When potassium cyanide is added to a silver nitrate solution, the precipitate formed is found to be silver argenticyanide, Ag[Ag(CN)_{2}], the silver salt of the extremely stable complex, rather than the simple salt, silver cyanide, AgCN [‹cf.› Bodländer, ‹Z. anorg. Chem.›, «39», 223 (1904)]. Ag[Ag(CN)_{2}] is even less soluble than silver chloride, the solubility-product constant for [Ag^{+}] × [Ag(CN)_{2}^{−}] being 2.25E−12. An excess of only 2E−6 mole, or about 0.15 milligram, of potassium cyanide (cyanide-ion) per liter is sufficient to prevent the precipitation of silver cyanide (silver argenticyanide) from a 0.1 molar solution of KAg(CN)_{2}, and, conversely, at least this minute excess of potassium cyanide is used in the preparation of a clear 0.1 molar solution of KAg(CN)_{2}, by the addition of potassium cyanide to silver nitrate, until the silver cyanide, first precipitated, is just redissolved (Bodländer, ‹loc. cit.›). This excess, as just explained, is more than sufficient to prevent the precipitation of silver chloride from the cyanide solution, even by a large excess of potassium or sodium chloride. Unless one takes into account, in the manner indicated, this marked influence of a minute excess of cyanide-ion in decidedly reducing the concentration of silver-ion in these solutions, one could be led, wrongly, to infer from the value of the instability constant of the complex ion and that of the solubility-product constant of silver chloride, that silver chloride should still be precipitated by the addition of sodium chloride to a solution of KAg(CN)_{2}. [450] For this reason potassium cyanide is an excellent cleansing agent for stained silverware (sulphide stains), and, since it is an intense poison, cleaning powders should be examined for it. [451] For the quantitative relations see Lucas, ‹Z. anorg. Chem.›, «41», 192 (1904). [452] See Bodlaender, ‹Z. phys. Chem.›, «39», 597 (1902); ‹Ber. d. chem. Ges.›, «36», 3933 (1903). [453] Potassium cyanide is a powerful reducing agent (see p. 89) and is readily oxidized to potassium cyanate. The action, presumably, takes the following course (see Chapters XIV and XV): 2 Cu^{2+} + 4 HO^{−} + KNC^{±} → 2 Cu^{+} + 4 HO^{−} + KNC^{2+} → 2 Cu^{+} + 2 HO^{−} + KNCO + H_{2}O. [454] Put [Cu^{+}] = ‹x›, and [CN^{−}] = 3 ‹x›, and, neglecting the degree of ionization, [Cu(CN)_{3}^{2−}] = 0.1, ‹x› being so small that it need not be subtracted from 0.1. Then ‹x› × (3 ‹x›)^3 = 0.1 × 0.5E−27, and ‹x› = 3.7E−8. [455] Treadwell and Girsewald, ‹Z. anorg. Chem.›, «38», 92 (1904). [456] Euler, ‹Ber. d. chem. Ges.›, «36», 3404 (1903).