[502] Mercuric sulphide is somewhat soluble in potassium and sodium sulphides, forming the salts Me_{2}HgS_{2}, and the complex ion HgS_{2}^{2−}. A liter of 0.1 molar Na_{2}S dissolves, at 25°, 1.9 grams (0.0082 mole) of HgS [Knox, ‹Trans. Faraday Society›, «4», 36 (1908)]. While the oxide (hydroxide) shows no perceptible tendency toward acid ionization, mercuric salts, it will be recalled, show in many cases an abnormally small tendency to form the mercuric-ion (see p. 115), and the latter also shows a particularly great tendency towards forming very stable complex ions of all kinds (‹e.g.› HgI_{4}^{2−}, in K_{2}HgI_{4}, HgCl_{4}^{2−} in K_{2}HgCl_{4}, Hg(CN)_{4}^{2−}, etc.). Knox found for [Hg^{2+}] × [S^{2−}]^2 / [HgS_{2}^{2−}] = ‹k›, the approximate value of ‹k› to be 1 / 10^{53}. Bismuth sulphide is also very sparingly soluble in sodium or potassium sulphide, but not in ammonium sulphide. Solid salts, KBiS_{2} and NaBiS_{2} are known [Knox, ‹J. Chem. Soc.› (London), «95», 1760 (1909)]. [503] See the table, p. 104. [504] See the discussion of the reaction, given below. [505] See Chap. XVI, for the interpretation of the reduction as an ionic reaction. [506] Bunsen, ‹Ann.› (Liebig), «192», 305 (1878). Brauner and Tomicek, ‹J. Chem. Soc.› (London), «53», 145 (1888). Usher and Travers, ‹ibid.›, «87», 1370 (1905). [507] Neher, ‹Z. anal. Chem.›, «32», 45 (1893). [508] Neher, ‹loc. cit.› [509] The theory of the relations favoring the precipitation expressed in equation (1) as against the reduction expressed in equation (2), forms a second interesting problem. [510] Intermediate derivatives, such as H_{3}AsSO_{3} (p. 246), could be the result of the ionization of As(OH)_{5}, or of AsCl_{5}, in stages (see p. 106). Neher, ‹loc. cit.›, McCay, ‹loc. cit.› [511] Neher (‹loc. cit.›) suggested that the favorable action of the large excess of hydrochloric acid might well be due to the formation of AsCl_{5}. McCay, ‹J. Am. Chem. Soc.›, «24», 661 (1902), discusses the ionization of arsenic acid as a base, in connection with the precipitation of As_{2}S_{5}. [512] As(OH)_{5} is considered to be an ‹extremely weak› base and AsCl_{5} to be an ionizable salt.