Part 7
Acids which, like those mentioned in the foregoing examples, contain one atom of replaceable hydrogen are called monobasic; those which contain two such atoms (_e.g._ sulphuric acid, H_{2}SO_{4}; tartaric acid, H_{2}C_{4}H_{4}O_{6}),[7] dibasic; those which contain three such atoms (_e.g._ phosphoric acid, H_{3}PO_{4}; citric acid, H_{3}C_{6}H_{5}O_{7}),[7] tribasic; and so on with acids of higher basicity. Acids of greater basicity than unity are frequently termed polybasic.
[Footnote 7: See footnote, p. 26.]
Besides containing replaceable or basic hydrogen, acids are further characterised by the property of combining with alkaloids to form salts; _e.g._--
Sulphuric Acid. Quinia. H_{2}SO_{4} + 2C_{24}H_{24}N_{2}O_{2} =
Quinia Sulphate. (C_{20}H_{24}N_{2}O_{2})_{2} . H_{2}SO_{4}
Acetic Acid. Morphia. HC_{2}H_{3}O_{2} + C_{17}H_{19}NO_{3} =
Morphia Acetate. C_{17}H_{19}NO_{3} . HC_{2}H_{3}O_{2}
=Dibasic Acids.= See ACID.
=Fatty Acids.= Acids separable from fats or oils; _e.g._ stearic acid, oleic acid, butyric acid, &c.
=Inorganic Acids.= Same as MINERAL ACIDS (which _see_).
=Mineral Acids.= Acids chiefly or wholly derived from the mineral kingdom. In _medicine_, sulphuric, hydrochloric, and nitric acids, are commonly so called.
=Monobasic Acids.= See ACID.
=Organic Acids.= Acids formed by, or derived from organic substances; _e.g._ acetic acid, tartaric acid, uric acid, &c.
=Polybasic Acids.= See ACID.
=Pyro-acids.= Acids resulting from the decomposition by heat of other acids, _e.g._ gallic acid, when heated, yields pyro-gallic acid.
=Tribasic Acids.= See ACID.
=ACIDIFICA'TION.= [Eng., Fr.] _Syn._ ACIDIFICA'TIO, L. In _chemistry_, the act, process, or state of acidifying, or of making, becoming, or impregnated with acid.
=ACIDIM'ETER.= _Syn._ ACIDOM'ETER; ACIDIME'TRUM, &c., L.; ACIDIMÈTRE, Fr. An instrument or apparatus employed in acidimetry.
The ordinary acidimeters of the chemist are small tubes, constructed to hold exactly 1000 grains of distilled water, at 60° Fahr., within the limits of their scale, which is accurately graduated into 100 divisions. They are used to contain the alkaline solutions (TEST-LIQUORS, NORMAL or STANDARD SOLUTIONS) employed in the following processes.
Beaumé's Acidimeter, and others of the same class, are HYDROMETERS, and are described under that 'head.'
=ACIDIM'ETRY.= _Syn._ ACIDOM'ETRY; ACIDIME'TRIA, &c., L.; ACIDIMÉTRIE, Fr. The estimation of the strength or quantity of acid, in a free state, contained in any liquid. It is the reverse of 'alkalimetry.' Acidimetrical assays are understood to refer to the relative strengths of the same acids (_i. e._, the quantity of real acid of the same kind contained in the solutions examined), and not to the comparative strengths of acids of different composition or names.
_Acidimetrical processes._ These are founded chiefly on the capacity of the acids to saturate the bases; and, in some of the liquid acids, on the specific gravity.
_a._ VOLUMETRICALLY:--
1. The sample of the acid to be examined (100 gr., or any convenient aliquot part thereof) is placed in a suitable glass vessel, and if it be one of the stronger acids, diluted with six or eight times its weight of water, or if solid (as oxalic, or citric acid), dissolved in a like quantity. This liquid is then exactly neutralised with an alkali.
This point is usually determined, by the addition of a small quantity of litmus solution, which turns just blue when the solution is neutralised, but when a carbonate is used for the alkaline solution, the acid must be boiled a short time after each addition to expel the carbonic acid. The quantity of the alkaline solution consumed for this purpose represents an equivalent quantity of acid, and thus gives us the acid content of the sample under examination. The common practice is to dissolve one equivalent of the alkaline test in grains or grammes in water, and to make up the solution to exactly 1000 parts by measure (_i. e._, 1000 'water-grains' or grammes), so as to accurately fill the 100 divisions of an acidimeter; when the quantity, in grains or grammes, of the sample tested, bears the same proportion to the equivalent number of the acid under examination, that the number of acidimeter divisions of the test-liquor consumed bear to the per-centage of acid sought. Thus:--suppose 50 gr. of a sample of sulphuric acid take 25 acidimeter divisions (300 parts or water-grains measure) of the test-liquid to neutralise it, what is its content of real acid?
The equivalent of sulphuric acid is 49 (half its atomic weight); so, by the rule of proportion,
50 : 49 :: 25 : 24-1/2
It therefore contains 24-1/2 parts of real sulphuric acid, in 50.
If the 1000 parts or grain-measures, instead of the number of the acidimeter divisions, be taken for the calculation, it will, of course, be necessary to point off the first right-hand figure of the result as a decimal. Thus; repeating the above example--
50 : 49 :: 250 : 24·5
Or, since the equivalent of the test-liquid is 100, it will bear the same proportion to the equiv. of the acid examined as the number of the acidimeter divisions of the test-liquid consumed in neutralising 100 gr., do to the per-centage sought. Thus:--50 gr. of hydrochloric acid take 45 acidimeter divisions to effect neutralisation, what is its real strength?--The equiv. of hydrochloric acid is 36·5: therefore--
100 : 36·5 :: 45 : 16·425%
and, since only 50 gr. (instead of 100 gr.) were examined--
16·425 × 2 = 32·85%
Some operators prefer employing 100 gr. instead of the equivalent weights of the given tests in making their test-solutions, in which case each gr. or 1000th part represents 1/10th, and each acidimeter degree 1 gr. of the alkali or carbonate employed; when a similar proportion will obtain to that first above given.
In technical analysis it is more convenient if the number of acidimeter divisions of the 'test-liquid' consumed express the per-centage strength of the acid, without further calculation. For this purpose the number of grains of the acid taken for the assay should correspond to the equivalent number of such acid (see _Table_ I, below); or to some convenient aliquot part of it, as the 1/2, 1/4, 1/5, or 1/10th; the per-centage answer, in the last case, being doubled, quadrupled, &c., according to the aliquot part taken. The reason of this is obvious.
For the test-solutions, ammonia, and the dry and crystallised carbonates and bicarbonates of potash and soda, are used, and are made by dissolving in water their constituents except ammonia, of which 1000 grains, or one litre, of solution of specific gravity 0·992 contains exactly one equivalent.
53 grains (or grammes) of pure anhydrous carbonate of soda, prepared by gradually heating to redness the crystallised salt, constitute one equivalent (half the atomic weight), and 69 grains (or grammes) of pure dry carbonate of potash. Of the crystallised salt 143 grains of carbonate of soda will be required, and 84 grains (grammes) of the crystallised bicarbonate of soda, and 100 of the crystallised bicarbonate of potash. Occasionally solutions containing in one thousand parts, 50 of pure carbonate of lime or chalk, or 28 of pure caustic lime, are used.
Besides these, a process known as Kiefer's is practised, and an ammoniacal solution of oxide of copper is employed as the 'test-liquor,' and the 'point of neutralisation' is known by the turbidity observed as soon as the free acid present is completely saturated.
The normal solution or test-liquor is prepared by adding to an aqueous solution of sulphate of copper, pure ammonia water, until the precipitate, which at first forms, is just redissolved, carefully avoiding excess. Or better, by adding a rather strong solution of sulphate of copper, to a quantity of a rather strong solution of ammonia containing exactly 17 gr., or one equiv. of pure ammonia, as long as the precipitate which forms is redissolved on agitation; the resulting liquid being afterwards diluted with pure distilled water, until it accurately measures 1000 water-grains, or fills 100 divisions of an acidimeter, at 60° Fahr. In either case, the strength of the resulting 'test-solution' must be carefully determined by means of standard sulphuric acid, and adjusted, if necessary.
This method answers well with all the stronger acids (excepting oxalic acid), even when dilute; and it has the advantage of not being affected by the presence of a neutral metallic salt with an acid reaction, as sulphate of copper, or of zinc.
Besides this process a solution of lime in sugar may be used, as proposed by M. Peligot, and made as follows:--
Pure caustic lime is carefully slaked by sprinkling with water, and 50 grains (or grammes), made up by water to a milky solution, and 100 grains of pure sugar candy dissolved in 1000 grains of water, are added, and the whole well shaken. It is allowed to settle in a closed bottle, and the clear solution poured off and diluted, until 1000 grains neutralise exactly 100 grains of pure hydrochloric acid of sp. gr. 1·1812. Of course it only answers with acids whose calcium salts are readily soluble in water.
_b._ GRAVERMETRICALLY:--
The test-liquors or standard solutions of the above methods are made up so as to _weigh_ exactly 1000 grains, instead of to 'measure' 100 acidimeter divisions. Every grain of the test-liquor thus represents 1/10th gr. of alkali; and every 10 gr., 1 gr. of alkali; or respectively, 1/10th per cent. and 1 per cent. The vessel used for containing the solutions is carefully weighed whilst empty, and 1000 gr. being placed in the opposite scale, the test-solution, containing exactly one equivalent of base, is poured in, and the whole made up with distilled water (if necessary) so as to restore the balance to an equilibrium. After the process of neutralisation, the acidimeter, with its contents, is again placed in the scales; its previous weight still remaining there. The number of grains required to restore the equilibrium of the balance (_i.e._, the loss of weight), gives the exact weight of the test-liquor consumed. In all other respects the process is the same as in the 'volumetrical method' already described.
Another method for estimating the strength of the sample of acid is by weighing the amount of carbonic acid expelled during saturation. (Method of Fresenius and Will.) This depends on the weight of gaseous carbonic acid which a given weight of the acid-sample under examination is capable of expelling from pure bicarbonate of soda (or of potash), which is estimated by the loss of weight in the acidimeter, or apparatus, after the gas, rendered perfectly dry by passing through sulphuric acid, has escaped into the air.
TABLE I.--_Weights of the respective acids equivalent to the given weight of the principal bases, hydrogen being taken as unity._
{51 Acetic acid (anhydrous). {60 " " (crystallised or glacial). {99 Arsenious acid (dry). {35 Boracic acid (anhydrous). 17 gr. of pure ammonia.[8] } {62 " " (crystallised). 31 " anhydrous soda.[9] } {22 Carbonic acid (dry). 40 " hydrate of soda.[9] } {67 Citric acid (crystallised). 53 " dry carbonate of soda.[10] } {85 Gallic acid (dried at 212°). 143 " crystallised carbonate of } {94 " " (crystallised). soda.[11] } {127-1/2 Hydriodic acid (dry or gaseous). 84 " crystallised bicarbonate } {27 Hydrocyanic acid (anhydrous). of soda. } {36-1/2 Hydrochloric acid (dry or gaseous). 47 " anhydrous potassa.[9] } {109 " " (liquid, sp. gr. 1·162). 56 " hydrate of potassa.[9] } are {166-1/2 Iodic acid. 69 " dry carbonate of potassa.[10]} exactly {54 Nitric acid (anhydrous). 100 " crystallised bicarbonate } neutralised {63 " " (liquid, _monohydrated_, sp. gr. of potassa. } by { 1·517 to 1·521). 50 " {pure chalk. } {67-1/2 " " (liquid, _sesquihydrated_, sp. gr. {pure marble. } { 1·5033 to 1·504). 28 " pure caustic lime. } {72 " " (liquid, _binhydrated_, sp. gr. 37 " hydrate of lime (fresh). } { 1·486). 44 " dry carbonic acid (when } {90 " " (liquid, sp. gr 1·42). the bicarbonate of } {36 Oxalic acid (anhydrous). potassa or soda is } {63 " " (crystallised). used for testing in } {72 Phosphoric acid (anhydrous). the process of Fresenius } {81 " " (glacial). and Will). } {50 Succinic acid (dry or anhydrous crystals). 22 " dry carbonic acid (when } {59 " " (ordinary crystals). a dry carbonate is } {40 Sulphuric acid (anhydrous). used). } {49 " " (liquid, _monohydrated_, sp. { gr. 1·8485). {75 Tartaric acid (crystallised). {212 Tannic acid (carefully dried).
[Footnote 8: 1000 water-grains measure of pure liquor of ammonia, sp. gr. 0·992, contains exactly 17 gr., or 1 equiv. of pure gaseous ammonia. A standard liquor of this strength may be most conveniently prepared by cautious dilution of a stronger solution, until a hydrostatic bead, corresponding to the sp. gr., floats indifferently in the middle of the new solution, at 60° Fahr. By keeping two hydrostatic beads in the solution--the one made barely to float, and the other barely to sink--we shall always be able to detect any change of strength or temperature which it may suffer; since the "loss of a single hundredth part of a grain of ammonia per cent., or the difference of a single degree of heat, will cause the beads to" vary their positions. To preserve its integrity it must be kept in a well-stoppered bottle. (See below.)]
[Footnote 9: These substances, as well as 'test-solutions' containing them, must be perfectly free from carbonic acid, and must be carefully preserved to prevent the absorption of carbonic acid from the atmosphere. Mohr states that a dilute solution of either of them is best preserved in a flask or bottle well closed with a cork fitted with a small bulb tube (resembling a chloride of calcium tube), filled with a finely triturated mixture of sulphate of soda and caustic lime, and bearing a very thin open tube in the exit aperture. Fresenius, and most other foreign chemists, prefer 'test-solutions' of pure soda. With test-solutions containing caustic alkalies, exact neutralisation of an acid is not only more easily effected, but more readily perceived, particularly when either solution is tinted with litmus.]
[Footnote 10: Prepared by gradually heating the pure crystallised carbonate to redness. From being uniform in composition, and easily procured or prepared, they are much employed; preference being usually given to the soda-salt.]
[Footnote 11: The crystals must be free from attached water, but not the least effloresced.]
_Oper._ A determined amount of the acid under examination is accurately weighed into the flask _A_ (see _engr._); and if it be a concentrated acid, or a solid, it is mixed with or dissolved in 6 or 8 times its weight of water. The little glass tube (_e_) is then nearly filled to the brim with pure bicarbonate of soda, in powder, and a fine silken thread is tied round the neck of the tube, by means of which it can be lowered down into the flask (_A_), so as to remain perpendicularly suspended when the cork is placed in the latter; the cord being held between the cork and the mouth of the flask. The flask (_B_) is next about half filled with oil of vitriol, and the tubes being arranged in their places, as represented in the _engr._; and time having been allowed for the mixture of acid and water to cool completely, after the increase of heat caused by mixing, the whole apparatus is very accurately weighed. The cork in the flask (_A_) is then slightly loosened, so as to allow the little tube containing the bicarbonate of soda to fall into the acid, and is again instantly fixed AIR-TIGHT in its place. The evolution of carbonic acid now commences, and continues until the acid in the flask (_A_) is neutralised. When this takes place, which is easily seen by no bubbles being emitted on shaking the apparatus, the flask (_A_) is put into hot water (120° to 130° Fahr.), and kept there, with occasional agitation, until the renewed evolution of gas has completely ceased. The little wax stopper is then taken off the tube (_a_), the apparatus taken out of the hot water, wiped dry, and suction applied, by means of a perforated cork, or a small india-rubber tube, and the mouth, to the end of the tube (_d_), until the sucked air no longer tastes of carbonic acid. The whole is then allowed to become quite cold, when it is replaced in the balance (the other scale still containing the original weights), and weights added to restore the equilibrium.
The loss of weight represents the exact quantity of dry carbonic anhydride, or anhydrous carbonic acid gas, that has been expelled from the bicarbonate of soda, by the action of the acid in the sample examined.
The quantity of real acid it contained is then deduced by the following calculation:--One equivalent of gaseous carbonic anhydride, or anhydrous carbonic acid (= 44) bears the same proportion to one equivalent of the acid in question, as the amount of the carbonic anhydride expelled does to the amount of the acid sought. Thus, suppose a dilute sulphuric acid expels 3 gr. of carbonic anhydride, the arrangement is--
44 : 49 :: 3 : 3·349
Consequently the sample operated on contained 3·5 (nearly) grains of true sulphuric acid.
Instead of the above calculation, we may multiply the weights of the respective acids required to expel 1 gr. of carbonic acid (as exhibited in the following table) by the number of gr. of dry carbonic acid evolved during the above operation. The product represents the per-centage strength, when 100 gr. of the acid have been examined. When only 50, 25, 20, or 10 gr. have been tested, this product must, of course, be doubled, quadrupled, &c., as the case may be.
TABLE II.
Multipliers. Acetic acid (anhydrous) 1·159 " " (hydrated or glacial) 1·364 Citric acid (crystallised) 1·523 Hydrochloric acid (dry or gaseous) ·829 " " (sp. gr. 1·16) 2·478 Nitric acid (anhydrous) 1·227 " " (sp. gr. 1·5) 1·523 " " (sp. gr. 1·42) 2·045 Oxalic acid (crystallised) 1·432 Sulphuric acid (anhydrous) ·909 " " (sp. gr. 1·8485) 1·114 Tartaric acid (anhydrous) 1·500 " " (crystallised) 1·705
Even this easy calculation may be avoided, in technical analysis, by simply taking for the assay such a weight of the respective acids as is capable of disengaging exactly 10 gr. of dry carbonic acid from the bicarbonate. In this case, the loss of weight in grains, from the operation, multiplied by 10, at once indicates the exact per-centage strength sought. The proper weight of any acid to be taken to give per-centage results is found by simply dividing ten times the equiv. of that acid by 44. For, taking sulphuric acid as an example,
as-- 44: 49 :: 10 : 11·1318
or 11·13 nearly.
On this principle are obtained the weights to be taken, as given in--
TABLE III.
Grains. Acetic acid (anhydrous) 11·59 " " (hydrated or glacial) 13·64 Citric acid (crystallised) 15·23 Hydrochloric acid (dry or gaseous) 8·29 " " (sp. gr. 1·16) 24·78 Nitric acid (anhydrous) 12·27 " " (sp. gr. 1·5) 15·23 " " (sp. gr. 1·42) 20·45 Oxalic acid (crystallised) 14·32 Sulphuric acid (anhydrous) 9·09 " " (sp. gr. 1·845) 11·14 Tartaric acid (anhydrous) 15·00 " " (crystallised) 17·05
2. A convenient modification of the preceding method of acidimetry consists in using the common apparatus figured in the margin and employing fused chloride of calcium to dry the evolved carbonic acid gas, instead of concentrated sulphuric acid. The mode of conducting the process and obtaining the results is precisely the same as in that last explained, and need not, therefore, be repeated. In this case, however, suction must be applied to the small tube (_g_), instead of (_d_) in the accompanying engraving.
_Obs._ These methods, though apparently complicated, are not difficult to perform, when once well understood. The application of heat after the completion of the operation is indispensable, as, if it were neglected, from 0·3 to 0·4 of a gr. of carbonic acid would be retained in the liquid. The bicarbonate of soda must be pure, and perfectly free from any neutral carbonate or sesquicarbonate of soda. To ensure this, the bicarbonate of commerce is reduced to a uniform powder, put into a glass jar, and covered with its own weight of cold distilled or rain water, and allowed to stand for twenty-four hours, with frequent stirring. It is then placed upon a funnel, the tube of which is stopped with loose cotton, so as to allow the lye to drain off. It is next washed several times with small quantities of cold distilled or rain water, and after being dried by pressure between some sheets of blotting-paper, without the aid of heat, is kept for use in a well-closed glass bottle. Before use, it may be tested to ascertain its purity. If pure, it neither reddens turmeric paper, nor gives a brick-red precipitate with a solution of bichloride of mercury. Pure bicarbonate of potassa may be used instead of bicarbonate of soda; but in either case it is always proper to use an excess, so as to leave some undecomposed carbonate after the operation has ended. The presence of a little sodium chloride or sulphate in the bicarbonate will not interfere in the least, but the absence of every trace of neutral carbonate is a _sine quâ non_.