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. If potassium cyanide is added to an ammoniacal solution, containing both cadmium and copper, until the color of the solution is just discharged, and if two or three drops excess of the cyanide is then used, the addition of ammonium sulphide will precipitate pure cadmium sulphide (‹exp.›), while ammonium sulphide, added to a portion of the original ammoniacal solution, will precipitate a dark mixture of cupric and cadmium sulphide (‹exp.›), in which the yellow color of cadmium sulphide is masked by the black precipitate of cupric sulphide. «Cobalticyanide and Nickelocyanide Ions.»—In much the same way, in the identification of nickel in the presence of cobalt, advantage may be taken of the fact that cobalt forms an extremely stable cobalticyanide[458] ion, [Co(CN)_{6}^{3−}], which does not permit of the precipitation of cobaltic hydroxide, whereas nickel does not form such an ion, but only forms a not very stable nickelocyanide ion, [Ni(CN)_{4}^{2−}], which is readily decomposed by bromine and alkali, nickelic hydroxide being precipitated. [p230] The following are the chief actions involved in the precipitation of the latter:[459] Ni(CN)_{4}^{2−} ⥂ Ni^{2+} + 4 CN^{−} 2 Ni^{2+} + Br_{2} ⥂ 2 Ni^{3+} + 2 Br^{−} Ni^{3+} + 3 HO^{−} ⥂ Ni(OH)_{3} ↓