Note 449 — Chapter XII · THE COPPER AND SILVER GROUPS (Continued).—THE THEORY OF COMPLEX IONS
Reading edition
The solubility-product constant for silver chloride at 25° is 2E−10. If the concentration of chloride-ion be made 1.0 by the addition of potassium chloride to a 0.05 molar solution of KAg(CN)2, then the concentration of silver-ion, necessary for the precipitation of the chloride, would be KS.P. / [Cl−] = 2E−10 gram-ion. Neglecting the fact that the complex salt is not completely ionized and putting [Ag(CN)2−] = 0.05, and calling x the concentration of the cyanide-ion just necessary to prevent the precipitation of the chloride, we have:
[Ag+] × [CN−]2 / [Ag(CN)2−] = 2E−10 × x2 / 0.05 = 10−21.
We find x = 5E−7 mole, or approximately 0.03 milligram potassium cyanide (cyanide-ion) per liter. This minute quantity of free cyanide, if not originally present in the solution used, would be formed by the liberation of potassium cyanide from the complex (according to KAg(CN)2 + KCl → AgCl + 2 KCN) as soon as 2.5E−7 mole, or 0.036 milligram, of silver chloride per liter have been formed, a quantity too small to be perceptible.
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.