Besides the complications mentioned, and provided against in the way discussed, there is still one more complication in the use of hydrogen sulphide: this is in the matter of the precipitation of ‹arsenic› sulphide from solutions containing ‹arsenic in the pentavalent condition›. Since the interpretation of this complication and the explanation of the methods for avoiding the errors, which may arise therefrom, are necessarily intimately connected with the chemical behavior of arsenic acid, this subject will be considered in the discussion of the arsenic group (Chapter XIII). FOOTNOTES: [395] Auerbach, ‹Z. phys. Chem.›, «49», 220 (1904). [396] See footnote 1, p. 201. [397] Knox (in Abegg's laboratory), ‹Trans. Faraday Soc.›, «4», 44 (1908). [398] The concentration of the hydrogen-ion is really a little greater than that of the hydrosulphide-ion, as a result of the ionization of the latter, but the amount of hydrogen-ion formed in this way (about 1E−15) is so minute, compared with that formed by the primary ionization, that it is negligible. [399] We can obtain the relation, directly, from H_{2}S ⇄ 2 H^{+} + S^{2−} and [H^{+}]^2 × [S^{2−}] / [H_{2}S] = K = 1.1E−22. For a given pressure of the hydrogen sulphide, [H_{2}S], expressing its solubility (about 0.1 molar at 25°), is constant, and therefore [H^{+}]^2 × [S^{2−}] = a constant, as given in equation (IV). Putting [H_{2}S] = 0.1, we have [H^{+}]^2 × [S^{2−}] = 0.1 × 1.1E−22 = 1.1E−23. [400] On account of the great mass of water, we compare (see equation, p. 176) [H^{+}] × [HO^{−}] = 1.2E−14 (at 25°) with [H^{+}] × [S^{2−}] / [HS^{−}] = 1.2E−15. [401] The calculation was made by the method used by Knox (‹loc. cit.›) for a molar solution. The degree of ionization of the salt was not considered and the correct ionization constant for ammonium hydroxide was used, 1.8E−5 in place of 2.3E−5. The latter, evidently, was used by Knox as the result of overlooking a correction, which Bredig made in his (Bredig's) first calculations of the constant; ‹cf.› Bredig, ‹Z. phys. Chem.›, «13», 293, footnote. The same erroneous constant is found in Kohlrausch and Holborn, ‹loc. cit.›, p. 194. [402] For further values and for the method of calculation, see Knox, ‹loc. cit.› [403] [S_{2−}] = 1.1E−23 / [H^{+}]^2, according to equation (IV), p. 201. [404] Knox's work leads to that conclusion. [405] The precipitation of sulphides, from a solution containing much more of the hydrosulphide-ion than of the sulphide-ion, is comparable with the precipitation of mercuric oxide, HgO, and of silver oxide, Ag_{2}O, by sodium or potassium hydroxide.