The acceleration of the precipitation of the pentasulphide by the presence of a large excess of hydrochloric acid forms a problem of peculiar interest and importance, and no complete explanation of it has yet been offered.[509] The following considerations lead to one explanation, that has been suggested. Arsenic acid, by virtue of its close relations to antimonic, stannic and arsenious acids, may be assumed to have extremely weak basic, as well as pronounced acid, properties. For its ionization, we would have 3 H+ + AsO43− ⇄ H3AsO4 (+ H2O) ⇄ As(OH)5 ⇄ As5+ + 5 HO−. Further, the precipitation of As2S5 may be assumed to result, ultimately,[510] from the action of the sulphide-ion S2− on the positive ion As5+ (2 As5+ + 5 S2− ⇄ As2S5 ↓). The favorable action of the hydrochloric acid might, consequently, be thought to result from the fact, that it facilitates the ionization of arsenic acid as a base and the formation of a salt[511] AsCl5. It could thus greatly increase the concentration of the ion As5+ and facilitate its combination with the sulphide-ion.
Treatment of a solution of arsenic acid with a concentrated acid, yielding a large concentration of hydrogen-ion, would carry the series of actions, represented in the above ionization equation for arsenic acid, decidedly toward the right—suppressing the arseniate-ion AsO43− and increasing the concentration of the arsenic-ion As5+. Since we cannot apply the equilibrium laws (or the principle of the solubility-product) to solutions as concentrated as the one under discussion, a quantitative theoretical treatment of the subject cannot be given. The following may be suggested: The action of the acid would be favorable to the precipitation of As2S5 by suppressing the arseniate-ion AsO43− and thus increasing the concentration of the hydroxide As(OH)5, available for ionization as a base and for the production of the ion As5+. But the further favorable effect of the hydrochloric acid, in converting the hydroxide into a salt AsCl5 and increasing thereby the concentration of As5+, would be very largely offset by the action of the acid in suppressing the [p250] sulphide-ion ([S2−] = k / [H+]2; see p. [201]). For systems to which the equilibrium laws could be applied, the concentration of As5+ (except for the suppression of the ion AsO43−) would grow, approximately, with the fifth power[512] of the concentration of the hydrogen-ion, and the concentration of the sulphide-ion would decrease, approximately, proportionally to the square of the concentration of the hydrogen-ion. Further, the precipitation of As2S5, in a system to which the principle of the solubility-product were applicable, would depend on the relation of the product [As5+]2 × [S2−]5 to the solubility-product constant; it is evident that the value for [As5+]2 would increase proportionally to the tenth power of [H+] and the value of [S2−] decrease proportionally to the tenth power of the same factor [H+]. The two effects would consequently offset each other under such conditions. However, the equilibrium laws cannot legitimately be applied to such concentrated solutions and the relation has been developed only to indicate opposing factors, which must be taken into account. An experimental study of the problem would be extremely interesting.[513] Since it involves the question of the minute basic ionization of a moderately strong acid (H3AsO4), which may be open to measurement (see Chap. XVI), the problem is one of particular interest and importance.
The analytical precautions, taken to insure the precipitation, by hydrogen sulphide, of arsenic sulphide, when arsenic is present in quinquivalent form, are based on the observations described; in quantitative analysis, for the sake of securing a precipitate of uniform composition, the aim is to precipitate the pure pentasulphide and a considerable excess of hydrochloric acid is used. In qualitative analysis, where the composition of the precipitate is a matter of indifference and a large excess of acid would seriously interfere with the precipitation of certain sulphides (e.g. CdS, see p. [211]), a smaller excess of acid is used and the precipitation of arsenic sulphide is insured by prolonged treatment of a solution with hydrogen sulphide at a high temperature.
Chapter XIII Footnotes
[492] The weak basic properties of the hydroxides of the aluminium group, as compared with those of the zinc group, a chemical difference, and the resulting great instability of the carbonates of the former group, are used in the separation of the aluminium from the zinc group, by barium carbonate; but the physical element of extreme insolubility of the trivalent hydroxides enters also as an important factor (see footnote 3, p. [194]).
[493] See p. [247], in regard to the behavior of arsenic acid in this respect.
[494] See p. [246], footnote 3, in regard to the action of sodium sulphide on mercuric and bismuth sulphide.
[495] Vide Nilson, J. prakt. Ch., 14, 150 (1876).
[496] Such a solution does not react alkaline to phenolphthaleïn.
[497] Hydrogen sulphide rapidly destroys the indicator and the experiment is best carried out by preparing 50 c.c. of a saturated aqueous solution of hydrogen sulphide, containing 1 or 2 c.c. of normal hydrochloric acid, and by adding a considerable excess of methyl orange to the solution immediately before the addition of potassium hydrosulphide solution, which has been prepared as described in the text.