CHAPTER XVI

«SYSTEMATIC ANALYSIS FOR ACID IONS»

The systematic analysis for acid ions is made on a plan differing in an important particular from the systematic analysis for metal ions. The latter, as has been seen, are divided into groups, which, by precipitation or solution of characteristic salts, are successively separated from subsequent groups, before the isolated groups are analyzed. That is, in general, a group of metal ions is examined in the absence of the ions of all other groups. Acid ions are also divided into groups, but, as a rule, the groups are not separated from each other for analysis. The reason for this difference in procedure is found, chiefly, in the fact that foreign acid ions interfere[589] to a smaller degree with the specific tests for the ions of a group, than is the case in the analysis for metal ions.

«Grouping of Acid Ions.»—While there is general agreement, among the most important systems of analysis, in the grouping of metal ions, there is notable variation in the way in which acids are grouped by different authors. We shall confine ourselves here to the consideration of two different modes of general procedure and not discuss differences in minor details.

If acids were present only in the form of the free acids or of their alkali salts, the division into groups could naturally and profitably be made to include groups, which are identified by reactions carried out in ‹neutral› or ‹alkaline› solution, as well as by such as are made in ‹acid› solutions. Now, cations other than the alkali ions are liable to interfere with tests designed for alkaline or neutral solutions. For instance, a group of acid ions, in which the phosphate-ion is included, is characterized by the fact that the acids form barium salts, which are soluble in acid but not in neutral or alkaline solutions. The absence or presence of such a group may be recognized, if no cations other than the alkali metal ions are present, by the addition of barium chloride to the solution and by careful neutralization of any free acid, by ammonium [p300] hydroxide. Barium phosphate and the barium salts of the other acids of the group will be precipitated, if their ions are present. It is clear, however, that a number of metal ions must interfere with the test. For instance, a solution of aluminium nitrate or of ferric chloride, treated with barium chloride, and with ammonium hydroxide to neutralize the acid present in the solution as a result of the hydrolysis of the salt, would give a precipitate of aluminium hydroxide or of ferric hydroxide, and ‹not of barium salts›. The formation of a precipitate under these conditions evidently will not constitute any basis whatever for reaching a conclusion as to the presence or absence of acid ions, such as can form precipitates of barium salts under the same circumstances.

«Systematic Analysis for Acid Ions, Based on the Removal of Metal Ions other than the Alkali Metal Ions.»—On account of this kind of interference, by cations other than those of the alkali group, with a number of group and specific tests for anions, that may be made in neutral or alkaline solutions, provision is made, in most systems of analysis, for the removal of such ions by proper treatment of the substance under examination with sodium carbonate. Interfering metal ions are thereby converted into carbonates or hydroxides which are insoluble in water, while the acids form sodium salts which pass into solution in water. Occasionally, recourse is also taken to hydrogen sulphide to remove ions of the arsenic and copper groups.

The treatment with sodium carbonate, while advantageous in certain cases, is ‹not uniformly successful›[590]: it is also frequently complicated by the presence of amphoteric bases or of organic substances, and frequently demands treatments and tests beyond those made with the solution thus prepared. Furthermore, if a group is found to be present, say a group of acid ions forming barium or calcium salts insoluble in water, but soluble in acids,[591] all the acids of the group, which have not been found to be present or absent in the analysis for metal ions,[592] must be specifically [p301] tested for, ‹although they may all be absent› and the only representative of the group present may be an ion previously found in the analysis for metal ions.[593] Then, too, when the group is found to be absent by the group test, more sensitive tests for some of the acid ions of the group must still be made to insure their complete absence.[594]

While much may be said in favor of the systematic analysis for acid ions, based on the preparation of a solution containing only the alkali salts of the anions, and ‹while one should be familiar with the plan and be able to have recourse to it at will›, yet the drawbacks mentioned suggest another basis for the analysis.

«Systematic Analysis for Acid Ions in Acid Solution.»—If the systematic analysis for acid ions is carried out entirely in ‹acid solutions›, interference of cations with tests for anions is rarely met with and in those rare cases may be easily provided against. Such a method of systematic analysis in acid solution is frequently more direct and more convenient than a method based on the removal of cations other than the alkali metal ions. Almost all of the most characteristic tests for anions, as it is, are carried out in acid solutions, and only a very few good tests, which must be made in neutral or alkaline solution, are sacrificed by placing the emphasis on those carried out in acid solutions. The method, on the whole, has proved a time-saving and convenient one, without loss in the trustworthiness of the results.

«Desirability of Experience with Both Methods.»—It is desirable to have experience with both methods, and to learn by such experience, when to have recourse to the one or the other method. Suggestions as to the choice of method are given in the Laboratory Manual, p. 119.

«The Groups of Acid Ions.»—The arrangement of the acid ions into groups, for analysis in acid solution, does not differ in any essential respect from the arrangement based on the use of solutions of sodium salts of the acid ions—only one group test, which must be made in neutral solutions, is omitted when the acid solution is used, and the individual members of this group are tested [p302] for, specifically. As that is also very frequently necessary when solutions of sodium salts are used, no notable sacrifice in convenience is made.

Since the grouping for both methods of analysis may be made the same, the following grouping of acid ions has been adopted. Only the group characteristics are given; the members of the groups are described in detail in the Laboratory Manual (Part III).

I. ‹Ions of Amphoteric Acids and of Related Acids.› This group includes those acids whose amphoteric character, or whose ready reduction by hydrogen or ammonium sulphide, leads to their being found, or indicated, in the systematic analysis for metal ions.

II. ‹The Carbonate Group.› This group includes those acids whose physical properties (insolubility), or the physical properties of decomposition products of which (carbon dioxide is a decomposition product of carbonic acid), usually lead to their discovery in the course of the preparation of solutions or of the analysis for cations.

III. ‹The Sulphate Group.› The ‹barium salts› of this group of acid ions are ‹insoluble in acid solution›. Barium nitrate, added to a solution acidified with nitric acid, is the group reagent.

IV. ‹The Chloride Group.› The anions of this group form ‹silver salts›, which are ‹insoluble in nitric acid›. Silver nitrate, added to a solution acidified with nitric acid, is the group reagent.

V. ‹The Phosphate Group.› A test for this group, as a whole, can be made only if cations other than the alkali metal ions are absent: the ‹barium salts› of the acid ions of the group are ‹insoluble in neutral›, but ‹soluble› in strongly ‹acid, solutions›. Barium nitrate, used with a neutral solution, is the group reagent in the absence of metal ions other than the alkali metal ions. In the presence of other cations, the three members of the group, which are not found in some other group,[595] namely: phosphate, borate and fluoride ions, are tested for specifically, and the group test omitted. Phosphate-ion is tested for in nitric acid solution, in the same solution as is used for the tests for groups III and IV. [p303]

VI. ‹The Nitrate Group.› The salts of the acids of this group are readily soluble in water and specific tests for the acid ions are made; there is no group test.

VII. ‹The Group of Organic Acids.› This group need only be considered when a test for organic matter reveals its presence.

«Applications of Physico-Chemical Principles and Theories.»—The physico-chemical principles and theories, which have been developed in the previous chapters, naturally apply also to the reactions by which acid ions are identified. In many cases such characteristic reactions are identical with reactions studied in connection with the metal ions. For instance, the precipitation of silver chloride, used to identify the silver-ion (reagent, chloride-ion), may be used, with certain precautions, to identify chloride-ion as well (reagent, silver-ion).

In the following, only a few typical and interesting applications of the principles and theories to acid ions will be given; numerous other applications will suggest themselves in connection with the laboratory work on the acids.

«Fractional Precipitation of Salts with a Common Ion.» For saturated solutions of silver chloride, bromide and iodide, we have, according to the principle of the solubility-product[596]:

[Ag^{+}]_{1} × [Cl^{−}]_{1} = K_{AgCl} = 1E−10;

[Ag^{+}]_{2} × [Br^{−}]_{2} = K_{AgBr} = 4E−13;

[Ag^{+}]_{3} × [I^{−}]_{3} = K_{AgI} = 3E−16.

For a solution saturated simultaneously with the three silver salts, the value of the concentration of the silver-ion is the same in the three solubility-products (see p. 164). Consequently, for such a solution, with which the three solid salts are in equilibrium, ‹the ratios of the concentrations of the anions› must be: [Cl^{−}] : [Br^{−}] : [I^{−}] = K_{AgCl} : K_{AgBr} : K_{AgI} = 3 × 10^5 : 1300 : 1. That is, if silver nitrate is added to a mixture of iodides, bromides and chlorides, ‹silver iodide› must be precipitated first, until the concentration of bromide-ion, in solution, is 1300 times as great as the concentration of the iodide-ion left in solution. Then ‹bromide› and traces of iodide of silver will be precipitated, ‹until the concentration of chloride-ion› is 300,000 times as great as the concentration of iodide-ion and some 250 times as great as the concentration of the bromide-ion. In other words, if silver nitrate is added gradually to such a mixture, iodide-ion and bromide-ion will be almost completely removed from solution ‹before a precipitate of silver chloride can be in equilibrium› with the solution. This gives us a convenient and rapid method of detecting chlorides, if present, [p304] in more than small quantities, with iodides and bromides. Silver nitrate, a few drops at a time, is added to the solution and the mixture vigorously shaken after each addition. As long as a yellow (AgI) or yellowish (AgBr) silver salt is precipitated on the addition of silver nitrate to the supernatant liquid (the precipitate settles quickly), silver nitrate is added as before; when the color becomes quite pale, the solution is filtered and silver nitrate added a drop at a time; if a ‹pure white precipitate› results finally, chloride-ion is present in the mixture (‹exp.›).

«Complex Ions.» Instances of the rôle of complex ions in the analysis for acid ions are numerous. One of the most interesting illustrations is the application of the equilibrium conditions for the complex silver-ammonium-ion (p. 224) to the separation of silver chloride, bromide and iodide. A rather more convenient and more sensitive method[597] for detecting the three halide ions in the presence of each other than the method just considered, may be discussed from this point of view.

The condition of equilibrium between silver-ion, ammonia and silver-ammonium-ion is expressed in the relation:

[Ag^{+}] × [NH_{3}]^2 / [Ag(NH_{3})_{2}^{+}] = K = 1 / 10^7.

The concentration of silver-ion, which may exist in an ammoniacal solution, evidently must decrease rapidly with increasing concentrations of the free ammonia. Now, let us imagine only sufficient free ammonia, in solution, added to a mixture of silver chloride, bromide and iodide, to keep the concentration of silver-ion, which can exist in the solution, say at [Ag^{+}] = 6E−9, which is just 1 / 100th of the concentration of silver-ion in a saturated aqueous solution of silver bromide. Such a solution of ammonia, in contact with the three silver salts mentioned, will dissolve silver chloride, if sufficient is present, until [Cl^{−}] = K_{AgCl} / 6E−9 = 0.017 molar. At the same time, silver bromide would be dissolved until [Br^{−}] = K_{AgBr} / 6E−9 = 0.000,06 molar. In other words, ‹silver chloride could be dissolved in some quantity›, while silver bromide is dissolved only in traces (the ratio of [Cl^{−}] : [Br^{−}] is again about 250 : 1). When such an ammoniacal extract is acidified with nitric acid, almost pure (white) silver chloride would be precipitated and only traces of bromide would be lost. After the extraction of the chloride, an ‹increased concentration of ammonia› would lead, similarly, to a solution in which ‹silver bromide would dissolve readily› and only traces of the iodide be lost, and thus a separation of bromide and iodide may be effected.

In Hagar's method, the concentration of ammonia, required to dissolve silver chloride with but traces of bromide, is attained by the use of a solution of ammonium sesqui-carbonate,[598] in which free ammonia is present only in small concentration, as a result of the hydrolysis of the salt. After the [p305] extraction of the chloride by this solution, the bromide is extracted with a 5% solution of ammonia.

«Complex Ions of Acid Ions with Other Acids.»—In the study of complex ions we found that positive ions (silver, cupric, etc.) may form complex positive ions with ammonia[599] or complex negative ions with acid ions (‹e.g.› with cyanide-ion). In the study of the acid ions we also meet instances of complexes formed by the ‹union of two acids› to form a new complex acid. Ammonium phosphomolybdate, an important salt that is extremely useful in detecting the presence of phosphate-ion, is the most interesting instance of the salt of such an acid, which is met in elementary qualitative analysis.[600]

Ammonium phosphomolybdate, (NH_{4})_{3}PO_{4}, 12 MoO_{3}, is the salt of a complex phosphomolybdic acid, formed from phosphoric acid, O:P(OH)_{3}, and molybdic acid, O_{2}Mo(OH)_{2}, by a loss of water, much as potassium dichromate is formed from potassium acid chromate [KO(CrO_{2})OH + HO(CrO_{2})OK ⇄ KO(CrO_{2})O(CrO_{2})OK]. The only difference between the two actions lies in the fact that, in the case of the dichromate, anhydride formation occurs between two molecules of a single acid; in the case of the phosphomolybdate, anhydride formation takes place between molecules of different acids, and a much larger number of molecules is involved. If we suppose the combination between the two acids to proceed symmetrically,[601] we may consider the following to be the action:

O:P(OH)_{3} + 3 [HO(MoO_{2})OH + HO(MoO_{2})OH + HO(MoO_{2})OH + HO(MoO_{2})OH] ⇄ O:P[O(MoO_{2})O(MoO_{2})O(MoO_{2})O(MoO_{2})OH]_{3} + 12 H_{2}O.

Intermediate complex acids, containing less molybdic acid, are no doubt formed first (the action is a relatively slow one), and the action proceeds until the formation of an insoluble salt leads to the final precipitation of all of the phosphate in this form. The precipitate shows the characteristic behavior of an acid anhydride—‹alkalies dissolve it readily› and form phosphate and molybdate—‹e.g.› ammonium hydroxide forms [NH_{4}]_{2}HPO_{4} and (NH_{4})_{2}MoO_{4} (‹exp.›). Dichromates, in a similar way, are converted by alkalies into chromates, an action which may readily be followed by the change in color (‹exp.›).

«Oxidation and Reduction.»—While fractional precipitation of silver iodide, bromide and chloride, and fractional solution of the silver salts in ammonia are convenient methods for detecting the three halide ions in the presence of one another, the ‹most accurate› and ‹most convenient› methods for this purpose depend on the different sensitiveness which iodide, bromide and chloride ions exhibit towards ‹oxidizing› agents. Of the three halogens, iodine shows the smallest tendency to form its ion (see the table, p. 294), chlorine the greatest. ‹Vice versa›, of the three halide ions, iodide-ion is most readily, chloride-ion least readily, oxidized. Treatment with a mild oxidizing agent, such as ferric-ion [p306] (see Chap. XIV and Laboratory Manual under iodide-ion), suffices to oxidize iodide-ion to iodine: 2 Fe^{3+} + 2 I^{−} ⥂ 2 Fe^{2+} + I_{2}. Bromide-ion and chloride-ion are left practically unaffected by this agent (see Chap. XIV). A somewhat stronger oxidizing agent, chromic acid (or its ion Cr^{6+}, see Chap. XV), oxidizes bromide-ion and leaves chloride-ion practically unaffected: 2 Cr^{6+} + 6 Br^{−} ⥂ 2 Cr^{3+} + 3 Br_{2}. This method of fractional oxidation forms one of the most convenient and sensitive methods for detecting the three halide ions in the presence of one another.[602]

We shall discuss here only one other oxidation-reduction reaction, taken in connection with the laboratory work—the ‹oxidation of hydroiodic acid by exposure to the air› and the resistance to oxidation shown by an ‹iodide›, such as potassium iodide, under the same conditions. The following method of proximate analysis of the chief relations involved may also be used to interpret the contrast in the behavior of hydroiodic acid and that of hydrobromic or hydrochloric acid (Laboratory Manual, ‹q. v.›). In all of these cases the actual relations are rendered more complex in consequence of secondary reactions, than is indicated in the text that follows: it is intended only to outline the most effective of the factors involved and to illuminate the qualitative results observed.

«Oxidation of Hydroiodic Acid by Air.»—The oxidation of hydroiodic acid, or of potassium iodide, by the oxygen of the air may be considered (Chapters XIV and XV) to involve primarily the action

4 I^{−} + O_{2} + 2 HOH ⇄ 2 I_{2} + 4 HO^{−}. (1)

The condition for equilibrium will be

[I^{−}]^4 × [O_{2}] / ([I_{2}]^2 × [HO^{−}]^4) = K_{equil.} (2)[603]

‹A system in which› I^{−} ‹is directly in equilibrium with› I_{2} (for which [I^{−}]_{1}^2 : [I_{2}]_{1} = K_{I^{−}, Iodine} = 5.6E29, at room temperature (p. 298)) ‹and in which, at the same time›, HO^{−} ‹is directly in equilibrium with› O_{2} (for which at room temperature [HO^{−}]_{1}^4 : [O_{2}]_{1} = K_{HO^{−}, Oxygen} = 8.2E49 (p. 298)) ‹would also represent a condition of equilibrium for the› «four» ‹components›. We find thus

K_{equil.} = K_{I^{−}, Iodine}^2 / K_{HO^{−}, Oxygen} = (5.6E29)^2 / (8.2E49) = 4E9. (3)

With the aid of this constant and of equation (2) we can obtain, at least, an approximate interpretation of the results of the exposure of hydroiodic acid and of potassium iodide to the influence of atmospheric oxygen.[604] We may [p307] calculate, first, what concentration of free iodine would be ‹required› to ‹prevent› «oxidation» of «hydroiodic acid», in molar solution, by the oxygen of the air, ‹i.e.› to establish equilibrium. We will call ‹x› that concentration of I_{2}. As hydroiodic acid is a very strong acid, ionized to the extent of about 80% in molar solution, we may, with sufficient accuracy for our purpose, consider it completely ionized and put [I^{−}] = 1 and [H^{+}] = 1. Since at 25° [H^{+}] × [HO^{−}] = 1.2E−14 (p. 104), we may put [HO^{−}] = 10^{−14}. The concentration of oxygen in the air, at room temperature, may be considered to be approximately [O_{2}] = (1/5) × (1 / 23.9). Inserting all these given values in equation (2), we have

[I^{−}]^4 × [O_{2}] / ([I_{2}]^2 × [HO^{−}]^4) = 1 × (1/5) × (1 / 23.9) / (‹x›^2 × (10^{−14})^4) = 4 × 10^9. (4)

Solving for ‹x›, we find ‹x› = 10^{22} = [I_{2}]. That is, free iodine of this enormous concentration would be required to prevent oxidation of hydroiodic acid in molar solution by the oxygen of the air at room temperatures. It is obvious that hydroiodic acid must be extremely sensitive to oxidation by exposure to air.

One might estimate, in a similar way, the extent to which hydroiodic acid, of a given concentration, would be oxidized by air before equilibrium would be reached. The process would involve simultaneous changes in three factors—iodide-ion is destroyed, iodine is formed and hydroxide-ion increases, as the result of the neutralization of hydrogen-ion by the hydroxide-ion formed in the action (see above). The solution of the equilibrium equation is too involved for the elementary purposes of this discussion: it leads to the same qualitative conclusion as was just reached.

«Oxidation of Potassium Iodide by Air.» We may now ask what the relations would be, if we used a molar solution of potassium iodide in place of the free acid. [I^{−}] and [O_{2}] would have the same value as before. The solution being originally neutral, [HO^{−}] would at first have the value √(1.2E−14) = 1.1E−7. But when potassium iodide is exposed to the air, if iodine is liberated, the solution becomes ‹alkaline›[605] (HO^{−} is formed according to equation (1)) and the concentration of HO^{−} consequently ‹grows continuously greater›. We will, therefore, formulate the problem as follows: ‹how much iodine[606] must be liberated, by oxidation of iodide-ion, in molar potassium iodide solution in order to establish equilibrium?› For every ‹two› molecules of iodine liberated, ‹four› HO^{−} ions are formed (equation (1)). If we call ‹y› the concentration of iodine at the point of equilibrium, then 2 ‹y› is the concentration of [HO^{−}] at that point, formed by the oxidation process. Inserting the given values[607] in equation (2), we have

[I^{−}]^4 × [O_{2}] / ([I_{2}]^2 × [HO^{−}]^4) = 1 × (1/5) × (1 / 23.9) / (‹y›^2 × (2 ‹y›)^4) = 4E9.

[p308]

Solving for ‹y›, we find ‹y› = 0.007. That is, in molar solution, about 1.4% of the iodide[608] would be oxidized (carbonic acid and other acids being excluded); in 5 c.c. (see Lab. Manual, p. 73) 9 ‹milligrams of iodine[609] would be liberated› to reach a condition of equilibrium.[610]

It is thus clear that the conditions for equilibrium between a solution of an iodide and air would be satisfied, in the case of an alkali iodide, by the liberation of a mere trace of iodine, whereas, as was previously shown, in the case of hydrogen iodide, a very large proportion of iodine must be liberated before equilibrium could obtain. A careful comparison of the two developments shows that the difference in result[610] is plainly due to the higher oxidizing power, the higher potential of oxygen (p. 280), in acid solutions, containing only a minute concentration of hydroxide-ion, as compared with its efficiency in neutral or slightly alkaline solution.

FOOTNOTES:

[589] In the few cases when there is interference, it is provided against.

[590] ‹Cf.› Fresenius, ‹Qualitative Analysis›, p. 520.

[591] The group includes phosphate, borate, fluoride, oxalate, silicate, arsenite, arseniate, chromate and tartrate ions.

[592] The ions of the amphoteric acids, arsenic and arsenious acids, and chromate-ion, which is reduced by hydrogen sulphide to chromium-ion, are found in the systematic analysis for metal ions.

[593] The ions of the amphoteric acids, arsenic and arsenious acids, and chromate-ion, which is reduced by hydrogen sulphide to chromium-ion, are found in the systematic analysis for metal ions.

[594] See Fresenius, ‹loc. cit.›, p. 511, footnote, and p. 520.

[595] Other acid ions which would show the group test—precipitation of a barium salt in a neutral solution—are determined in other groups, as follows: arsenite, arseniate and chromate ions in the group of amphoteric acids, etc. (I); carbonate and silicate ions in the carbonate group (II); and oxalate and tartrate ions in the group of organic acids (VII).

[596] The constants refer to 18°. The subindices are used to distinguish the (unequal) concentrations of the silver-ion and of the halide ions in the different solutions referred to in the text.

[597] Hagar's method. See Fresenius, ‹loc. cit.›, pp. 356 and 378.

[598] See Fresenius, ‹loc. cit.›, pp. 356 and 378, for the preparation of the solution and for details of the method, and see Smith, ‹General Inorganic Chemistry›, p. 566, as to the nature of the sesqui-carbonate.

[599] They also form complex ions with substances related to ammonia, such as the organic amines.

[600] Ammonium arsenomolybdate is an analogous salt (see Laboratory Manual, Part III). A similar complex acid, phosphotungstic acid, is used in alkaloidal analysis.

[601] The exact structure of the complex acid is not known.

[602] In the Laboratory Manual a second, similar method is also given.

[603] It is considered that water has a constant concentration in a dilute solution and that for its active components [H^{+}] × [HO^{−}] is a constant (p. 176).

[604] In the calculation which follows, which is meant merely for a rough survey, no account is taken of the formation of complex ions I_{3}^{−}, or of the tendency of hydroiodic acid to decompose spontaneously into iodine and hydrogen: 2 H^{+} + 2 I^{−} ⇄ H_{2} + I_{2}, a reaction which could also be studied profitably with the aid of the equilibrium constants for I_{2} ⇄ 2 I^{−} and for H_{2} ⇄ 2 H^{+}. The value of the iodide constant is also uncertain (see p. 273).

[605] 4 K^{+} + 4 I^{−} + O_{2} + 2 HOH ⇄ 2 I_{2} + 4 K^{+} + 4 HO^{−}.

[606] The formation of complex ions I_{3}^{−} and other secondary reactions (formation of hypoiodite, iodate, etc.) are ignored.

[607] ‹y› has so small a value that we may consider [I^{−}] ‹practically› unchanged.

[608] 0.007 I_{2} = 0.014 I^{−}.

[609] A mole of I_{2} = 2 × 127 = 254 grams; (0.007 × 254 × 5) / 1000 = 0.009 gram.

[610] The tendency of iodine to form hypoiodous acid, iodates, etc., is not taken into consideration here and involves another relation.

INDEX

Numbers marked (†) refer to subjects illustrated by experiments, «heavy» numbers refer to tables.

Acetic acid, 98, 101†, 111–114†

Acid ions, systematic analysis for, 299 ‹et seq.› groups of, 301

Acids, 81, «82», 100, «104»

Acid salts, 103

Alkali group, 158, 159

Alkaline earth group, 158, 162

Aluminate-ion, 172

Aluminium group, 158, 188 ‹et seq.›, 195

Aluminium hydroxide, 171 ‹et seq.›, 196

Aluminium-ion, 158, 172, 188 ‹et seq.›

Ammonia, complex ions of, 216 ‹et seq.›, 224 ionization of, 87

Ammonium hydroxide, conductivity of, 77 dissociation of, 160 ionization of, 78†, 114 strength as base, 78†, 79†, 168†

Ammonium-ion, 161

Ammonium salts, dissociation of, 34†, 159 effect on ammonium hydroxide, 168†

Amphoteric acids, group of, 302

Amphoteric hydroxides, 171 ‹et seq.›, 187, 188, 196

Antimony, ions of, 174

Argenticyanide-ion, 225

Arrhenius's theory of ionization, ‹see› Ionization

Arsenic acid, 247, 250, 283

Arsenic group, 158, 210, 242 ‹et seq.›

Arsenic, ions of, 158, 174, 242 ‹et seq.›

Arsenic pentasulphide, 247

Arsenious sulphide, colloidal, 125†

Association, 19, 64

Aurocyanide-ion, 232

Avogadro's hypothesis, 33, 34

Avogadro-van 't Hoff Hypothesis, 15, «37»

Azolitmin, «79»

Barium carbonate, as reagent, 193

Barium-ion, 162

Bases, 79†, «81», 105, «106»

Basic salts, 106, 194

Bismuth-ion, 158

Boiling-point, 17, 36, 38

Borates, hydrolysis of, 90†

Bromine, 252†

Cadmium-ion, 158 complex ions of, 224, 228† sulphide, 209† ‹et seq.›

Calcium-ion, 162

Carbonate group, 302

Carbonic acid, 90, 100

Chemical activity of ions, 72†, 78†, 87†, 232 of acids, 81, «82», 105† of bases, 79†, «81» of molecules, 74, 83, 232 of salts, 74†, 107, 116†

Chemical equilibrium, 90 ‹et seq.›, 96† and direction of action, 112 and path of action, 235 and physical equilibrium, 139 ‹et seq.› of electrolytes, 98, 111–115†

Chemical equilibrium, law of, 91, 94 and ionization of strong electrolytes, 108 factors of, 95† ‹et seq.› limitations to, 95

Chemometer, 253

Chloride group, 302

Chloride-ion, oxidation of, 275, 276†

Chlorine, 46, 252†, 275

Chromium-ion, 158, 188 ‹et seq.›

Chromic acid, 287

Cobalt-ion, 158, 192

Cobalticyanide-ion, 229

Cobalt sulphide, 192

Colloidal condition, 125† ‹et seq.› definition of, 130 relations to analysis, 131, 136† solution theory of, «128» suspension theory of, 129

Colloids, electric charges on, 131† precipitation of, 133† protective action of, 136, 137†

Complex ions, 88, 216 ‹et seq.› applications in analysis, 220†, 230† of acid ions, 305 of ammonia, 216† ‹et seq.› of cyanide-ion, 225† ‹et seq.› of halide ions, 237 of oxide-ion, 238 of sulphide-ion, 238, 246 organic, 238†

Concentration, definition of, 91

Conductivity, 44†, 46, 47†, 49, 51, 56 partial, ‹see› Mobilities

Copper, equilibrium with cupric-ion, 258, 264† oxidation of, 257† ‹et seq.› solution-tension of, 259†

Copper group, 158, 199 ‹et seq.›, 210, 216 ‹et seq.›

Cupric-ion, complex ions of, 224† equilibrium with copper, 258, 264† reduction of, 257† sulphide, 212

Cuprous-ion, complex ions of, 228†

Cyanide-ion, complex ions of, 88†, 225† ‹et seq.›, 232

Dialysis, 128

Dielectric constants, 62, «63»

Diffusion, of gases, 29† of ions, 59† of solutes, 9†

Dissociation of complex ions, 219† electrolytic, ‹see› Ionization gaseous, 34†, 36, 96†

Dry salts, 74†

Electrolysis, 46, 58

Electrolytes, ‹see› Ionogens

Electron theory, 42

Equilibrium, ‹see also› Chemical equilibrium and Physical equilibrium constants, «95», «98», 233, 236, «298» oxidation and reduction, 258, 265 ‹et seq.›, 267†, 273 ‹et seq.›

Faraday's law, 58

Ferric hydroxide, 127†, 170†

Ferric-ion, 88†, 251†, 252†, 269†

Ferricyanide-ion, 88†, 230†

Ferrocyanide-ion, 88†, 230†

Ferrous-ion, 88†, 251†, 252†, 269†

Formaldehyde, 289† ‹et seq.›

Fractional oxidation, 305, 306

Fractional precipitation, 163, 165†, 303

Fractional solution, 304

Freezing-point, 17, 36, 38

Fused salts, 75†

Gold, 158, 242 ‹et seq.› colloidal, 126† ‹see also› Aurocyanide-ion

Groups of metal ions, 157

Groups of acid ions, 301

Heat, 75, 76†

Heterogeneous equilibrium, ‹see› Physical equilibrium

Hydrogen, oxidation of, 277, 278, 280†

Hydrogen chloride, 34, 37, 42, 72†, 73, 84

Hydrogen-ion, action on indicators, 79, 105† chemical activity, 72–74†, 81, «82», 278 ‹et seq.› concentration for precipitation by H_{2}S, 213 mobility, 54†, 56

Hydrogen sulphide, ionization of, 199, 245 oxidation of, 251†, 254† precipitation by, 90†, 199† ‹et seq.›, 203†

Hydrol, 175

Hydrolysis of salts, 127, 178† ‹et seq.›, 190†

Hydroxide-ion, action on indicators, 78†, 79 chemical activity, 79, «81», 168† mobility, 54†, «56»

Indicators, «79», 165, 214

Instability constants, 219, 224, 226

Iodide-ion, 88†, 272†, 305

Iodine, 273, 305

Ionization and chemical activity, 69, 72†, 79†, 90† ‹et seq.›, 116†, 232 and conductivity, 44†, 77†, 115† and dielectric constants, 62–«64» and electron theory, 42 and Faraday's law, 58 and osmotic pressure, 67 and solvents, 61 constants, 98, 100, «104», «106», «108» degree of, 50 exceptional, of certain salts, 115† in stages, 100, 102† of acids, 69, 98, 100, «104», 108 of bases, 69, 105, «106», 108 of colloids, 132 of salts, 69†, 74†, 75†, 107, «108», 115† theory of Arrhenius, 40, 41, 51 theory of Clausius, 51

Ionogens, 69

Ion-product, ‹see› Solubility-product

Ions, 42 charges on, 41, 58 combination with solvents, 42, 65 composition of, 69†, 89† migration of, 45†, 53, 54† mobilities of, «56»

Kinetic theory, of gases, 26 and osmotic pressure, 26

Lead-ion, 158 sulphide, 212, 213

Light, 76

Litmus, «79»

Magnesium hydroxide, 168†

Magnesium-ion, 162

Manganous-ion, 158

Mass action, law of, 93, 96†

Mercuric chloride, 57†, 115†

Mercuric cyanide, 115†, 116†, 231†

Mercuric-ion, 158, 257

Mercurous-ion, 158

Mercury, 257

Methyl orange, «79»

Methyl violet, 213

Mobilities of ions, 53†, «56»

Molecular weights, 33, 36, «37»

Nernst's formula, 261 ‹et seq.›, 296 ‹et seq.›

Nickel-ion, 158 cyanide-ion, 229 sulphide, 192

Nitrate group, 302

Nitric acid, 288†

Organic acids, group of, 302

Organic substances, complex ions of, 238 oxidation of, 289† ‹et seq.›

Osmosis, 10†, 22, 24†

Osmotic pressure, 8† ‹et seq.› and ionization, 40, 67 definition of, 10 indirect determination of, 16 laws of, «12–20» measurement of, 10 theories of, 32

Oxalates, complex ions of, 241†

Oxidation, 251† ‹et seq.›, 282† ‹et seq.› by electric current, 252† definition of, 251 of organic compounds, 289† production of current by, 253† relation to theory of ionization, 251† ‹et seq.›

Oxidation-reduction, reversibility of, 256†, 268† ‹et seq.› interpretations of, 251, 282, 286

Oxygen, oxidation by, 277, 278, 280†, 305

Oxygen-hydrogen cell, 280†

Permanganic acid, 287†

Perpetuum mobile, 12

Phases, 118

Phenolphthaleïn, «79»

Phosphate group, 302

Phosphomolybdates, 305

Phosphoric acid, 102†, 134

Physical equilibrium, 118† ‹et seq.› and chemical equilibrium, 139 ‹et seq.› applications of law of, 120 law of, 118

Platinum, 158, 242 ‹et seq.›

Potassium hydroxide, 77†−81†, «106»

Potassium-ion, 158, 161

Potential differences, 261 ‹et seq.› and concentrations, 261, 263† and osmotic pressures, 261 sign of, 261

Precipitates, ‹see also› Precipitation solution of, 151† washing of, 148

Precipitation, 122†, 145† ‹et seq.› and ionization, 74†, 90, 152†, 220 ‹et seq.› fractional, 163, 165† influence of a common ion, 144†, «146», «147» influence of electrolytes, 144†, 150† of electrolytes, 145†

Primary ionization, 101, 102†

Purple of Cassius, 126†, 134†

Reduction, ‹see also› Oxidation by electric current, 252† definition of, 252 interpretations of, 251, 282, 286 production of current by, 253†

Salt-effect, 82, 109, 110†

Secondary ionization, 101, 102†, 246

Silver, colloidal, 127†

Silver-ammonium-ion, 217 ‹et seq.›

Silver bromide, 224, 303

Silver chloride, colloidal, 138† precipitation of, 150 solubility of, 221

Silver chromate, 141, 165†

Silver group, 157, 199 ‹et seq.›, 216 ‹et seq.›

Silver iodide, 224, 303

Silver-ion, 158 as oxidizing agent, 290† complex ions of, 216†, 225†

Sodium hydroxide, «81», «106», 173

Sodium-ion, 158, 161

Solubility, 121, 123, «146», «147», 153, «155»

Solubility-product principle, 141 ‹et seq.› applications of, 145†, 147, 149, 151 derivation of, 139 ‹et seq.›

Solution, theories of, 8, 32

Solution of electrolytes, 151

Solutions, concentrated, 15, 32, 142 dilute, ‹see› Osmotic pressure non-aqueous, 62, 73†, 84 supersaturated, 121†

Solution-tension, electrolytic, 258 ‹et seq.› constants, 259, 266, «294», «295»

Solvents and ionization, 61, «64» and solubility of electrolytes, 154, «155»

Strength of acids, «104»

Strength of bases, 78†, «106»

Strontium-ion, 158, 162

Sulphate group, 302

Sulphide-ion, complex ions of, 238, 246 concentration of, «202» oxidation of, 251†, 254†

Sulphides, precipitation of, 199†, 203† ‹et seq.› solubilities of, 203 ‹et seq.›, 212

Sulpho-acids, 244 ‹et seq.›

Sulpho-bases, 244† ‹et seq.›

Sulpho-salts, 243 ‹et seq.›

Sulphuric acid, ionization of, 103†

Supersaturation, 121†

Systematic analysis, of metal ions, 157 ‹et seq.› of acid ions, 299 ‹et seq.›

Tartaric acid, complex ions of, 238†

Tin, ions of, 158, 174, 242 ‹et seq.›

Unsaturated compounds, 64

Uranyl salts, 286

Valence, 59 and precipitating power of ions, 135

Van 't Hoff's hypothesis, ‹see› Avogadro

Van 't Hoff's theory of solution, 12 ‹et seq.› apparent exceptions to, 18

Vapor tension, 17, 36, 38

Velocity of action, 92

Water, ‹see also› Hydrolysis composition of, 175 dielectric constant of, 63 formation by electrolytic oxidation, 280, 282† ionization by, 37, 41, 47†, 61, 73†, 74† ionization of, 53, «104», «106», 176 ‹et seq.› oxonium ions of, 238 secondary ionization of, 246, 278

Zinc, 257†, 266†

Zinc group, 158, 188 ‹et seq.›, 210

Zinc-ion, 266

Zinc sulphide, 204† ‹et seq.›

TRANSCRIBER'S NOTE.

Original printed spelling and grammar is generally retained. Footnotes were renumbered and relocated to the ends of chapters. Notes to tables were left in their original locations. Some mathematical and chemical formulas and equations were modified in format or rearranged. Text originally printed in italics is herein ‹marked thus›. Originally bold text is «herein like this». Original small caps is herein all capitals.

This book uses many uncommon Unicode characters, and careful selection of the ebook reader software and font used to view it is necessary. Some of the uncommon characters not already mentioned are: U+2296 ⊖; U+2295 ⊕; U+221E ∞; U+221B ∛; U+2212 −, Minus Sign; U+21C4 ⇄; U+2192 →; U+2572 ╲; and U+2571 ╱. The use of a monospaced font is important for the data tables, and for some chemical formulas. On the other hand, the Thin Space U+2009 is used aesthetically in most of the chemical and mathematical equations, and an exact monospaced font will not display Thin Space correctly. Adobe's "Source Code Pro" is an unusual "monospace font" that does display Thin Space, the chemical formulas, and the data tables correctly. DejaVu Sans Mono is another good choice, but it does not display Thin Space correctly.

The notation "D_{x}" signifies "D subscript x". The notation "D^{y}" signifies "D superscript y". In unambiguous cases where the superscripted text is a single digit exponent (0–9), this is simplified to "D^y". The archaic form of scientific notation exemplified by "0.0_{4}13", is herein simplified either to decimal form—"0.000013" in this example—or to modern scientific E notation, "1.3E−5". In this, "E" means "times ten raised to the power of". Furthermore, E notation has also been substituted herein for most numbers originally printed like this: "a × 10^{b}".

The name "van't Hoff" was changed to "van 't Hoff" throughout; likewise "Van't" to "Van 't". The hyphen is used inconsistently throughout the book, in words such as "hydrogen-ion" versus "hydrogen ion" or "non-ionized" versus "nonionizied". These have been retained. The word "difficulty" was sometimes employed as an adverb; herein it is converted to "difficultly" in this usage.

Page 45: Changed "permangante" to "permanganate".

Page 81: Removed the unmatched right parenthesis from "the difference in ionization between potassium hydroxide and ammonium hydroxide).".

Page 104: In the table, "Hydrogen sulphide" was changed to "H_{2}S" to meet maximum line-length constraints for this ebook. Furthermore, the table notes were reordered and renumbered to match the sequence of note anchors in the table. The anchor in the table title originally linked to a footnote instead of a table note; this footnote was converted to a table note (the first one).

Page 106: The label for the sixth note to the table "The Ionization Constants of Bases" was changed from "3" to "F".

Page 117: The chemical reaction schema, originally comprising two balanced equations and two unbalanced equations using vertical arrows has been rearranged into four balanced equations, with horizontal arrows. Such rearrangements have been silently performed elsewhere. On page 217, a schema of three equilibrium equations was separated into three equations, and also removed from its embedded location within its paragraph line.

Page 117: The printed symbol that might be described as "normal leftward arrow over rightward dark arrow (or heavy arrow)" has been represented herein with a more readily available character "⥂"—the Unicode character with hexadecimal number 2942 (U+2942), rightward arrow over short leftward arrow. The same character is used herein for the printed symbol which might be described as "downward dark arrow left beside upward short arrow right"—the equations having been rearranged into a horizontal format—and also for "rightward dark arrow over leftward arrow". A different character U+2943 "⥃" represents "normal rightward arrow over heavy leftward arrow".

Page 125: In the reaction of arsenious oxide with hydrogen sulphide, changed "H_{3}S" to "H_{2}S".

Page 146: In the table heading, "[CH_{3}COO^{-}]" was changed to "acetate" to save space. The same thing was done in the table on page 146.

Page 157 "Mendelejeff" changed to "Mendeléeff".

Page 188, second footnote (#380): The printed structure for "glycocoll" a.k.a. glycine was shown roughly as follows: "H_{3}N.CH_{2}COO" with a line drawn between the "N" and the right-hand "O". This has been herein modernized to "H_{3}N^{+}.CH_{2}.COO^{−}", showing explicitly the zwitterion structure (the second full stop added to emphasize that "COO" is a unit). The html and mobile versions reproduce the original structure.

Page 197: Changed "[AlO^{3−}] = ‹y›" to "[AlO_{3}^{3−}] = ‹y›".

Page 226 etc.: Both forms "Bodlaender" and "Bodländer" are retained.

Page 239: The letters originally underscored are herein marked with U+2017, Double Low Line, e.g. "‗OH‗".

Page 246: Changed "saponifying esters (p. 801)" to "saponifying esters (p. 81)".

Page 252: Substituted "ε^{−}" for a symbol that might be described as "circled epsilon".

Page 283: In the equation showing the oxidation of zinc by cupric ion, the upward arrow originally shown beside the symbol for copper metal is changed to downward arrow.

Page 310: The reference for "mobility" of "Hydroxide-ion" was changed from "156" to "56".