«Cuprocyanide and Cadmicyanide Ions.»—Very many of the metal ions are capable of forming complexes with cyanide-ion, of greater or smaller degrees of stability, and, as is the case for the complex ions formed by metal ions with ammonia, a metal ion is frequently able to form more than one complex with cyanide-ion.[452] A number of these complex cyanide ions are of particular interest in qualitative analysis. For instance, we make use of the difference in the stability of the cuprocyanide and the cadmicyanide ions as offering us the most convenient method of recognizing cadmium in the presence of copper. Excepting for the sulphide and the oxide, cadmium does not form salts and compounds of characteristic colors, and, except in color, the salts resemble the corresponding copper salts very much in their physical and chemical behavior. Copper and cadmium consequently show the same group reactions in systematic analysis. The more intense colors of the copper compounds—the black sulphide, the intensely blue cupric-ammonium-ion—mask the cadmium reactions. But cupric-ion may be converted, by potassium cyanide, into a complex ion of extreme stability, from the solutions of which hydrogen sulphide and alkali sulphides ‹fail to precipitate any sulphide of copper, while cadmium sulphide may be precipitated from the solutions of the much less stable complex cadmicyanide-ion›.
When potassium cyanide is added to the deep blue ammoniacal solution of cupric-ammonium sulphate [Cu(NH_{ 3})_{ 4}]SO_{ 4}, the cupric-ion is reduced[453] [p229] to cuprous-ion, and the latter is converted, by an excess of cyanide, into the extremely stable complex ion Cu(CN)_{ 3}^{ 2−} and its salt, potassium cuprocyanide K_{ 2}[Cu(CN)_{ 3}]. The instability constant of the complex ion is: [Cu^{ +}] × [CN^{ −}]^3 / [Cu(CN)_{ 3}^{ 2−}] = 0.5E−27, and the concentration of cuprous-ion, in a 0.1 molar solution, is approximately[454] 3.7E−8. Without an excess of cyanide, traces of cuprous sulphide may still be precipitated, but ‹a few drops excess will prevent the precipitation entirely›.[455]
With an excess of potassium cyanide, cadmium forms the salt K_{ 2}[Cd(CN)_{ 4}], yielding the ion [Cd(CN)_{ 4}^{ 2−}]. The instability constant[456] of the complex ion is [Cd^{ 2+}] × [CN^{ −}]^4 / [Cd(CN)_{ 4}^{ 2−}] = 1.4E−17. The concentration of cadmium-ion, in a 0.1 molar solution of the salt, is then approximately[457] 8E−5.