The ammonium-ion, appearing with the same coefficient on both sides of the last equation, evidently takes no direct part in the action and we have more simply: Sn^{4+} + 3 S^{2−} ⇄ SnS_{3}^{2−}. For the condition of equilibrium between the complex and its components we have:[501][502] [Sn^{4+}] × [S^{2−}]^3 / [SnS_{3}^{2−}] = K. [p247] «Sulphurization of Sulphides.»—Since the solubility of the arsenic group of sulphides in ammonium sulphide solution—the reagent commonly used in analysis—depends on the formation of soluble sulpho-salts, due consideration must be taken of the fact that some of the ‹lower› sulphides—notably stannous, aurous and platinous sulphides—do not possess acid-forming properties in any marked degree; even antimonous sulphide is soluble only in a considerable excess of ammonium sulphide. In order, then, to insure a more complete separation of the arsenic from the copper group, precautions are taken to sulphurize the lower sulphides to higher, stronger acid-forming, sulphides, in the course of the separation. For this purpose so-called "yellow" ammonium sulphide, containing persulphides of ammonium, (NH_{4})_{2}S_{2}, etc., is used in place of a solution of ammonium sulphide and ammonium hydrosulphide. Stannous sulphide, for instance, is dissolved by the reagent as ammonium sulphostannate: SnS + (NH_{4})_{2}S_{2} ⇄ (NH_{4})_{2}SnS_{3}. «Behavior of Arsenic Acid toward Hydrogen Sulphide.»—In conclusion, special consideration must still be given to the behavior of arsenic acid, H_{3}AsO_{4}. As its name indicates, it is an acid, and, in fact, a rather strong acid, of the order of strength[503] of phosphoric acid, H_{3}PO_{4}, which it resembles in composition and in many of its properties. As a strong acid, arsenic acid, when it is ionized, yields chiefly negative arseniate ions, H_{2}AsO_{4}^{−}, HAsO_{4}^{2−} and AsO_{4}^{3−}. Any basic properties, which it may and, most likely, does possess, must be extremely weak. It is, therefore, not surprising to find that unusual difficulties are experienced in precipitating arsenic sulphide, by hydrogen sulphide, from arseniate [p248] solutions, in as much as hydrogen sulphide is an agent for the precipitation of the sulphides of ‹cations›. Arsenious acid, the hydroxide of the lower oxide, on the other hand, is a much weaker acid and shows more pronounced basic (amphoteric) properties, and arsenic trisulphide is precipitated, without difficulty, from solutions of arsenious acid in hydrochloric acid. We have: 2 As^{3+} + 3 S^{2−} ⥂ As_{2}S_{3} ↓. When arsenic acid is reduced to arsenious acid by sulphurous acid, by iodides (see Chap. XVI) or by hydrogen sulphide (see below), no further difficulty in precipitating a sulphide (As_{2}S_{3}) is experienced.