cases may amount to 2 or 3 per cent. of the rock, is weighed and deducted from the weight of the impure silica. It is retained in the crucible.
~Alumina, &c.~--The filtrate from silica is treated by the basic acetate method. That is, it is first treated by a cautious addition of a solution of soda, almost to the point of producing a precipitate, in order to neutralise the excess of acid; 2 or 3 grams of sodium acetate are added, and the whole boiled for a minute or so. The precipitate is filtered off and washed only slightly. Save the filtrate. The precipitate is dissolved in hydrochloric, or, perhaps better, in nitric acid; and is reprecipitated by adding an excess of ammonia and boiling. The precipitate is filtered and washed with water containing 2 per cent. of ammonium nitrate. Both filtrates are evaporated separately to a small bulk, a drop or two of ammonia being added to the second towards the finish. They are next filtered into a 6 or 8-ounce flask through a small filter, the second filtrate coming after, and serving in a manner as wash water for the first[113]. The two washed alumina precipitates are dried and placed in the platinum crucible containing the residue from silica after treatment with hydrofluoric acid. They are then ignited in an oxidising atmosphere at a high temperature for about 10 minutes. The weight, including that of the residue from the silica, is noted as that of "alumina, &c."
The weighed oxides are next fused with bisulphate of potash for some hours. The bisulphate should have been first fused, apart, until the effervescence from the escape of steam has stopped. The melt is dissolved out with cold water and dilute sulphuric acid, and any insoluble residue is filtered off, washed, ignited and weighed. The filtrate is reserved for determinations of iron and titanium. The residue, after weighing, may be treated with hydrofluoric and sulphuric acids for any silica,[114] which would be determined by loss. It may be tested for barium sulphate by treatment with hot strong sulphuric acid; in which this salt dissolves, but is again insoluble (and so comes out as a white precipitate) on diluting with cold water; the acid also must be cold before adding the water. The filtrate containing the iron is reduced with sulphuretted hydrogen, boiled till free from that gas, filtered and titrated with a standard solution of permanganate of potassium. The iron found is calculated to ferric oxide by dividing by .7. The iron solution after titration serves for the determination of titanium oxide (TiO_{2}). This is done colorimetrically, by adding peroxide of hydrogen free from hydrofluoric acid, and comparing the brown colour produced with that produced by the addition of a standard solution of titanium to an equal volume of water containing sulphuric acid.[115] The alumina is determined by difference. From the weight of the combined precipitate which has been recorded as "Alumina, &c.," deduct (1) the residue, insoluble, after fusion with bisulphate; (2) the ferric oxide; (3) the titanium oxide; and (4) the phosphoric oxide (P_{2}O_{5}), the amount of which is subsequently determined in a separate portion. This gives the alumina.
~Manganous oxide, &c.~--The filtrate from the "alumina, &c." contained in a 6 or 8-ounce flask, which it nearly fills, is made slightly alkaline with ammonia and treated with a small excess of ammonium sulphide; the flask is then corked and placed on one side for some time (a day or so) so that the manganese sulphide may separate. The precipitate is filtered off and washed with water containing ammonium chloride and a few drops of ammonium sulphide. The filtrate is reserved for lime, &c. The precipitate is digested with sulphuretted hydrogen water, to which one-fifth of its volume of strong hydrochloric acid has been added; this dissolves the sulphides of zinc and manganese; any black residue should be tested for copper and perhaps nickel. The solution is evaporated to dryness, taken up with a little water and treated with a small excess of solution of carbonate of soda. It is boiled and again evaporated, washed out with hot water and filtered on to a small filter, dried, ignited, and weighed as Mn_{3}O_{4}. It is calculated to MnO. It may contain, and should be tested for oxide of zinc, which, if present, must be deducted. If the dish becomes stained during evaporation, take up with a few drops of hydrochloric and sulphurous acids, evaporate, and then treat with carbonate of soda.
~Lime, &c.~--The filtrate from the manganese sulphide is boiled, and without cooling, treated with ammonium oxalate in solution, which also should be heated to boiling. The liquid is filtered off and reserved for magnesia. The precipitate is dissolved in very little hydrochloric acid and reprecipitated by adding ammonium oxalate and ammonia to the boiling solution. The filtrate and washings from this are reserved for magnesia. The precipitate is either dissolved in dilute sulphuric and titrated with permanganate of potash as described under Lime (p. 322); or it is ignited and weighed as oxide. In this last case it may be examined for barium and strontium, the former of which will rarely be present.
~Magnesia.~--The filtrate from the first lime precipitate is treated with sodium phosphate and ammonia, and allowed to stand overnight. It is then filtered. The precipitate is dissolved in hydrochloric acid; the solution is filtered into the beaker containing the solution from the second lime precipitate. Ammonia and sodium phosphate are again added, and the precipitate, after standing, is filtered off, washed with water containing ammonia; it is then dried, ignited and weighed as magnesium pyrophosphate. This is calculated into magnesia.
~Potash and Soda.~--Weigh out .5 gram of the dried ore, and mix with an equal quantity of ammonic chloride; and to the mixture add gradually 4 grams of calcium carbonate ("precipitated"). Introduce into a platinum-crucible and cover loosely. Heat, at first, gently; and then at a red heat for from forty to sixty minutes. Transfer to a porcelain dish, and digest with 60 or 80 c.c. of water; warm and filter: to the filtrate add ammonic carbonate and ammonia, and filter; evaporate the filtrate to dryness, adding a few drops more of ammonic carbonate towards the end; when dry, heat gently, and then raise the temperature to a little below redness. Dissolve in a small quantity of water, add a drop of ammonic carbonate, and filter through a small filter into a weighed platinum dish. Evaporate, ignite gently, and weigh. The residue contains the soda and potash of the mineral as chlorides.
To determine the proportion of potassium, dissolve this residue in a little water, add platinum chloride in excess, evaporate to a paste, extract with alcohol, decant through a small weighed filter, wash with alcohol, and dry at 100° C. Weigh. The substance is potassium platinic chloride (2KCl.PtCl_{4}). Its weight, multiplied by 0.1941, will give the weight of potash (K_{2}O).
To find the proportion of soda, multiply the weight of the potassium platinic chloride by 0.306; this gives the weight of potassium chloride. Deduct this from the weight of the mixed chlorides first got; the difference will be the sodium chloride, which weight, multiplied by 0.53, will give the weight of soda (Na_{2}O).
~Ferrous Oxide.~--When a qualitative test shows both ferric and ferrous oxide to be present, the proportion of the ferrous oxide must be separately determined. The finely ground mineral mixed with dilute sulphuric acid is treated on a water bath with hydrofluoric acid. Solution is best effected in an atmosphere of carbonic acid. In about an hour the decomposition is complete, and the solution is diluted with cold water, and titrated with the solution of bichromate or of permanganate of potassium. The iron found is multiplied by 1.286, and reported as ferrous oxide. To find the proportion of ferric oxide, the ferrous iron found is multiplied by 1.428, and this is deducted from the weight of ferric oxide obtained by precipitation with ammonia. The ammonia precipitate contains the whole of the iron as ferric oxide; hence the necessity for calculating the ferrous oxide as ferric, and deducting it.
~Phosphoric Oxide (P_{2}O_{5}).~--Weigh up 5 grams of the finely-divided and dry sample, and digest with 10 or 20 c.c. of nitric acid; evaporate to dryness on the water-bath; take up with a little dilute nitric acid; dilute with water; and filter. Add a few grams of ammonic nitrate and 10 c.c. of ammonium molybdate solution, heat nearly to boiling, and allow to settle; filter off, and wash the yellow precipitate. Dissolve with dilute ammonia, add "magnesia mixture," and allow to stand overnight. Filter, wash with dilute ammonia, dry, ignite, and weigh as pyrophosphate of magnesia. The weight, multiplied by 0.6396, gives the weight of phosphoric oxide.
~Soluble Silica.~--Some silicates are acted on by hydrochloric acid, and leave on evaporation a residue; which, when the soluble salts have been washed out, consists generally of the separated silica with perhaps quartz and unattacked silicates. It should be ignited, weighed and boiled with a solution containing less than 10 per cent. of caustic soda: this dissolves the separated silica. The liquor is diluted, rendered faintly acid, and filtered. The residue is washed, ignited and weighed. The loss gives the soluble silica.
~Estimation of Silica in Slags~ (Ferrous silicates).--Take 1 gram of the powdered slag, treat with aqua regia, evaporate to dryness, extract with hydrochloric acid, filter, dry, ignite, and fuse the ignited residue with "fusion mixture," then separate and weigh the silica in the usual way. Slags are for the most part decomposed by boiling with aqua regia, but it will be found more convenient and accurate to first extract with acids and then to treat the residue as an insoluble silicate.
~Estimation of "Silica and Insoluble Silicates" in an Ore.~--Take 2 grams of the powdered mineral, evaporate with nitric acid (if sulphides are present), treat the dried residue (or the original substance if sulphides are absent) with 10 or 20 c.c. of hydrochloric acid; again evaporate to dryness, take up with dilute hydrochloric acid, filter, wash, ignite, and weigh.
~Estimation of Silicon in Iron.~--Place 2 grams of the metal (borings or filings) in a four-inch evaporating dish, and dissolve (with aid of heat) in 25 c.c. of dilute nitric acid. Evaporate to complete dryness, take up with 20 c.c. of hydrochloric acid, and allow to digest for one hour. Boil down to a small bulk, dilute with a 5 per cent. solution of hydrochloric acid, boil, and filter. Wash with acid and water, dry, ignite in a platinum crucible, and weigh the SiO_{2}. This, multiplied by 0.4673, gives the weight of the silicon. The percentage is calculated in the usual way.
PRACTICAL EXERCISES.
1. A certain rock is a mixture of 70 per cent. of quartz, 25 per cent. of potash-felspar, and 5 per cent. of potash-mica. What per cent. of silica will it contain?
2. Two grams of a mixture of silica and cassiterite left, after reduction in hydrogen, 1.78 grams. Assuming all the oxide of tin to have been reduced, what will be the percentage of silica?
3. The formula of a compound is 2FeO.SiO_{2}. What percentage of silica will it contain?
4. Two grams of a sample of cast-iron gave 0.025 gram of silica. Find the percentage of silicon in the metal.
5. What weights of quartz and marble (CaCO_{3}) would you take to make 30 grams of a slag having the formula CaO.SiO_{2}?
CARBON AND CARBONATES.
Carbon compounds enter so largely into the structure of organised bodies that their chemistry is generally considered apart from that of the other elements under the head of _Organic Chemistry_. Carbon occurs, however, among minerals not only in the oxidised state (as carbonates), but also in the elementary form (as in diamond and graphite), and combined with hydrogen, oxygen, &c. (as in petroleums, bitumens, lignites, shales, and coals). In small quantities "organic matter" is widely diffused in minerals and rocks. In shales and clays it may amount to as much as 10 or 20 per cent. (mainly as bituminous and coaly matters).
The assayer has only to take account of the organic matter when it is of commercial importance, so that in assays it is generally included under "loss on ignition."
In coals, shales, lignites, &c., the carbon compounds are, on heating, split up into oils and similar compounds. The products of distillation may be classified as water, gas, tars, coke, and ash. The assay of these bodies generally resolves itself into a distillation, and, in the case of the shales, an examination of the distillates for the useful oils, paraffin, creosote, &c., contained in them.
Elementary carbon is found in nature in three different forms, but these all re-act chemically in the same way. They combine with oxygen to form the dioxide.[116] The weight of oxygen required to burn a given weight of any form of carbon is the same, and the resulting product from all three has the same characteristic properties. Carbon dioxide is the common oxide of carbon. A lower oxide exists, but on burning it is converted into the dioxide. Wherever the oxidation of carbon takes place, if there is sufficient oxygen, carbon dioxide (carbonic acid) is formed; this re-action is the one used for the determination of carbon in bodies generally. The dioxide has acid properties, and combines with lime and other bases forming a series of salts called carbonates.
The carbon-compounds (other than carbonates, which will be subsequently considered) occurring in minerals are generally characterised by their sparing solubility in acids. The diamond is distinguished from other crystals by its hardness, lustre, and specific gravity. It may be subjected to a red heat without being apparently affected, but at a higher temperature it slowly burns away. Graphite, also, burns slowly, but at a lower temperature. The other bodies (coals, shales, &c.) differ considerably among themselves in the temperature at which they commence to burn. Some, such as anthracite, burn with little or no flame, but most give off gases, which burn with a luminous flame. They deflagrate when sprinkled on fused nitre, forming carbonate of potash. In making this test the student must remember that sulphur and, in fact, all oxidisable bodies similarly deflagrate, but it is only in the case of carbon compounds that carbonate of potash is formed. Carbon unites with iron and some of the metals to form carbides; combined carbon of this kind is detected by the odour of the carburetted hydrogen evolved when the metal is treated with hydrochloric acid; for example, on dissolving steel in acid.
The natural carbon compounds, although, speaking generally, insoluble in hydrochloric or nitric acids, are more or less attacked by aqua regia. The assayer seldom requires these compounds to be in solution. The presence of "organic matter"[117] interferes with most of the reactions which are used for the determination of the metals. Consequently, in such cases, it should be removed by calcination unless it is known that its presence will not interfere. When calcination is not admissible it may be destroyed by heating with strong sulphuric acid and bichromate or permanganate of potash or by fusion with nitre.
Carbon may be separated from other substances by conversion into carbon dioxide by burning. In most cases substances soluble in acids are first removed, and the insoluble residue dried, weighed, and then calcined or burned in a current of air. The quantity of "organic matter" may be determined indirectly by the loss the substance undergoes, but it is better to determine the "organic carbon" by confining the calcination in a tube, and collecting and weighing the carbon dioxide formed. Each gram of carbon dioxide is equivalent to 0.2727 gram of carbon.
Instead of a current of oxygen or air, oxide of copper may be more conveniently used. The operation is as follows:--Take a clean and dry piece of combustion tube drawn out and closed at one end, as shown in the figure (fig. 70), and about eighteen inches long. Fit it with a perforated cork connected with a ~U~-tube (containing freshly-fused calcium chloride in coarse grains) and a set of potash bulbs (fig. 71) (containing a strong solution of potash), the exit of which last is provided with a small tube containing calcium chloride or a stick of potash. Both the ~U~-tube and bulbs should have a loop of fine wire, by which they may be suspended on the hook of the balance for convenience in weighing. They must both be weighed before the combustion is commenced; to prevent absorption of moisture during weighing, &c., the ends are plugged with pieces of tube and glass rod.
Fill the combustion tube to a depth of about eight inches with some copper oxide, which has been recently ignited and cooled in a close vessel. Put in the weighed portion for assay and a little fresh copper oxide, and mix in the tube by means of an iron wire shaped at the end after the manner of a corkscrew. Put in some more oxide of copper, and clean the stirrer in it. Close loosely with a plug of recently ignited asbestos, place in the furnace, and connect the ~U~-tube and bulbs in the way shown in the sketch (fig. 72).
See that the joints are tight, and then commence the combustion by lighting the burners nearest the ~U~-tube; make the first three or four inches red hot, and gradually extend the heat backwards the length of the tube, but avoid too rapid a disengagement of gas. When gas ceases to come off, open the pointed end of the tube and draw a current of dried air through the apparatus.
The carbon dioxide is absorbed in the potash bulbs, and their increase in weight multiplied 0.2727 gives the amount of carbon in the substance taken.
The increase in weight in the calcium chloride tube will be due to the water formed by the oxidation of the combined hydrogen. If this last is required the increase in weight multiplied by 0.111 gives its amount.
COALS.
The determination of the actual carbon in coals and shales is seldom called for; if required, it would be performed in the way just described.[118] The ordinary assay of a sample of coal involves the following determinations--moisture, volatile matter, fixed carbon, ash, and sulphur. These are thus carried out:--
~Determination of Moisture.~--Take 3 grams of the powdered sample and dry in a water-bath for an hour or so. The loss is reported as moisture. Coals carry from 1 to 2 per cent. If the drying is carried too far, coals gain a little in weight owing to oxidation, so that it is not advisable to extend it over more than one or two hours.
~Determination of Volatile Matter.~--This determination is an approximate one, and it is only when working under the same conditions with regard to time, amount of coal taken, and degree of heat used, that concordant results can be arrived at. It is a matter of importance whether the coal has been previously dried before heating or not, since a difference of 2 per cent. may be got by working on the dried or undried sample. Take 2 grams of the powdered, but undried, sample of coal, place in a weighed platinum crucible, and support this over a good Bunsen burner by means of a thin platinum-wire triangle. The heat is continued until no further quantity of gas comes off and burns at the mouth. This takes only a few minutes. The cover is tightly fitted on, and when cold the crucible is weighed. The loss in weight, after deducting the moisture, gives the "volatile matter," and the residue consists of "fixed carbon" and "ash."
~Determination of Ash.~--The coke produced in the last operation is turned out into a porcelain dish and ignited over a Bunsen burner till the residue is free from particles of carbon. Calcination is hastened by stirring with a platinum wire. The operation may be done in a muffle, but this gives results a few tenths of a per cent. too low. The dish is cooled in a dessicator, and weighed. The increase in weight gives the amount of "ash," and the difference between this and the weight of the coke gives the "fixed carbon."
The assay is reported as follows:--
Moisture at 100° C. ---- per cent. Volatile matter ---- " Fixed carbon ---- " Ash ---- " contains sulphur ---- per cent.
~Determination of Sulphur.~--The sulphur exists in the coal partly in organic combination, partly as metallic sulphide (iron pyrites, marcasite, &c.), and, perhaps, as sulphate. So that the sulphur determination must be separately reported, since a portion will go off with the volatile matter, and the remainder would be retained and weighed with the coke.
The sulphur is thus determined:--Take 1 gram of the coal and mix with 1.5 gram of a mixture of 2 parts of calcined magnesia and 1 part of carbonate of soda, and heat in a platinum crucible for one hour or until oxidation is complete. Turn out the mass and extract it with water and bromine, filter, acidulate with hydrochloric acid, boil off the bromine, and precipitate with baric chloride (estimating gravimetrically as given under _Sulphur_). Another method is as follows:--Take 1 gram of the coal and drop it gradually from a sheet of note paper on to 5 grams of fused nitre contained in a platinum dish. Extract with water, acidify with acetic acid, and estimate volumetrically as described under _Sulphur_.
~Calorific Effect of Coals.~--The heat-giving value of a coal is best expressed in the number of pounds of water, previously heated to the boiling point, which it will convert into steam. This is generally termed its evaporative-power. It may be determined by means of the calorimeter (fig. 73). This consists of a glass cylinder marked to hold 29.010 grains of water. The instrument consists of a perforated copper stand, provided with a socket and three springs. The socket holds a copper cylinder which is charged with 30 grains of the dried coal mixed with 300 grains of a mixture of 3 parts of potassium chlorate and 1 part of nitre. The charge is well packed in the cylinder and provided with a small fuse of cotton saturated with nitre. Fill the glass cylinder to its mark with water and take the temperature with a thermometer marked in degrees Fahrenheit. Ignite the fuse and immediately cover with the outer copper cylinder (extinguisher-fashion), which will be held in its place by the springs. The stop-cock should be closed before this is done. Place the apparatus quickly in the cylinder of water. When the action is over open the stop-cock and agitate the water by raising and lowering the instrument a few times. Again take the temperature. The rise in temperature, plus 10 per cent. for the heat used in warming the apparatus and lost by radiation, gives the evaporative-power.
The following is an example:--
Temperature before experiment 67.0° F. Temperature after " 79.0° " -------- Rise 12.0° " + 1/10th 1.2° " -------- Gives 13.2° "
One pound of the coal will evaporate 13.2 pounds of water.
SHALES, ETC.
The assay of these is carried out in the same way as that of coals, but the volatile matters are separately examined, and, in consequence, a larger quantity of material must be used. For the moisture, volatile matter, fixed carbon and ash, the determinations are the same, but a special distillation must be made to obtain a sufficient quantity of the volatile products for subsequent examination. Take 500 or 1000 grams of the well-sampled and powdered shale, and introduce into a cast-iron retort as shown in fig. 74. Lute the joint with fire-clay, place the cover on, and bolt it down. The bolts should have a covering of fire-clay to protect them from the action of the fire. Place the retort in a wind furnace, supporting it on a brick, and pack well around with coke. Build up the furnace around and over the retort with loose fire-bricks, and heat gradually.
As soon as water begins to drip, the tube of the retort is cooled by wrapping a wet cloth around it, and keeping wet with water. The water is kept from running into the receiver by a ring of damp fire-clay. A quantity of gas first comes over and will be lost, afterwards water and oily matters. The retort must be red hot at the close of the distillation, and when nothing more distils off, which occurs in about two or three hours, the wet cloth is removed, and the tube heated with a Bunsen burner to drive forward the matter condensed in it into the receiver, and thus to clean the tube. It can be seen when the tube is clean by looking up through it into the red-hot retort. The receiver is then removed, and the retort, taken from the furnace, is allowed to cool. When cold it is opened, and the fixed carbon and ash weighed, as a check on the smaller assay.
The distillate of water and oil is warmed, and will separate into two layers, the upper one of which is oil, and the lower water. These are measured, and if the specific gravity of the oil is taken, its weight may be calculated. If the two liquids do not separate well, the water may be filtered off, after cooling, through a damped filter. The separation is, however, best effected in a separator (fig. 75). The liquids are poured into this, allowed to settle, and the lower layer drained off. The volume of the water is measured and its weight calculated in per cents. on the amount of shale taken.
~Examination of the Oil.~--A sufficient quantity of the oil must be got, so that if one distillation does not yield enough, the requisite quantity must be obtained by making two or more distillations. The oils are mixed, and the mixture, after having had its volume and specific gravity ascertained, is placed in a copper retort, and re-distilled with the aid of a current of steam. The residue in the retort is coke.
The distillate is separated from the water by means of the separator, and shaken for ten minutes with one-twentieth of its bulk of sulphuric acid (sp. g. 1.70). The temperature should not be allowed to rise above 40°. Allow to stand, and run off the "acid tar."
The oil is now shaken up with from 10 c.c. to 20 c.c. of sodic hydrate solution (sp. g. 1.3), allowed to stand, warmed for half-an-hour, and the "soda-tar" run off.
On mixing this soda-tar with dilute acid, the "crude shale oil creosote" separates, and is measured off.
The purified oil is next re-distilled in fractions, which come over in the following order:--"Naphtha," "light oil," "heavy oil," and "still bottoms." For the first product, which is only got from certain shales, the receiver is changed when the distillate has a specific gravity of 0.78. For the second product the process is continued till a drop of the distillate, caught as it falls from the neck of the retort on a cold spatula, shows signs of solidifying. This is "crude light oil."
The receiver is changed, and the "heavy oil" comes over; towards the end a thick brown or yellow viscid product is got. The receiver is again changed, and the distillation carried to dryness.
The "crude light oil" is washed cold with 2 per cent. of sulphuric acid (concentrated), and afterwards with excess of soda. Thus purified it is again distilled to dryness, three fractions being collected as before. Naphtha, which is added to the main portion, and measured; "light oil," which is also measured; and "heavy oil," which is added to that got in the first distillation. This last is poured into a flat-bottom capsule, and allowed to cool slowly. The temperature may with advantage be carried below freezing-point. The cooled cake is pressed between folds of linen, and the paraffin scale detached and weighed.
The results may be reported thus:--
Naphtha, sp. g. ---- Light oil, sp. g. ---- Heavy oil, sp. g. ---- Paraffin scale ---- Coke, &c. ----
The results are calculated in per cents. on the oil taken. Some workers take their fractions at each rise of 50° C. The composition of average shale, as given by Mills, is as follows:--Specific gravity, 1.877; moisture, 2.54.
Gas } Volatile matter, water, ammonia } 23.53 Oil } Fixed carbon 12.69 Ash 63.74 _____ 99.96
The ash is made up of silica, 55.6; ferric oxide, 12.2; alumina, 22.14; lime, 1.5; sulphur, 0.9; soluble salts (containing 0.92 per cent. sulphuric oxide), 8.3.
Total sulphur in shale 1.8 per cent. " " in ash 1.3 "
For further information on these assays, and for the assay of petroleums, bitumens, &c., the student is referred to Allen's "Commercial Organic Analysis," Vol. II.
~Determination of Organic Carbon in a Limestone.~--Take 1 or 2 grams and dissolve with a very slight excess of dilute hydrochloric acid, evaporate to dryness, and determine the carbon in the residue by combustion with copper oxide.
~Estimation of Carbon in a Sample of Graphite (Black-lead).~--Weigh up 1 or 2 grams in a dish and calcine in the muffle till the carbon is burnt off. Weigh the residue, and calculate the carbon by difference.
~Determination of Carbon in Iron.~--The carbon exists in two states--free (graphite) and combined. The following process estimates the total carbon:--The carbon existing as graphite may be separately estimated in another portion by the same process, but using hydrochloric acid to dissolve the iron instead of the copper solution:--Weigh up 2 grams of the iron (or a larger quantity if very poor in carbon), and attack it with 30 grams of ammonic-cupric chloride[119] dissolved in 100 c.c. of water. Let the reaction proceed for a quarter-of-an-hour, and then warm until the copper is dissolved. Allow to settle, and filter through a filtering-tube. This is a piece of combustion tube drawn out and narrowed at one end, as shown in fig. 76. The narrow part is blocked with a pea of baked clay, and on this is placed half-an-inch of silica sand (previously calcined to remove organic matter), then a small plug of asbestos, and then a quarter-of-an-inch of sand. The tube is connected with a pump working at a gentle pressure, and the solution is filtered through the tube with the aid of a small funnel (fig. 77). The residue is washed, first with dilute hydrochloric acid, and then with distilled water. The tube is dried by aspirating air through it, and gently warming with a Bunsen burner. The tube is then placed in a small combustion-furnace, and connected with calcium chloride and potash bulbs, as shown in fig. 78. The potash bulb to the right of the figure must be weighed. A slow stream of air is drawn through the apparatus, and the heat gradually raised; in from thirty minutes to one hour the combustion will be complete. The potash bulbs are then disconnected and weighed, and the increase multiplied by 0.2727 gives the weight of carbon.
CARBONATES.
Carbon dioxide, which is formed by the complete oxidation of carbon, is a gas with a sweetish odour and taste, having a strong affinity for alkalies, and forming a series of compounds termed carbonates. The gas itself occurs in nature, and is sometimes met with in quantity in mining. The carbonates occur largely in nature, forming mountain masses of limestone, &c. Carbonates of many of the metals, such as carbonate of lead (cerussite), carbonate of iron (chalybite), carbonates of copper (malachite and chessylite), and carbonate of magnesia (magnesite), are common.
All the carbonates (those of the alkalies and alkaline earths excepted) are completely decomposed on ignition into the oxide of the metal and carbon dioxide; but the temperature required for this decomposition varies with the nature of the base. All carbonates are soluble with effervescence in dilute acids; some, such as chalybite and magnesite, require the aid of heat. The alkaline carbonates are soluble in water; the rest, with the exception of the bicarbonates, are insoluble therein.
Carbonates are recognised by their effervescence with acids--a stream of bubbles of gas are given off which collect in the tube, and possess the property of extinguishing a lighted match. The most characteristic test for the gas is a white precipitate, which is produced by passing it into lime or baryta-water, or into a solution of subacetate of lead.
The expulsion of carbon dioxide by the stronger acids serves for the separation of this body from the other acids and bases.
~Dry Assay.~--There is no dry assay in use. Any method which may be adopted will necessarily be applicable only to special compounds.
WET METHODS.
There are several methods in use which leave little to be desired either in speed or accuracy. We will give (1) a gravimetric method in which the estimation may be made directly by weighing the carbonic acid, or, indirectly, by estimating the carbon dioxide from the loss; (2) a volumetric one, by which an indirect determination is made of the gas; and (3) a gasometric method, in which the volume of carbon dioxide given off is measured, and its weight deducted.
~Direct Gravimetric Method.~--Fit up the apparatus shown in the diagram (fig. 79). The various tubes are supported by a fixed rod with nails and wire loops, and connected by short lengths of rubber-tubing. The first tube contains soda-lime. The small flask is fitted with a rubber-stopper perforated with two holes, through one of which passes the tube of a pipette holding 25 or 30 c.c. This pipette is to contain the acid. The substance to be determined is weighed out into the flask. The second tube contains strong sulphuric acid; the third, pumice stone, saturated with copper sulphate solution, and dried until nearly white (at 200° C.); the fourth contains recently fused calcium chloride; and the fifth, which is the weighed tube in which the carbonic acid is absorbed, contains calcium chloride and soda-lime,[120] as shown in fig. 80. The sixth also contains calcium chloride and soda-lime; its object is to prevent the access of moisture and carbonic acid to the weighed tube from this direction; it is connected with an aspirator.
Having weighed the ~U~-tube and got the apparatus in order, weigh up 1, 2, or 5 grams of the substance and place in the flask. Fill the pipette with dilute acid, close the clamp, and cork the flask. Then see that the apparatus is tight. Open the clamp and allow from 10 to 20 c.c. of the acid to run on to the assay. Carbonic acid will be evolved and will be driven through the tubes. The gas should bubble through the sulphuric acid in a moderate and regular stream. When the effervescence slackens the clamp is opened and the greater part of the remaining acid run in. When the effervescence has ceased the clamp is opened to its full extent and a current of air drawn through with an aspirator. A gentle heat is applied to the flask; but it should not be prolonged or carried to boiling. After the removal of the heat a gentle current of air is drawn through the apparatus for 30 or 40 minutes. The weighed ~U~-tube, which in the early part of the operation will have become warm if much carbonic acid was present, will by this time be cold. It is disconnected, plugged, and weighed. The increase in weight is due to the carbon dioxide of the sample.
_Example._--Ore taken 1 gram.
Weight of tube, before 42.6525 grams " " after 43.0940 " ------- Increase equals CO_{2} 0.4415 "
~Indirect Gravimetric, or Determination by Loss.~--Take a Geissler's carbonic-acid apparatus (fig. 81) and place in the double bulb some strong sulphuric acid. Put into the other bulb, the stopcock being closed, 3 or 4 c.c. of nitric acid diluted with water. Leave the apparatus in the balance-box for a few minutes and weigh. Introduce into the flask (through A) about 1 gram of the powdered substance and again weigh to find the exact amount added. Allow the acid to run gradually on to the carbonate, and when solution is complete, heat and aspirate. Cool and again weigh; the loss in weight is the carbonic acid.
For _Example_:--
Weight of apparatus and acids 85.494 grams " " marble 86.879 " ------ Equal to marble taken 1.385 "
Weight of apparatus and marble 86.879 grams " " minus carbonic acid 86.2692 " ------- Equal to carbonic acid 0.6098 "
1.385 : 100 :: 0.6098 : _x_ _x_ = 44.03 per cent.
The substance contains 44.03 per cent. of carbonic acid; a duplicate experiment gave 43.73 per cent.
This method is quicker, but less exact, than the direct gravimetric determination.
VOLUMETRIC METHOD.
This, which is of somewhat limited application, is based upon the determination of the quantity of acid required to decompose the carbonate. It consists in adding to a weighed quantity of the mineral a known amount of standard solution of acid which is in excess of that required to effect the decomposition. The quantity of residual acid is then determined by titrating with standard solution of alkali. This method has been described under _Lime_.
GASOMETRIC METHOD.
This method is the quickest of all, and the least troublesome after the apparatus has been once prepared. It yields fairly accurate results when worked in the manner described below; but if greater precautions are taken the results are exact. It depends on the measurement of the volume of gas given off on treating the weighed sample with acid. The apparatus described, page 52, is used. Weigh out a portion of the mineral which shall contain not more than 0.15 gram of carbonic acid (or 0.4 gram of carbonate of lime) and put it in the bottle. Put in the inner tube 10 c.c. of dilute hydrochloric acid (1--1), cork tightly, and read off the level of the liquid in the burette after adjusting the pressure. Turn the acid over on to the mineral. Run out the water so as to keep the level in the two burettes the same. When effervescence has ceased, rotate the contents of the bottle; finally, adjust the level in the burettes and read off the volume. The increase in volume is due to the evolved carbon dioxide. At the same time read off the "volume corrector."
Some of the carbon dioxide remains dissolved in the acid in the generating bottle, and the quantity thus dissolved will depend on the amount of carbonate as well as on the amount of acid present. Consequently, a measured quantity of acid should be used in each assay and a comparative experiment made with a known weight of pure carbonate of lime which will yield about the same volume of gas. The number of c.c. of gas got in the assay multiplied by 4.7 will give the number of milligrams of pure carbonate of lime that must be taken for the standard. With ordinary work the error rarely exceeds half a c.c.
The following example will illustrate the calculations:--
One gram of a mineral was taken, and yielded 49.0 c.c. of gas. The "volume corrector" reading was 100.4 c.c.
0.2405 gram of pure carbonate of lime was then taken, and treated in the same way; 50.5 c.c. of gas were got. The volume corrector still read 100.4 c.c.
0.2405 gram of carbonate of lime is equivalent to 0.1058 gram of carbon dioxide; then,
50.5 : 49.0 :: 0.1058 : _x_ _x_ = 10.26 per cent.
~Estimation of Carbonic Acid in the Air of Mines.~--According to a series of analyses by Angus Smith, the proportion of carbonic acid in the air of underground workings varied from 0.04 to 2.7 per cent. by volume. In places where men are working the proportion ought not to reach 0.25 per cent.
A simple method of determining whether a sample of air reaches this limit (0.25 per cent.) is described by Dr. C. Le Neve Foster in the "Proceedings of the Mining Association and Institute of Cornwall" for 1888. The apparatus used is an ordinary corked 8-ounce medicine bottle. This is filled with the air to be examined by sucking out its contents with a piece of rubber-tube. Half-an-ounce of dilute lime-water[121] (tinted with phenolphthalein) is poured in. If, on corking the bottle and shaking, the colour is not discharged, the air contains less than 0.25 per cent. of carbon dioxide. "If the colour fades slowly, and does not finally vanish till after a great deal of shaking, it may be assumed that the percentage of carbon dioxide does not greatly exceed one quarter; whereas, if the disappearance is rapid after a very few shakes, the contrary, of course, is the case." The dilute lime-water is measured out and carried in ordinary half-ounce phials. This method does not pretend to great accuracy, but as a method of distinguishing between good and bad air it is very convenient, and will be found useful.
For determining the actual proportion in the air the following plan is adopted:--Take a bottle which will hold about 50 ounces, and measure its capacity; fill the bottle with the air to be examined, pour in 100 c.c. of lime-water, and shake up for some time; add phenolphthalein, and titrate the remaining calcium hydrate with standard solution of oxalic acid.
The solution of oxalic acid is made by dissolving 2.25 grams of re-crystallised oxalic acid (H_{2}C_{2}O_{4}.2H_{2}O) in water and diluting to 1 litre. One c.c. = 0.001 gram of lime (CaO), or 0.0007857 gram of carbon dioxide.
Take 100 c.c. of the same lime-water, to which add the same amount of phenolphthalein as before. Titrate. The difference between the two readings gives the amount of "acid" equivalent to the lime-water neutralised by the carbon dioxide. The number of c.c. thus used up, when multiplied by 0.3989, gives the number of c.c. of carbon dioxide (at 0° C. and 760 mm.) in the volume of air taken. This volume, which is that of the bottle less 100 c.c., must in accurate work be reduced to the normal temperature and pressure.[122] The percentage by volume can then be calculated.
PRACTICAL EXERCISES.
1. In a gasometric determination 71.3 c.c. of gas were obtained from 0.2055 gram of mineral. The "volume corrector" reading was 102.2 c.c. 0.3445 gram of pure carbonate of lime gave 74.1 c.c. The "volume corrector" reading was 100.6. What is the percentage of carbon dioxide in the substance?
2. What volume of dry gas at 0° C. and 760 m.m. pressure should be obtained from 0.3445 gram of carbonate of lime? 1 c.c. of CO_{2} under these conditions weighs 1.97 milligrams.
3. A sample of coal is reported on as follows:--
Specific gravity 1.315 Moisture 1.001 Volatile matter 35.484 Fixed carbon 50.172 Ash 12.028 ------- 100.000
What is there about this requiring explanation?
4. Calculate the percentage of carbonic acid in a mineral from the following data:--
Weight of apparatus and acids 87.0888 grams " " " plus mineral 88.8858 " " " " after loss of carbonic acid 88.1000 "
5. A sample of pig iron contains 1.43 per cent. of "combined" and 2.02 per cent. of "free" carbon. Taking 2 grams of it for each determination, what weight of CO_{2} will be got on burning the residue from solution in ammonium cupric chloride, and what from the residue after solution in hydrochloric acid?
BORON AND BORATES.
Boron occurs in nature as boric acid or sassoline (H_{3}BO_{3}); borax or tincal (Na_{2}B_{4}O_{7}.10H_{2}O); ulexite or boronatrocalcite (2CaB_{4}O_{7}.Na_{2}B_{4}O_{7}); borocalcite (CaB_{4}O_{7}.4H_{2}O); boracite, 2Mg_{3}B_{8}O_{15}.MgCl_{2}, and some other minerals. Boric acid is also a constituent of certain silicates, such as tourmaline, axinite, and datholite.
The natural borates are used in the preparation of borax, which is largely employed as a preservative agent, for fluxing, and for other purposes.
There is only one series of boron compounds which have any importance. These are the borates in which the trioxide (B_{2}O_{3}) acts the part of a weak acid. The addition of any acid liberates boric acid, which separates out in cold solutions as a crystalline precipitate. Boric acid is soluble in alcohol and in hot water. On evaporating these solutions it is volatilised, although the anhydrous oxide is "fixed" at a red heat. The borates are mostly fusible compounds, and are soluble in acids and in solutions of ammonic salts.
~Detection.~--Boron in small quantities will escape detection unless specially looked for, but there is no difficulty in detecting its presence. Heated in the Bunsen-burner flame with "Turner's test," it gives an evanescent yellowish-green colour, due to fluoride of boron (BF_{3}). "Turner's test" is a mixture of 5 parts of bisulphate of potash and 1 part of fluor spar. Boric acid itself imparts a characteristic green colour to the flame, which gives a spectrum made up of four well-marked and equidistant lines, three in the green and one in the blue. Solutions of boric acid give with "turmeric paper," which has been dipped into it and dried, a characteristic red tint. This is a very delicate test, but in trying it a blank experiment should be carried out alongside with a solution made up of the same re-agents which have been used in liberating the boric acid in the sample.
~Solution and Separation.~--The solution presents no difficulty, but the separation is troublesome. The best method is that of Gooch; who, if necessary, first fuses with carbonate of soda, and after the removal of chlorides and fluorides (by nitrate of silver or a lime salt), evaporates the aqueous extract with nitric or acetic acid to dryness in a retort and, subsequently, with repeated doses of 10 c.c. each of methyl alcohol. The distillate contains the boron as boric acid. Half a gram of the trioxide (B_{2}O_{3}) is completely carried over by two evaporations, each with 10 c.c. of the alcohol; but if water or foreign salts are present, more than this is required. In ordinary cases six such evaporations are sufficient for 0.2 gram of the oxide.[123]
GRAVIMETRIC DETERMINATION.
Before the introduction of Gooch's process it was usual to determine the boron trioxide "by difference." If the alcoholic distillate containing the boric acid is digested with about 1 gram (a known weight) of lime for ten or fifteen minutes, the alcohol can be evaporated off without danger of loss. Either calcium nitrate or acetate (which will be formed at the same time) yields lime upon subsequent ignition. Consequently, the increase in weight, after ignition, upon that of the lime taken gives the amount of boron trioxide present. The trioxide contains 31.4 per cent. of boron (B). Since magnesia does not form a soluble hydrate it cannot satisfactorily be used instead of lime.
The apparatus required is shown in fig. 82. It consists of a small retort or evaporating vessel made out of a pipette of 200 c.c. capacity. This is heated by means of a paraffin-bath at 130° or 140° C. It is connected with an upright condenser, at the lower end of which is a small flask which serves as a receiver.
The quantity of the borate taken should contain not more than 0.2 gram of the trioxide. Insoluble compounds are "dissolved in nitric acid at once, or, if necessary, first fused with sodium carbonate." With soluble and alkaline borates sufficient nitric acid is added to render it faintly acid. The solution is then introduced into the retort.
"The lime, to retain the boric acid in the distillate, is ignited in the crucible in which the evaporation of the distillate is to be made subsequently." It is then cooled in the desiccator for ten minutes, and weighed. The lime is transferred to the receiving flask and slaked with a little water. The retort is lowered into the bath so that "only the rear dips below the surface." The evaporation is carried to dryness, the retort being lowered further into the bath as the evaporation proceeds. Ten c.c. of methyl alcohol are introduced upon the residue, and the evaporation again started. Six such portions of alcohol are thus distilled and 2 c.c. of water are introduced and evaporated between the second and third, as also between the fourth and fifth distillations. If acetic acid is used instead of nitric in the first instance this addition of water is unnecessary.
The distillate is evaporated in the crucible ignited over the blowpipe, cooled in the desiccator for ten minutes and weighed. The increase in weight gives the boron trioxide. The results tend to be from 1 to 2 milligrams too high.
VOLUMETRIC METHOD.
This method is applicable to the indirect determination of boric acid in borax and similar compounds. It is based on the measurement of the quantity of normal solution of acid required to replace the boric acid, and, consequently, is rather a measure of the soda present. The process is an alkalimetric one, and is carried out as follows:--Weigh up 3 grams of the sample and dissolve in water. Tint with methyl orange, and run in from an ordinary burette normal solution of sulphuric acid until a pink tint is got. 100 c.c. of the normal solution of acid are equal to 7.0 grams of boron trioxide (B_{2}O_{3}), or 10.1 grams of anhydrous borax (Na_{2}B_{4}O_{7}).
~Examination of Borax.~--In addition to the determination just given, the following determinations are also required:--
~Water.~--Take about 2 grams and heat to tranquil fusion in a platinum crucible. Count the loss in weight as water.
~Sulphuric Oxide.~--Take 2 grams, dissolve in water, acidify with hydrochloric acid, filter, and precipitate with barium chloride. Wash the precipitate, ignite, and weigh as barium sulphate (see _Sulphur_).
~Chlorine.~--Take 2 grams, dissolve in water, acidify with nitric acid, filter, and add silver nitrate. Collect, wash, and weigh the precipitate as silver chloride.
~Alumina.~--Take 5 or 10 grams, dissolve in water, boil, add ammonia in slight excess, and filter off the precipitate when it has settled. Wash with hot water, ignite, and weigh as alumina (Al_{2}O_{3}).
FOOTNOTES:
[113] If the dishes show a manganese stain, wash them out with a few drops of hydrochloric and sulphurous acids. Pass the acid liquor through the same small filter but collect the liquor apart. Make ammoniacal and again pass through the filter, this time collecting the liquid with the main filtrate.
[114] This rarely amounts to more than 1 milligram.
[115] To make this, dissolve 1 gram of titanium oxide by fusing for some time with an excess of bisulphate of potash and dissolve out with cold water and sulphuric acid. Dilute to 1 litre, having previously added not less than 50 c.c. of strong sulphuric acid: 1 c.c. will contain .01 gram of TiO_{2}. For the assay take 10 c.c. of this, add 2 c.c. of peroxide of hydrogen and dilute to 100 c.c. Run this from a burette into the flask until the colour equals that of the assay. Each c.c. equals 1 milligram of TiO_{2}. Fluorides must be absent.
[116] C + O_{2} = CO_{2}
[117] For example, soluble organic acids formed by partial oxidation with nitric acid.
[118] For coals, and other bodies containing sulphur, chromate of lead should be used instead of oxide of copper; and the temperature should be limited to dull redness.
[119] This may be prepared by dissolving 534 grams of ammonium chloride and 854 grams of crystallized cupric chloride (CuCl_{2}.2H_{2}O) in hot water and crystallizing.
[120] Soda-lime is made by dissolving 100 grams of "soda" in water, and carefully slaking 200 grams of lime with it. Evaporate to dryness in an iron dish and ignite at a low red heat in a crucible. Use the small lumps.
[121] Made by diluting 1 part by measure of saturated lime-water up to 10 with recently boiled distilled water.
[122] See under _Gasometric Assays_.
[123] See "A Method for the Separation and Estimation of Boric Acid," by F.A. Gooch, _Chemical News_, January 7, 1887.
APPENDIX A.
TABLE OF ATOMIC WEIGHTS AND OTHER CONSTANTS.
---------+------------+----------+----------+--------- | | | | Symbols.| Names. | Atomic | Specific | Melting | | Weights. | Gravity. | Points. ---------+------------+----------+----------+--------- | | | | C. Ag | Silver | 107.9 | 10.5 | 1000° Al | Aluminium | 27.0 | 2.7 | 700° As | Arsenic | 75.0 | 5.9 | Au | Gold | 197.3 | 19.2 | 1200° B | Boron | 11.0 | 2.7 | Ba | Barium | 137.0 | 4.0 | Be | Beryllium | 9.0 | 2.1 | Bi | Bismuth | 208.9 | 9.8 | 270° Br | Bromine | 80.0 | 3.2 | -25° C | Carbon | 12.0 | | Ca | Calcium | 40.0 | 1.6 | Cd | Cadmium | 112.0 | 8.6 | 315° Ce | Cerium | 140.2 | 6.7 | Cl | Chlorine | 35.5 | | Co | Cobalt | 59.0 | 8.5 | Cr | Chromium | 52.1 | 7.3 | Cs | Caesium | 132.9 | 1.9 | 25° Cu | Copper | 63.4 | 8.9 | 1090° Di | Didymium | 142.3 | 6.5 | Er | Erbium | 166.3 | | F | Fluorine | 19.0 | | Fe | Iron | 56.0 | 7.8 | Ga | Gallium | 69.0 | 5.9 | 30° Ge | Germanium | 72.3 | | H | Hydrogen | 1.0 | | Hg | Mercury | 200.0 | 13.6 | -40° I | Iodine | 126.8 | 4.9 | 106° In | Indium | 113.7 | 7.4 | 175° Ir | Iridium | 193.1 | 22.4 | K | Potassium | 39.1 | 0.86 | 62.5° La | Lanthanum | 138.2 | 6.1 | Li | Lithium | 7.0 | 0.59 | 180° Mg | Magnesium | 24.3 | 1.7 | Mn | Manganese | 55.0 | 8.0 | Mo | Molybdenum | 96.0 | 8.6 | N | Nitrogen | 14.0 | | Na | Sodium | 23.0 | 0.97 | 95.6° Nb | Niobium | 94.0 | 4.1 | Ni | Nickel | 58.7 | 8.9 | O | Oxygen | 16.0 | | Os | Osmium | 191.7 | 22.4 | P | Phosphorus | 31.0 | 1.8 | 44° Pb | Lead | 206.9 | 11.4 | 334° Pd | Palladium | 106.6 | 11.4 | 1350° Pt | Platinum | 195.0 | 21.5 | 2000° Rb | Rubidium | 85.5 | 1.5 | 38.5° Rh | Rhodium | 103.5 | 12.1 | Ru | Ruthenium | 101.6 | 11.4 | S | Sulphur | 32.0 | 2.0 | 115° Sb | Antimony | 120.0 | 6.7 | 425° Se | Selenium | 79.0 | 4.8 | 100° Si | Silicon | 28.4 | 2.0 | Sn | Tin | 119.0 | 7.3 | 235° Sr | Strontium | 87.6 | 2.5 | Ta | Tantalum | 182.6 | | Te | Tellurium | 125.0 | 6.2 | 480° Th | Thorium | 232.6 | 7.8 | Ti | Titanium | 48.0 | 5.3 | Tl | Thallium | 204.2 | 11.9 | 294° U | Uranium | 239.6 | 18.4 | V | Vanadium | 51.4 | 5.5 | W | Tungsten | 184.0 | 19.1 | Y | Yttrium | 89.1 | | Yb | Ytterbium | 173.0 | | Zn | Zinc | 65.3 | 6.9 | 423° Zr | Zirconium | 90.6 | 4.1 | _________|____________|__________|__________|_________
The atomic weights in this table are in accord with the numbers given by F.W. Clarke (Dec. 6, 1890), chief chemist of the United States Geological Survey.
Nitric Acid.
_Table showing the percentage, by Weight, of Real Acid_ (HNO_{3}) _in Aqueous Solutions of Nitric Acid of different Specific Gravities. Temperature_, 15° C.
-------+-------++-------+-------++-------+------- 1.530 | 100.0 || 1.405 | 66.0 || 1.205 | 33.0 1.527 | 99.0 || 1.400 | 65.0 || 1.198 | 32.0 1.524 | 98.0 || 1.395 | 64.0 || 1.192 | 31.0 1.520 | 97.0 || 1.390 | 63.0 || 1.185 | 30.0 1.516 | 96.0 || 1.386 | 62.0 || 1.179 | 29.0 1.513 | 95.0 || 1.380 | 61.0 || 1.172 | 28.0 1.509 | 94.0 || 1.374 | 60.0 || 1.166 | 27.0 1.506 | 93.0 || 1.368 | 59.0 || 1.159 | 26.0 1.503 | 92.0 || 1.363 | 58.0 || 1.152 | 25.0 1.499 | 91.0 || 1.358 | 57.0 || 1.145 | 24.0 1.495 | 90.0 || 1.353 | 56.0 || 1.138 | 23.0 1.492 | 89.0 || 1.346 | 55.0 || 1.132 | 22.0 1.488 | 88.0 || 1.341 | 54.0 || 1.126 | 21.0 1.485 | 87.0 || 1.335 | 53.0 || 1.120 | 20.0 1.482 | 86.0 || 1.329 | 52.0 || 1.114 | 19.0 1.478 | 85.0 || 1.323 | 51.0 || 1.108 | 18.0 1.474 | 84.0 || 1.317 | 50.0 || 1.102 | 17.0 1.470 | 83.0 || 1.311 | 49.0 || 1.096 | 16.0 1.467 | 82.0 || 1.304 | 48.0 || 1.089 | 15.0 1.463 | 81.0 || 1.298 | 47.0 || 1.083 | 14.0 1.460 | 80.0 || 1.291 | 46.0 || 1.077 | 13.0 1.456 | 79.0 || 1.284 | 45.0 || 1.071 | 12.0 1.452 | 78.0 || 1.277 | 44.0 || 1.065 | 11.0 1.449 | 77.0 || 1.270 | 43.0 || 1.060 | 10.0 1.445 | 76.0 || 1.264 | 42.0 || 1.053 | 9.0 1.442 | 75.0 || 1.257 | 41.0 || 1.047 | 8.0 1.438 | 74.0 || 1.251 | 40.0 || 1.041 | 7.0 1.435 | 73.0 || 1.244 | 39.0 || 1.034 | 6.0 1.431 | 72.0 || 1.238 | 38.0 || 1.028 | 5.0 1.427 | 71.0 || 1.232 | 37.0 || 1.022 | 4.0 1.423 | 70.0 || 1.225 | 36.0 || 1.016 | 3.0 1.418 | 69.0 || 1.218 | 35.0 || 1.010 | 2.0 1.414 | 68.0 || 1.212 | 34.0 || 1.004 | 1.0 1.410 | 67.0 || | || | -------+-------++-------+-------++-------+--------
HYDROCHLORIC ACID.
_Table showing the percentage, by Weight, of Real Acid_ (HCl) _in Aqueous Solutions of Hydrochloric Acid of different Specific Gravities. Temperature_, 15° C.
-----------+---------++----------+---------++----------+--------- | 1.2000 | 40.78 || 1.1410 | 28.54 || 1.0798 | 16.31 | | 1.1982 | 40.37 || 1.1389 | 28.13 || 1.0778 | 15.90 | | 1.1964 | 39.96 || 1.1369 | 27.72 || 1.0758 | 15.49 | | 1.1946 | 39.55 || 1.1349 | 27.32 || 1.0738 | 15.08 | | 1.1928 | 39.14 || 1.1328 | 26.91 || 1.0718 | 14.68 | | 1.1910 | 38.74 || 1.1308 | 26.50 || 1.0697 | 14.27 | | 1.1893 | 38.33 || 1.1287 | 26.10 || 1.0677 | 13.86 | | 1.1875 | 37.92 || 1.1267 | 25.69 || 1.0657 | 13.45 | | 1.1857 | 37.51 || 1.1247 | 25.28 || 1.0637 | 13.05 | | 1.1846 | 37.11 || 1.1226 | 24.87 || 1.0617 | 12.64 | | 1.1822 | 36.70 || 1.1206 | 24.46 || 1.0597 | 12.23 | | 1.1802 | 36.29 || 1.1185 | 24.06 || 1.0577 | 11.82 | | 1.1782 | 35.88 || 1.1164 | 23.65 || 1.0557 | 11.41 | | 1.1762 | 35.47 || 1.1143 | 23.24 || 1.0537 | 11.01 | | 1.1741 | 35.07 || 1.1123 | 22.83 || 1.0517 | 10.60 | | 1.1721 | 34.66 || 1.1102 | 22.43 || 1.0497 | 10.19 | | 1.1701 | 34.25 || 1.1082 | 22.02 || 1.0477 | 9.78 | | 1.1681 | 33.84 || 1.1061 | 21.61 || 1.0457 | 9.38 | | 1.1661 | 33.43 || 1.1041 | 21.20 || 1.0437 | 8.97 | | 1.1641 | 33.03 || 1.1020 | 20.79 || 1.0417 | 8.56 | | 1.1620 | 32.62 || 1.1000 | 20.39 || 1.0397 | 8.15 | | 1.1599 | 32.21 || 1.0980 | 19.98 || 1.0377 | 7.75 | | 1.1578 | 31.80 || 1.0960 | 19.57 || 1.0357 | 7.34 | | 1.1557 | 31.40 || 1.0939 | 19.16 || 1.0337 | 6.93 | | 1.1536 | 30.99 || 1.0919 | 18.76 || 1.0318 | 6.52 | | 1.1515 | 30.58 || 1.0899 | 18.35 || 1.0298 | 6.11 | | 1.1494 | 30.17 || 1.0879 | 17.94 || 1.0279 | 5.51 | | 1.1473 | 29.76 || 1.0859 | 17.53 || 1.0259 | 5.30 | | 1.1452 | 29.36 || 1.0838 | 17.12 || 1.0239 | 4.89 | | 1.1431 | 28.95 || 1.0818 | 16.72 || 1.0200 | 4.01 | -----------+---------++----------+---------++----------+---------
AMMONIA.
_Table showing the percentage, by Weight, of Real Ammonia_ (NH_{3}) _in Aqueous Solutions of Ammonia of different Specific Gravities. Temperature_, 14° C.
----------+--------++----------+--------++----------+-------- 0.8844 | 36.0 || 0.9145 | 23.6 || 0.9534 | 11.6 0.8852 | 35.6 || 0.9156 | 23.2 || 0.9549 | 11.2 0.8860 | 35.2 || 0.9168 | 22.8 || 0.9563 | 10.8 0.8868 | 34.8 || 0.9180 | 22.4 || 0.9578 | 10.4 0.8877 | 34.4 || 0.9191 | 22.0 || 0.9593 | 10.0 0.8885 | 34.0 || 0.9203 | 21.6 || 0.9608 | 9.6 0.8894 | 33.6 || 0.9215 | 21.2 || 0.9623 | 9.2 0.8903 | 33.2 || 0.9227 | 20.8 || 0.9639 | 8.8 0.8911 | 32.8 || 0.9239 | 20.4 || 0.9654 | 8.4 0.8920 | 32.4 || 0.9251 | 20.0 || 0.9670 | 8.0 0.8929 | 32.0 || 0.9264 | 19.6 || 0.9685 | 7.6 0.8938 | 31.6 || 0.9277 | 19.2 || 0.9701 | 7.2 0.8948 | 31.2 || 0.9289 | 18.8 || 0.9717 | 6.8 0.8957 | 30.8 || 0.9302 | 18.4 || 0.9733 | 6.4 0.8967 | 30.4 || 0.9314 | 18.0 || 0.9749 | 6.0 0.8976 | 30.0 || 0.9327 | 17.6 || 0.9765 | 5.6 0.8986 | 29.6 || 0.9340 | 17.2 || 0.9781 | 5.2 0.8996 | 29.2 || 0.9353 | 16.8 || 0.9790 | 4.8 0.9006 | 28.8 || 0.9366 | 16.4 || 0.9807 | 4.6 0.9016 | 28.4 || 0.9380 | 16.0 || 0.9823 | 4.2 0.9026 | 28.0 || 0.9393 | 15.6 || 0.9839 | 3.8 0.9036 | 27.6 || 0.9407 | 15.2 || 0.9855 | 3.4 0.9047 | 27.2 || 0.9420 | 14.8 || 0.9873 | 3.0 0.9057 | 26.8 || 0.9434 | 14.4 || 0.9890 | 2.6 0.9068 | 26.4 || 0.9449 | 14.0 || 0.9907 | 2.2 0.9078 | 26.0 || 0.9463 | 13.6 || 0.9924 | 1.8 0.9089 | 25.6 || 0.9477 | 13.2 || 0.9941 | 1.4 0.9100 | 25.2 || 0.9491 | 12.8 || 0.9959 | 1.0 0.9111 | 24.8 || 0.9505 | 12.4 || 0.9975 | 0.6 0.9122 | 24.4 || 0.9520 | 12.0 || 0.9991 | 0.2 0.9133 | 24.0 || | || | ----------+--------++----------+--------++----------+--------
SULPHURIC ACID.
_Table showing the percentage, by Weight, of Real Acid_ (H_{2}SO_{4}) _in Aqueous Solutions of Sulphuric Acid of varying Specific Gravity. Temperature_, 15° C.
--------+--------++--------+--------++--------+-------- 1.838 | 100.0 || 1.568 | 66.0 || 1.247 | 33.0 1.840 | 99.0 || 1.557 | 65.0 || 1.239 | 32.0 1.841 | 98.0 || 1.545 | 64.0 || 1.231 | 31.0 1.841 | 97.0 || 1.534 | 63.0 || 1.223 | 30.0 1.840 | 96.0 || 1.523 | 62.0 || 1.215 | 29.0 1.838 | 95.0 || 1.512 | 61.0 || 1.206 | 28.0 1.836 | 94.0 || 1.501 | 60.0 || 1.198 | 27.0 1.834 | 93.0 || 1.490 | 59.0 || 1.190 | 26.0 1.831 | 92.0 || 1.480 | 58.0 || 1.182 | 25.0 1.827 | 91.0 || 1.469 | 57.0 || 1.174 | 24.0 1.822 | 90.0 || 1.458 | 56.0 || 1.167 | 23.0 1.816 | 89.0 || 1.448 | 55.0 || 1.159 | 22.0 1.809 | 88.0 || 1.438 | 54.0 || 1.151 | 21.0 1.802 | 87.0 || 1.428 | 53.0 || 1.144 | 20.0 1.794 | 86.0 || 1.418 | 52.0 || 1.136 | 19.0 1.786 | 85.0 || 1.408 | 51.0 || 1.129 | 18.0 1.777 | 84.0 || 1.398 | 50.0 || 1.121 | 17.0 1.767 | 83.0 || 1.388 | 49.0 || 1.113 | 16.0 1.756 | 82.0 || 1.379 | 48.0 || 1.106 | 15.0 1.745 | 81.0 || 1.370 | 47.0 || 1.098 | 14.0 1.734 | 80.0 || 1.361 | 46.0 || 1.091 | 13.0 1.722 | 79.0 || 1.351 | 45.0 || 1.083 | 12.0 1.710 | 78.0 || 1.342 | 44.0 || 1.075 | 11.0 1.698 | 77.0 || 1.333 | 43.0 || 1.068 | 10.0 1.686 | 76.0 || 1.324 | 42.0 || 1.061 | 9.0 1.675 | 75.0 || 1.315 | 41.0 || 1.053 | 8.0 1.663 | 74.0 || 1.306 | 40.0 || 1.046 | 7.0 1.651 | 73.0 || 1.297 | 39.0 || 1.039 | 6.0 1.639 | 72.0 || 1.289 | 38.0 || 1.032 | 5.0 1.627 | 71.0 || 1.281 | 37.0 || 1.025 | 4.0 1.615 | 70.0 || 1.272 | 36.0 || 1.019 | 3.0 1.604 | 69.0 || 1.264 | 35.0 || 1.013 | 2.0 1.592 | 68.0 || 1.256 | 34.0 || 1.006 | 1.0 1.580 | 67.0 || | || | --------+--------++--------+--------++--------+--------
APPENDIX B.
ESTIMATION OF SMALL QUANTITIES OF GOLD.[124]
In the case of small buttons of gold the weight can be determined more easily and accurately by measuring with the help of a microscope than by the actual use of a balance. Moreover, the method of measurement is applicable to the determination of quantities of gold too minute to affect even the most delicate balance.
For quantities of gold of from .5 to .005 milligram a microscope with 1/2 inch objective and B eyepiece is suitable. The measurements are made with the help of a scale engraved (or, better, photographed) on a circular piece of glass which rests on the diaphragm of the eyepiece. This scale and the object upon the stage can be easily brought into focus at the same time. The button of gold obtained by cupelling is loosened from the cupel by gently touching with the moistened point of a knife; it generally adheres to the knife, and is then transferred to a glass slide. The slide is placed on the stage of the microscope, illuminated from below; and the button is brought into focus, and so placed that it apparently coincides with the scale. The diameters in two or three directions (avoiding the flattened surface) are then read off: the different directions being got by rotating the eyepiece. The mean diameter is taken. The weight of the button is arrived at by comparing with the mean diameter of a _standard prill_ of gold of known weight. The weights are in the proportion of the cubes of the diameters. For example, suppose a prill has been obtained which measures 12.5 divisions of the scale, and that a standard prill weighing 0.1 milligram measures 11.1 divisions. The weight will be calculated as follows:
11.1^{3} : 12.5^{3} :: 0.1 : _x_
0.1×12.5×12.5×12.5 _x_ = -------------------- = 0.143 milligram. 11.1×11.1×11.1
The calculations are simplified by the use of a table of cubes. The standard prills used in the comparison should not differ much in size from the prills to be determined. They are prepared by alloying known weights of gold and lead, so as to get an alloy of known composition, say one per cent. gold. Portions of the alloy containing the weight of gold required (say 0.1 milligram) are then weighed off and cupelled on small smooth cupels, made with the finest bone-ash. Care must be taken to remove the cupels as soon as cupellation has finished. Several standard prills of the same size should be made at the same time, and their mean diameter calculated. The lead for making the gold-lead alloy is prepared from litharge purified by reducing from it about 10 per cent. of its lead by fusion with a suitable proportion of flour; the purified litharge is powdered, mixed with sufficient flour and reduced to metal.
In determining the gold contained in small buttons of silver-gold alloy obtained in assaying (and in which the silver is almost sure to be in excess of that required for parting), transfer the button from the cupel to a small clean porcelain crucible; pour on it a drop or two of nitric acid (diluted with half its bulk of water), and heat gently and cautiously until action has ceased. If the residual gold is broken up, move the crucible so as to bring the particles together, so that they may cohere. Wash three or four times with distilled water, about half filling the crucible each time and decanting off against the finger. Dry the crucible in a warm place; and when dry, but whilst still black, take the gold up on a small piece of pure lead. Half a grain of lead is sufficient, and it is best to hold it on the point of a blunt penknife, and press it on the gold in the crucible. The latter generally adheres. Transfer to a small smooth cupel and place in the muffle. When the cupellation has finished, the button of gold is measured as already described.
PRACTICAL NOTES ON THE IODIDE PROCESS OF COPPER ASSAYING.
For the following remarks and experiments we are indebted to Mr. J.W. Westmoreland, who has had considerable experience with the process. Having dissolved the ore he converts the metals into sulphates by evaporating with sulphuric acid. The copper is then separated as subsulphide by means of hyposulphite of soda, and the precipitate is washed, dried, and calcined. The resulting oxide of copper is then dissolved in nitric acid; and to the concentrated solution, a saturated solution of carbonate of soda is added in sufficient quantity to throw down a considerable proportion of the copper. Acetic acid is added to dissolve the precipitate, and when this is effected more of the acid is poured on so as to render the solution strongly acid. To this potassium iodide crystals are added in the proportion of ten parts of iodide to each one part of copper supposed to be present. The solution is then titrated with "hypo" as usual.
For the examination of technical products experiments made in sulphuric acid solutions have no value, since arsenic acid, which is generally present to a greater or less extent, affects the end reaction. In such solutions bismuth may also interfere.
The solution best suited for the assay is one containing acetate of soda and free acetic acid. The presence of acetate of soda counteracts the interference of arsenic and of bismuth.
The return of the blue colour after titration is due to the excessive dilution of the assay, or to an insufficiency of potassium iodide, or to the presence of nitrous fumes. The interference of an excess of sodium acetate is avoided by adding more iodide crystals to the extent of doubling the usual amount.
The interference of lead can be avoided by the addition of sulphuric acid or of phosphate of soda to the acid solution containing the copper, and before neutralising with carbonate of soda. The end reaction is, however, with care distinguishable without this addition. The following experiments, each containing .0648 gram of lead, were made by him in illustration:
---------------+-------------------+---------------+--------------------- Copper taken. | Reagent added. | Copper found. | End reaction. ---------------+-------------------+---------------+--------------------- .2092 gram | -- | .2077 gram | fairly satisfactory .2101 " | -- | .2092 " | " .2167 " | sulphuric acid | .2152 " | " .2117 " | " | .2108 " | " .2109 " | phosphate of soda | .2092 " | good, colourless .2205 " | " | .2174 " | rather yellow ---------------+-------------------+---------------+---------------------
_Effect of Sodium Acetate._--Each solution contained .3343 gram of copper.
a.b.c. d. e. f. g. grams. grams. grams. grams. grams. "Acetate" added -- 16.2 16.2 16.2 16.2 "Iodide" added 3.5 3.5 7.0 3.5 7.0 Copper found .3343 .3324 .3351 .3269 .3356
In these experiments, except with the excessive quantities of acetate of soda and the insufficiency of potassium iodide in the cases of c and f, there was no difficulty with the after-blueing.
METHOD OF SEPARATING COBALT AND NICKEL.
The following method of separating and estimating cobalt and nickel has been described by Mr. James Hope,[125] with whom it has been in daily use for several years with completely satisfactory results.
The quantity of ore taken should contain about .5 gram of the mixed metals. It is dissolved in hydrochloric acid or aqua regia, and the solution evaporated to dryness. The residue is taken up with dilute hydrochloric acid and hot water. The solution is filtered off from the silica, freed from second group metals by treatment with sulphuretted hydrogen and filtered, and after oxidation with nitric acid is separated from iron and alumina by the basic acetate method (page 233). The precipitate is redissolved in a little hydrochloric acid, and again precipitated by sodium acetate. The two filtrates are mixed and treated with a little acetic acid, and the cobalt and nickel are then precipitated as sulphides by a current of sulphuretted hydrogen. The precipitate is filtered off, washed, dried, and calcined, and the resulting oxides are weighed to get an idea as to the quantity of the two metals present.
The calcined precipitate is dissolved in a small covered beaker in aqua regia with the help of a few drops of bromine to remove any separated sulphur, and the solution evaporated to dryness with a few drops of sulphuric acid. The residue is dissolved in hot water, diluted to about 50 c.c., and heated to boiling. About 2 grams (four times the quantity of mixed metals present) of ammonium phosphate (AmH_{2}PO_{4}) are weighed off, dissolved in the smallest possible quantity of water, and boiled for a minute or two with a few c.c. of dilute sulphuric acid. This is added to the boiling-hot solution of cobalt and nickel, which is then treated cautiously with dilute ammonia until the precipitate partially dissolves. The addition of the ammonia is continued drop by drop with constant stirring, until the cobalt comes down as a pink precipitate of ammonium cobalt phosphate (AmCoPO_{4}). The beaker is placed on the top of a water bath with occasional stirring for five or ten minutes. The blue liquid containing the nickel is decanted through a small filter and the precipitate is dissolved with a few drops of dilute sulphuric acid. The resulting solution is treated with a small excess of ammonium phosphate and the cobalt again precipitated by the cautious addition of ammonia exactly as before. The precipitate containing the whole of the cobalt is filtered off and washed with small quantities of hot water. The filtrate is added to the previous one containing the greater part of the nickel.
The ammonium cobalt phosphate is dried, transferred to a platinum crucible, and ignited over a Bunsen flame for fifteen or twenty minutes. A purple coloured cobalt pyrophosphate (Co_{2}P_{2}O_{7}) is thus formed, and is weighed. It contains 40.3 per cent. of cobalt.
The mixed filtrates containing the nickel are placed in a tall beaker, and dilated if necessary to about 200 c.c. Ten c.c. of strong ammonia are added, and the solution, heated to 70° C., is ready for electrolysis. A battery of two 1-1/2 pint Bunsen cells is used. This is found capable of depositing from .15 to .20 gram of nickel per hour, and from two to three hours is generally sufficient for the electrolysis. The electrode with the deposited nickel is washed with distilled water, afterwards with alcohol as described under copper, and is then dried and weighed.
The following results obtained with this method by Mr. Hope illustrate the accuracy of the method. They were obtained by working on solutions containing known weights of the two metals:
-------------------------+------------------------- Taken. | Found. ------------+------------+------------+------------ Cobalt. | Nickel. | Cobalt. | Nickel. ------------+------------+------------+------------ .1236 gram | .1155 gram | .1242 gram | .1155 gram .1236 " | .0577 " | .1232 " | .0575 " .2472 " | .0577 " | .2449 " | .0585 " .3708 " | .0577 " | .3701 " | .0580 " .0618 " | .3465 " | .0619 " | .3454 " .0618 " | .2310 " | .0625 " | .2295 " .0618 " | .1155 " | .0621 " | .1155 " ------------+------------+------------+------------
FOOTNOTES:
[124] For fuller information see a paper on "The Estimation of Minute Quantities of Gold," by Dr. George Tate; read before the Liverpool Polytechnic Society, Nov. 1889.
[125] _Journal of the Society of Chemical Industry_, No 4, vol. ix. April 30, 1890.
APPENDIX C.
A LECTURE ON THE THEORY OF SAMPLING.
The problem of the sampler is essentially the same as that of the student of statistics. One aims at getting a small parcel of ore, the other a number of data, but each hopes to obtain what shall represent a true average applicable to a much larger mass of material. Ignoring the mechanical part of the problems, the sampling errors of the one and the deviations from the average of the other are the same thing.
It may be doubted whether many not specially trained in the study of statistics could answer such a question as the following:--Seven hundred thousand men being employed, there are, in a given year, one thousand deaths from accident. Assuming the conditions to remain unaltered, within what limits could one foretell the number of deaths by accident in any other year?
On the other hand, there is a widespread belief in the efficacy of what is called the law of averages. Even the ordinary newspaper reader is accustomed to look on the national death-rate or birth-rate as a thing capable of being stated with accuracy to one or two places of decimals, and he knows that the annual number of suicides is practically constant.
If a man played whist often and kept a record of the number of trumps n each hand, he would find fortune treated him quite fairly; in a year's play the average number would deviate very little from the theoretical average, _i.e._, one-quarter of thirteen. And a knowledge of this truth is useful, and that not merely in keeping ejaculations in due restraint. But every good player knows more than this: he has a sense of what variations in the number of trumps may reasonably be expected. For example, he will be prepared to risk something on neither of his opponents having more than five trumps, and will accept it as a practical certainty that no one has more than eight. Much of what is known as good judgment is based on a proper estimate of deviations from the average. The question has an important bearing on sampling, as may be seen from the fact that shuffling and dealing at cards are but modifications of the well-known mixing and quartering of the sampler.
Because of this bearing on sampling and for other reasons, I became many years ago much interested in the question, and gave to its solution perhaps more labour than it was worth. In books on Medical Statistics the answer to the question is stated in a mathematical formula, called Poisson's formula, which, in a modified form, I shall give further on. But this did not satisfy me, because I wanted to learn what a reasonably safe _limit of error_ actually meant, and this could be best learnt by experiment; so with the help of some friends I went in for a thorough course of penny-tossing.
Tossing a penny twenty times, an average result would be ten heads and ten tails. To find the deviations from this, we tossed two hundred twenties, _i.e._, four thousand times. Of the two hundred, thirty-three gave the exact average, viz.:--10 heads; sixty-four gave an error of one, viz.:--9 or 11 heads; forty-nine, an error of two; twenty-six, an error of three; twenty, an error of four; eight gave an error of five, and this limit was not exceeded. From these we may say that six is a reasonably safe limit of error. Ninety-seven cases, say one-half, gave an error not exceeding one; and the mean error is 1.8.
In other words, in twenty tosses you will not get more than 16 nor less than 4 heads; you are as likely as not to get 9, 10, or 11 heads; and lastly, if you lost in twenty throws all heads or tails over 10 your average loss would be 1.8 penny, or say roughly 2d. on the twenty throws.
It was necessary to compare these with another series containing a larger average, say that of 100 heads in 200 throws. I confess the labour of tossing pennies two hundred at a time was little to our taste. So from a bag of pennies borrowed from the bank, we weighed out samples containing two hundred, and for an evening we were busy counting heads and tails in these. The heads in sixty samples ranged from 80 to 114. One hundred heads occurred seven times. The extent and frequency of the errors is shown in the table.
------+-------+------+-------+------+------- Error.|No. of |Error.|No. of |Error.|No. of | Times.| | Times.| | Times. ------+-------+------+-------+------+------- 1 | 8 | 6 | 3 | 11 | 1 2 | 5 | 7 | 3 | 14 | 3 3 | 6 | 8 | 3 | 15 | 1 4 | 3 | 9 | 7 | 18 | 2 5 | 6 | 10 | 1 | 20 | 1 --------------------------------------------
We may call the limit of error 21. Twenty-nine results out of sixty, say one-half, had an error not exceeding 4; and the mean error is 5.6. In comparing these with the series 10 in 20 we must, working by rule, divide not by 10 but by 3.16, the square root of 10; for if we multiply an average by any number[126] the error is also multiplied but only by the square root of the number. The error varies as the square root of the number. Now
21/3.16 = 6.6 = limit of error for 10 in 20. 5.6/3.16 = 1.8 = mean error " " " 4/3.16 = 1.2 = probable error " " "
It will be seen that these calculated results agree fairly well with those actually obtained. The rule by which these calculations are made is important and will bear further illustration. To calculate the number of heads in 3200 throws, we have to find the limit of error on a true average of 1600 in 3200. This being 16 times the average of 100 in 200, the corresponding errors must be multiplied by 4. This gives
21×4 = 84 = limit of error. 5.6×4 = 22.4 = mean error. 4×4 = 16 = probable error.
The results I have actually obtained with these large numbers are hardly enough to base much on, but have a value by way of confirmation. Expecting 1600 heads, the actual numbers were 1560, 1596, 1643, 1557, 1591, 1605, 1615, 1545.
It will be seen that exactly half are within the probable error; but this, considering the small number of results, must be more or less of an accident; it is more to the point they are all well within the limits of error.
I have a large number of other results which with a single exception are all in accord with those given; and this exception only just overstepped the limits. It was like a case of nine trumps, which though in a sense possible, is very unlikely to happen in any one's experience.
But even now we are not quite in a position to answer the question with which we started. If you refer to it you will see that we are face to face with this problem: the limit of variation on the 1000 who died would be say 70,[127] ignoring decimals. But if we calculate on the number who did not die, viz.--699,000,[128] we shall get a variation 26 times as great as this. But it is evident the variation must be the same in each case. I submitted this kind of problem also to the test of experiment, the results of which gave me great faith in Poisson's formula.
Imagine two hundred pennies in a bag all heads up. Any shaking will spoil this arrangement and give a certain proportion of tails. And, further, the probable effect of shaking and turning will be to reduce the preponderance of heads or tails whichever may be in excess. This of course is the reason why we are so unlikely to get more than 120 of them in either position.
But if the two hundred pennies are increased to 20,000 by adding pennies which have tails on both sides, then the shaking or mixing would be less effective. We should still expect as an average result to get the 100 heads but in 20,000 instead of 200. The variation will be 28 or 29 on the 100 instead of 20. And this is a better limit in such cases. _Taking 28 as the limit of error on 100 instances_ and proportionally increasing the others so that _the mean error becomes 7.8 and the probable error 5.6_, we may now calculate the answer without gross mistake.
The probable variation on the 1000 deaths by accident will be 18, the mean variation will be 24.6, and the limits of variation 88.5. One such table showing in five years a mean number of deaths of about 1120 per annum gives an annual deviation of about 50 up or down of this. It will be seen at once that an improvement of 30 or 40 in any one year would be without meaning, but that an improvement of from 100 to 200 would indicate some change for the better in the circumstances of the industry. Before applying these principles to the elucidation of some of the problems of sampling it will be well to give Poisson's formula (in a modified form) and to illustrate its working.
Let _x_ equal the number of cases of one sort, _y_ the cases of the other sort, and _z_ the total. In the example, _z_ will be the 700,000 engaged in the industry; _x_ will be the 1000 killed by accidents, and _y_ will be the 699,000 who did not so die. The limit of deviation or error calculated by Poisson's formula will be the square root of 8_xy_/_z_. Replacing _x_, _y_ and _z_ by the figures of the example we get the square root of (8×1000×699000)/700,000, which works out to the square root of 7988.57, or 89.3. Which means that we may reasonably expect the number of deaths not to vary from 1000 by more than 89, _i.e._, they will be between 1090 and 910. It will be seen that this number is in very satisfactory agreement with 88.5 given by the rougher calculation based on my own experiments.
To come to the question of sampling. Consider a powder of uniform fineness and fine enough to pass through an 80 sieve. For purposes of calculation this may be assumed to be made up of particles of about one-eighth of a millimetre across (say roughly 1/200 of an inch); cubed, this gives the content as about 1/500 (strictly 1/512) of a cubic m.m. Now one cubic m.m. of water weighs 1 milligram; therefore 500 such particles if they have the specific gravity of water weigh 1 milligram, and otherwise weigh 1 milligram multiplied by the sp. gr.: 500 particles of ruby silver (Pyrargyrite)[129] will weigh 5.8 milligrams and will contain nearly 3.5 milligrams of silver.
Now suppose a portion of 3.2667 grams (1/10 Assay Ton) of silver ore to contain 500 such particles of ruby silver and no other material carrying silver: such an ore would contain 35 ozs. of silver to the ton. But the limits of variation on 500 particles would be 28[130] multiplied by the square root of 5, or 62 particles. Thus the limit of sampling error would amount to just one-eighth of the silver present, or say to rather more than 4 ozs. to the ton; the mean sampling error would be rather more than a quarter of this, or say about 1.3 ozs. to the ton.
On the other hand, if one took for the assay a charge six times greater (say about 20 grams), the number of particles would be 3000 and the limits of variation would be 28 multiplied by the square root of 30, or 153 particles, which is very closely 1/20 of the silver present, or say 1.75 ozs. to the ton, whilst the mean error would amount to about .5 ozs. to the ton.
To work these examples by Poisson's formula let us assume the gangue to have a mean sp. gr. of 3. Then 500 particles would weigh 3 milligrams; and 3.2609[131] grams would contain 543,500 particles. There would be then 500 of ruby silver and 543,500 of gangue, together 544,000, and the formula gives the square root of (8×500×543500)/544000, which works out to 63 particles as against 62 by the other method.
A practical conclusion from this is of course that either the ore must be powdered more finely or a larger portion than 3 grams must be taken for the assay. Moreover, it is evident that on such an ore no small sample must be taken containing less than several million particles.
Consider now a copper ore of the same uniform fineness containing particles of copper pyrites (sp. gr. 4) of which 1000 particles will weigh 8 milligrams, mixed with gangue of which 1000 particles weigh 6 milligrams.
If one gram of such ore contain .5 gram of copper pyrites (= .1725 gram copper) and .5 gram of gangue, these will contain 62,500 and say 83,500 particles respectively. Altogether 146,000 particles. With Poisson's formula this gives the limit of sampling error as the square root of (8×62500×83500)/146000 or 521 particles. But a variation of 521 on 62,500 is a variation of .83 per cent. The percentage of copper in the ore is 17.25 per cent., and .83 per cent. of this is .14 per cent. The limits of sampling error, therefore, are 17.11 per cent. and 17.39 per cent. Again, it must be remembered that the mean sampling error would be a little over one-quarter of this, or say from 17.2 per cent. to 17.3 per cent. The practical conclusion is that a powder of this degree of fineness is not fine enough. In the last place let us consider a similar iron ore containing 90 per cent. of hæmatite (sp. gr. 5) and 10 per cent. of gangue (sp. gr. 3), 1 gram of such ore will contain 90,000 particles of hæmatite weighing .9 gram and containing .63 gram of iron with say 16,500 particles of gangue weighing .1 gram. Altogether 106,500 particles.
Poisson's formula then gives the limits of variation as the square root of (8×90000×16500)/106500 or 334 particles. But 334 on 90,000 is 0.23 on 63.0, which is the percentage of iron present. The limits of sampling error then are 62.77 per cent. and 63.23 per cent. and the mean variation is from 62.94 per cent. to 63.06 per cent.
These examples are worthy of careful consideration, and it must be remembered that the calculations are made on the assumption that the ore is made up of uniform particles of mineral of such fineness as would pass easily through an 80 sieve, but which does not pretend to represent with great exactness the fineness of the powdered ore customary in practice. They show that having passed through such a sieve is no proof of sufficient powdering, not that all ores powdered and so sifted are unfit for assaying. This last would be an absurd and illogical conclusion.
If an ore be powdered to a fairly fine sand and then be passed through a series of sieves, say a 40, 60, and 80, in such a state that little or none remains on the first, but the others retain a large proportion; then of that which comes through the 80 sieve, perhaps two-thirds by weight may be even coarser than the powder I have used in the example. Of the rest most may be of about half this diameter; the weight of the really fine powder may be quite inconsiderable. On the other hand, if the grinding be continued until, on sifting, little or nothing that is powderable remains on the sieves; then in the sifted product the proportions will be very different. This last, of course, is the only right way of powdering. Also it is evident that so much depends on the manner of powdering that nothing precise can be stated as to the average coarseness of the powder. Suppose, however, by good powdering a product is obtained which may be represented by a uniform powder with particles 1/20th of a millimetre in diameter (say roughly 1/500 inch). Compared with the previous powder, the diameter has been divided by 2.5; their number, therefore, in any given weight has been increased by the cube of 2.5, which is 15.6. But the value of a sample varies as the square root of the number of particles. Hence the reduction in size and consequent increase in number has made the sample nearly four times better than before; and it will be seen that this brings the sampling error within tolerable limits.
There are one or two words of warning which should be given. In the first place, using a 90 sieve instead of an 80 must not be too much relied on; the powder I took in the example would pass through it. It is a question of good powdering rather than of fine sifting. In the second place, a set of, say half-a-dozen, assays concordant within 1 oz. where the theory gives 4 ozs. as the limit of error does not upset the theory: the theory itself states this as likely. It is the error you _may_ get in one or two assays out of a hundred, not the error you are _likely_ to get in any one assay, which is considered under the heading "limit of error."
Accepting the result just arrived at that a portion of 1 gram may be safely taken for an assay if the particles are 1-20th of a millimetre in diameter, the further question remains as to what weight of the original sample must be reduced to this degree of fineness. This may be answered on the principle that the same degree of excellence should be aimed at in each of a series of samplings. This principle is illustrated in the table on page 2.
A fine sand, such as would pass a 40 sieve but be retained on a 60 sieve, would be fairly represented by particles one-quarter of a millimetre in diameter. This being five times coarser, to contain the same number of particles must be 125 times (the cube of 5) as heavy; therefore 125 grams of it can be taken with the same degree of safety as 1 gram of the finer powder. Of such a sand about this weight should be taken and reduced to the finer powder. If the ore were in coarse sand, say in particles 1 millimetre in diameter, this would be four times as coarse as that last considered, and we should have to take 64 times as much of it: 64 times 125 grams is 8 kilos, or say roughly from 15 to 20 lbs. This should be crushed to the finer size and mixed; then from 100 to 150 grams should be taken and ground to the finest powder.
There is, however, a reason why, on the coarser stuff, a smaller proportion may safely be used. This becomes more evident if we consider a still coarser sample. A heap of ore in stones about 2 inches across would be 50 times coarser than the sand, and an equivalent sample would need to be 125,000 times heavier; this would amount to about 1000 tons. Experienced samplers would say that under such conditions so large a sample was hardly necessary.
This is because I have assumed in the calculations that the grains of copper pyrites, for example, were all copper pyrites and the particles of gangue were free from copper. This would be true or nearly so for the very fine powder, but far from true in the case of the ore heap. In the heap probably few of the stones would be pure ore and still fewer would be free from copper. The stones would differ among themselves in their copper contents only within certain comparatively narrow limits. And it is evident that, if replacing one stone by another, instead of resulting in the gain or loss of all the copper one or other contained, merely affected the result to one-tenth of this amount, then a sample of 1-100th of the weight (say 10 tons) would be equally safe.
It should be remembered, however, that while the man who samples on a large scale can safely and properly reduce the size of his samples on this account, yet the principle is one which counts less and less as the stuff becomes more finely divided, and ought to be ignored in the working down of the smaller samples which come to the assayer.
FOOTNOTES:
[126] The 10 in 20 multiplied by 10 = 100 in 200.
[127] Multiply the errors for 100 by the square root of 10.
[128] Multiply the errors for 100 by the square root of 6990.
[129] Sp. Gr. 5.8. Silver 60 per cent.
[130] Taking 28 as the limit of variation on 100.
[131] The weight of the ore less the weight of ruby silver in it.
INDEX.
Acid measures, 49
Acidimetry, 323
Acidity of ores, 168
Acids, 54 strength of, 54, 75, 436
Air of mines, carbonic acid in, 428
Alkalies, 330 determination of, 331 Lawrence Smith's method for, 333, 412 separation of, 332
Alkalimetry, 323
Alkaline earths, 320
Alumina, 314 determination of, 315 in mineral phosphates, 316 separation of, 314, 316
Amalgamation, 126
Ammonia, detection of, 341 determination of, 342 in natural waters, 353
Antimony, 225 detection of, 227 dry assay for, 226 gravimetric assay, 228 separation of, 228 volumetric assay, 229
Arsenic, 381 detection of, 381 dry assay for, 382 gravimetric assay, 383 in brimstone, 393 in crude arsenic, 388, 393 in mispickel, 125, 392 iodine, assay for, 386 separation by distilling, 384 uranium acetate, assay for, 389 Volhard's method applied to, 124
Assay book, 11 note, 12 results, 7 tons, 13, 131
Assaying, 1 methods, 15
Assays, check, 154 preliminary, 147
Atomic weights, 69 table of, 433
Barium, 326
Baryta, 326
Barytes, sulphur in, 378
Base bullion, sampling of, 157
Basic acetate separation, 233
Baumé's hydrometer, 77
Beryllia, 319
Bismuth, 220 colorimetric assay, 223 detection of, 221 gravimetric determination of, 222 in commercial copper, 208 separation of, 222
Black tin, 271 an analysis of, 287 assay of, 276 copper in, 204 examination of, 285 separation by vanning, 272
Blank assays, 34
Blende, sulphur in, 375 zinc in, 266
Book, assay, 11 laboratory, 10 sample, 9
Boracic acid. _See Boron_
Borax, examination of, 431
Boron, 429 direct determination of, 431
Brass, copper in, 194 zinc in, 265
Bromine and bromides, 361
Bronze, copper in, 194 tin in, 281
Burettes, 51
Burnt ore, silver in, 116, 118 sulphur in, 377
Cadmium, 269 gravimetric determination, 269 separation of, 269
Caesium, 339
Calcination, 22, 92, 139, 345
Calcium, 320 detection of, 321 gravimetric determination, 321 separation of, 321 titration with normal acid, 322 titration with permanganate, 322
Calculation of results, 7
Calculations from formulæ, 70
Calorific effect of coal, 419
Calorimeter, 419
Calx, 345
Carbon, 414 gravimetric determination, 416 in iron or steel, 423
Carbonates, 424
Carbonic acid in the air of mines, 428
Caustic potash = potassium hydroxide, 65
Caustic soda = sodium hydroxide, 66
Cerium, 318
Chalybite, iron in, 243
Charcoal, 21, 94
Check assays for gold, 154 for silver, 104, 113
Chlorine and chlorides, 359
Chromium, 307 gravimetric assay, 309 in chrome iron ore, 308 volumetric assay, 309
Clays, examination of, 316
Coals, 418
Cobalt, 259 detection of, 259 dry assay for, 251 gravimetric determination, 260 in hardhead, 288 separation from nickel, 442, 254, 258
Coke, 25
Common salt, examination of, 336
Concentrates, assay for gold of, 140
Colorimetric assays, 44
Copper, 175
Copper, bismuth in, 208 colorimetric assay for, 190, 203 commercial, arsenic in, 208, 388 commercial, copper in, 193 commercial, examination of, 205 cyanide assay for, 194 dry assay of, 176 dry assay, loss of, in, 176 electrolytic assay for, 190, 203 gold in, 206 iodide assay for, 199 iron in, 209, 249 lead in, 206 separation of, 183 silver in, 205 sulphur in, 207
Copper ores, solution of, 183 valuation of, 181
Copper pyrites, copper in, 179, 188, 198, 202 sulphur in, 376
Culm, 22
Cupel, 23, 142
Cupellation, loss, corrections for, 103 loss in gold, 145 loss in silver, 101 of gold lead alloys, 182 of silver lead alloys, 98, 110 temperature of, 143
Cyanicides, 169
Cyanide assay for copper, 194 for nickel, 255 for tin, 278
Cyanides, alkalinity of, 167 assay of, 167 commercial, 160 double, 161 gold-dissolving power, 162 prussic acid, 162 volumetric determination of, 163, 165
Cyanide liquors, alkalinity of, 167 assay of, 164, 165 assay of, for gold, 140 assay of, for zinc, &c., 169
Daniell cells, 185
Didymium, 319
Dollars to the ton, 9
Dry assays, 16
Drying, 5, 33
Earths, 314 the alkaline, 320
Electrodes, 187
Electrolysis for copper, 184 for nickel, 254
Equations, 69
Erbia, 319
Ferrous and ferric salts, 231
Filtration, 31
Finishing point, 42
Flasks, graduated, 49
Flatting, 149
Fluorine and fluorides, 363
Fluxes, 16, 93, 136, 138, 140
Formulæ, 68
Furnaces, 25
Galena, lead in, 217, 218
Gangue, 405 iron in the, 244
Gas-measuring apparatus, 52
Gases, measurement of, 44
Gay-Lussac's assay for silver, 119 assay for silver modified, 123
German silver, copper in, 194 nickel in, 255, 259
Gold, 126 amalgamation of, 126 in cyanide liquor, 140 loss of, in cupellation, 145 loss of, in parting, 154 preparation of, 63 silver in, 157 silver in, after parting, 154 test for, 126
Gold-lead alloys, cupellation of, 142 sampling of, 158
Gold ores assay with cyanide solutions, 141 calcination of, 139 concentrates, 140 fluxing, 136, 138, 140 sampling of, 127 size of assay charges, 127 tailings, 140
Gold-parting, 150 platinum in, 145, 154, 170, 171
Gold-zinc slimes, 142
Graduated vessels, 49
Gravimetric methods, 15, 27
Halogens, 358
Hardhead, 287 an analysis of, 289
Hot plate, 30
Hydrogen, preparation of, 62 reduction by, 280
Hydrometer, 77
Ignition, 32 in hydrogen, 280
Indicators, 42
Inquartation, 146
Iodine and iodides, 362
Iridium, 171
Iron, 231 bichromate assay for, 237, 243, carbon in, 423 colorimetric assay for, 247 ferrous and ferric, 231 gravimetric determination, 233 permanganate assay for, 236, 238 phosphorus in, 399 reduction of ferric solutions, 235, 241 separation of, 232 stannous chloride assay for, 244 volumetric assays for, 234
Iron ores, iron in, 244, 247 phosphates in, 399
Laboratory books, 9
Lanthanum, 319
Lawrence Smith's method for alkalies, 333, 412
Lead, 211 colorimetric assay for, 218 detection of, 211 dry assay for, 211 gravimetric determination of, 213 in commercial copper, 206 in commercial zinc, 214 in galena, 217, 218 separation of, 211, 213 volumetric determination of, 214
Litharge, use of, in dry assays, 20, 93
Lithium, 338
Lime, 320 milk of, 321 volumetric assays for, 322
Limestone, examination of, 329 lime in, 324
Limewater, 321
Loths, 9
Magnesia, magnesium, 328 mixture, preparation of, 64
Manganese, 298 colorimetric assay, 306 detection of, 299 gravimetric determination of, 300 separation of, 299 volumetric determination of, 300
Manganese peroxide, ferrous sulphate assay for, 301 iodine assay for, 302 = manganese dioxide, 298
Manganese ore, copper in, 204 manganese in, 300 peroxide in, 302
Matte, 18
Measuring, 49 flasks, 49 gases, 44, 52 gold buttons, 133, 440, liquids, 49 silver buttons, 106
Mechanical methods, 16
Mercury, 171 dry assay, 172 wet assay, 173
Metallic particles in ores, gold, 129 particles in ores, silver, 108 particles, tin, 278, 287
Micrometer, 133
Microscope, measuring with the, 440, 133
Mispickel, arsenic in, 125, 392 sulphur in, 376
Moisture, 7, 350
Molybdate separation for phosphates, 395 solution, preparation of, 60
Molybdenum, 311
Muffle, 25
Nessler's solution, 342
Nickel, 251 dry assay for, 251 electrolytic assay, 254 gravimetric determination of, 254 in German silver, 255, 259 separation from cobalt, 254, 258, 442 separation from iron, 258 separation from manganese, 258 separation of, 253 volumetric assay, 255
Niobium, 297
Nitre, 22 use of, in dry assays, 95
Nitrogen and nitrates, 400
Nitrometer, 403
Normal acid, normal solutions, 323
Ores, determining water in, 5, 351 drying, 5 powdering, 4, 109, 130, 448 quantities of, for an assay, 11, 27, 127 sampling, 1, 127, 444 with metallic particles, 3, 108, 129
Osmiridium, 171
Osmium, 171
Ounces to the ton, long, 107 to the ton, short, 132
Oxidation, 345
Oxides, 345 determination of oxygen in, 346
Oxidising agents, 22, 95, 345 effect of nitre, 95 effect of nitric acid, 56
Oxygen, 344 equivalent, 358 in natural waters, 344, 356 in ores, 348
Palladium, 171
Parting, 150 acids, 150 in flasks, 151 in glazed crucibles, 153 in special apparatus, 156 in test tubes, 152
Phosphate, assay of apatite for, 399 assay of iron ore for, 399
Phosphates, gravimetric assay, 396 volumetric assay, 397
Phosphorus and phosphates, 394 in iron, 399
Pipette, 50, 120
Platinum, 170 in gold, 145, 154, 170
Potash, commercial examination of, 338
Potassium, 336 gravimetric determination, 337
Potassium cyanide, 22, 65, 160 commercial assay of, 167 commercial, purity of, 161
Powdering, 4, 130, 448, 109
Precipitation, 30
Precipitates, drying, 32 igniting, 32, 34 washing, 31
Preliminary assays, 104, 147
Preparation of acids, 54 of other reagents, 59
Prill, 108, 129, 278, 287
Produce, 8
Pyrarsenate of magnesia, 383
Pyrites, iron in, 244 sulphur in, 370, 376
Pyrophosphate of magnesia, 397
Quantity to be taken for an assay, 11, 27, 127
Quartation, 146
Quartering, 2
Reagents, strength of, 54
Red lead for dry assays, 20, 22, 94
Reducing agents, 21, 94 effects of charcoal, &c., 94 effect of mineral sulphides, 95, 97, 98
Reduction by hydrogen, 280 of ferric solutions, 235, 242, 244
Regulus, 18
Report form, 12
Results, calculation of, 7, 13, 16, 38, 107, 131, 132 statement of, 7
Rhodium, 171
Roasting, 22, 345
Rolling, 149
Rubidium, 340
Ruthenium, 171
Sample book, 9
Sampling, 1 effect of powdering on, 449 errors, 447 gold ores, 127 metals, 157 theory of, 444
Scorification of silver ores, 88
Scorifier, 23, 89
Selenium, 379
Separation, as sulphides, 57 basic acetate, 233 molybdate, 395
Shales, bituminous, 420
Silicon and silicates, 405 in iron, 414
Silica in rocks, 409 in slags, 414
Silicates, alkalies in, 333, 412 beryllia in, 320 examination of, 409 titanium in, 411
Silver, 87 correction for cupellation loss, 103 detection of, 87 Gay-Lussac's assay, 119 Gay-Lussac's assay modified, 123 gravimetric determination of, 117 in bullion, 113 in burnt ore, 116, 118 in copper, 114, 205 in galena, 114 in lead, 113 in oxide of lead, 113 in silver precipitate, 115 loss in cupellation, 101 pure preparation of, 66 Volhard's assay, 121 volumetric methods, 119, 121, 123
Silver lead alloys, cupellation of, 98 sampling of, 157
Silver ore, crucible assay of, 90 metallic particles in, 108 scorification of, 88
Size of assay charges, 11, 27, 127
Slags, 19
Soda-lime, 425
Sodium, 334
Sodium cyanide, 160
Solution, 29
Solutions, normal, 323 standard, 36
Specific gravity, 75, 436
Speise, 19
Standard, 37 solutions, 36
Standardising, 37
Steel, carbon in, 423 chromium in, 310 manganese in, 300
Stoking, 25, 143
Strength of reagents, 54
Strontium, 324
Sulphates and sulphur, 367 gravimetric determination, 369 volumetric determination, 370
Sulphides, reducing action of, 9, 95
Sulphocyanate assay for silver, 121
Sulphur in blende, 375 in burnt ore, 377 in chalcocite, 376 in coal, 419 in copper, 207 in copper pyrites, 376 in mispickel, 376 in pyrites, 370, 376
Sulphuretted hydrogen, preparation, 57
Surcharge, 154
System in assaying, 28
Table, atomic weights, 433 comparing thermometers, 435 ounces to the long ton, 107 ounces to the short ton, 132 sp. g. ammonia, 438 sp. g. hydrochloric acid, 437 sp. g. minerals, 86 sp. g. nitric acid, 436 sp. g. sulphuric acid, 439 sp. g. water, 83
Tantalum, 297
Tartar, 20, 94
Tellurium, 379 improved test for, 150
Thallium, 219
Thorium, 317
Tin, 271 _See also Black tin_ assay for, by vanning, 273 copper in, 204 Cornish assay, 276 cyanide assay, 278 detection of, 279 gravimetric determination of, 281 iron in, 250 separation of, 280 volumetric assay for, 282
Tin arsenide, 284
Tin phosphide, 284
Tin slag, 290 an analysis of, 292 tin in, 290
Titanium, 292 detection of, 293 in black tin, 272, 287 in rocks, 411 separation, &c., 294
Titration, 35 indirect, 43, 72
Ton, assay, 13, 131, long, 2240 lbs. = 32,666.6 oz., 107 short, 2000 lbs = 29,166.6 oz., 132
Tungsten, 295
Tungstic acid, 295 gravimetric determination, 296 in black tin, 285 in wolfram, 296
Uranium, 312
Valuation, of copper ores, 181
Vanadium, 310
Vanning, 273
Volhard's assay applied to arsenic, 124 silver assay, 121
Volume-corrector, 53
Volumetric assay, 35, 38
Water, 7, 350 direct determination of, 351 examination of, 352 expansion of, 83 solids in, 354
Weighing, 47 small gold buttons, 131
Weights, 47
Wolfram, an analysis of, 296 tungstic acid in, 296
Yttria, 319
Zinc, 261 commercial, examination of, 268 commercial, iron in, 249 commercial, lead in, 214 dry assay, 261 gasometric assay, 266 gravimetric determination, 262 in blende, 266 in cyanide liquors, 169 in silver precipitate, 266 separation of, 262 volumetric assay, 263
Zirconia, 317
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* * * * *
THIRD EDITION, _Thoroughly Revised. Royal 8vo. With numerous Illustrations and 13 Lithographic Plates. Handsome Cloth. Price 30s._
~A PRACTICAL TREATISE ON~
~BRIDGE-CONSTRUCTION:~
~Being a Text-Book on the Construction of Bridges in Iron and Steel.~
~FOR THE USE OF STUDENTS, DRAUGHTSMEN, AND ENGINEERS.~
BY T. CLAXTON FIDLER, M. INST. C.E.,
~Prof. of Engineering, University College, Dundee.~
GENERAL CONTENTS.--PART I.--Elementary Statics. PART II.--General Principles of Bridge-Construction. PART III.--The Strength of Materials.