The equilibrium equations give us, thus, a comprehensive basis for the interpretation of the behavior of cyanide solutions containing silver. First, in accordance with the small value of the constant, we find it very much more difficult to obtain precipitates of silver salts in cyanide, than, say, in ammoniacal solutions; secondly, in accordance with the fact that a very small, but definite, concentration of the silver-ion may still persist in the [p228] system, we find it possible, in the absence of an excess of cyanide, to precipitate such an extremely insoluble silver salt as silver sulphide represents; and finally, in accordance with the form and constant of our equation, we find it possible, by using an excess of potassium cyanide, to suppress the silver-ion to the point where even this extremely insoluble salt can no longer exist.[451]
Cuprocyanide and Cadmicyanide Ions.
When potassium cyanide is added to the deep blue ammoniacal solution of cupric-ammonium sulphate [Cu(NH3)4]SO4, 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)32− and its salt, potassium cuprocyanide K2[Cu(CN)3]. The instability constant of the complex ion is: [Cu+] × [CN−]3 / [Cu(CN)32−] = 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 K2[Cd(CN)4], yielding the ion [Cd(CN)42−]. The instability constant[456] of the complex ion is [Cd2+] × [CN−]4 / [Cd(CN)42−] = 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.
Ni(CN)42− ⥂ Ni2+ + 4 CN−
2 Ni2+ + Br2 ⥂ 2 Ni3+ + 2 Br−
Ni3+ + 3 HO− ⥂ Ni(OH)3 ↓
Applications and Precautions in Analysis.
Ferrocyanide and Ferricyanide Ions.
Fe(CN)63− ⇄ Fe3+ + 6 CN− and
[Fe3+] × [CN−]6 / [Fe(CN)63−] = K.