Note 502 — Chapter XIII · THE ARSENIC GROUP. SULPHO-ACIDS AND SULPHO-SALTS

Mercuric sulphide is somewhat soluble in potassium and sodium sulphides, forming the salts Me2HgS2, and the complex ion HgS22−. A liter of 0.1 molar Na2S 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. HgI42−, in K2HgI4, HgCl42− in K2HgCl4, Hg(CN)42−, etc.). Knox found for [Hg2+] × [S2−]2 / [HgS22−] = k, the approximate value of k to be 1 / 1053. Bismuth sulphide is also very sparingly soluble in sodium or potassium sulphide, but not in ammonium sulphide. Solid salts, KBiS2 and NaBiS2 are known [Knox, J. Chem. Soc. (London), 95, 1760 (1909)].