[Cd^{ 2+}] × [NH_{ 3}]^4 / [Cd(NH_{ 3})_{ 4}^{ 2+}] = 1E−7
[Zn^{ 2+}] × [NH_{ 3}]^4 / [Zn(NH_{ 3})_{ 4}^{ 2+}] = 2.6E−10.
Cupri-ammonium-ion is far more intensely blue than cupric-ion and its color is used as one of the tests to identify copper in its salts. Nickel-ammonium-ion is also blue, a much paler blue, and its color must not be mistaken as indicating the presence of a dilute solution of cupric-ammonium-ion. The same kind of relations obtain for these complex ions as for silver-ammonium-ion. For instance, a salt like cupric phosphate, which is readily precipitated from cupric sulphate solutions, is not precipitated from the ammoniacal solutions containing an excess of ammonia (‹exp.›), while the very much less soluble[443] cupric sulphide is readily precipitated even from the ammoniacal solutions (‹exp.›). It may [p225] easily be shown, in the usual way,[444] that the sulphide of copper is very much less soluble than its phosphate (‹exp.›).
«The Complex Cyanide Ions.»—Metal ions are capable of forming complex ions, of importance in analytical work, with a number of components other than ammonia. Among the most important of these are the complex ions formed with cyanide-ion. The theory of the complex cyanide ions is entirely analogous to that of the complex metal-ammonium ions, but there is a difference in stability that makes their special consideration desirable, both for practical and for theoretical purposes. The complex ions of silver-ion and cyanide-ion will be discussed first.
«The Argenticyanide-Ion.»—When potassium cyanide is added to a solution of silver nitrate, a very insoluble precipitate of silver cyanide is obtained, but an excess of potassium cyanide readily redissolves the precipitate (‹exp.›). Since solution is effected in spite of the presence of an excess of the precipitating cyanide-ion, one is led to suspect that the other ion, the silver-ion, which is needed to form the precipitate, is suppressed by entering into some kind of complex with the excess of cyanide. As a matter of fact, the solution contains a salt, potassium argenticyanide KAg(CN)_{ 2}, in which the silver forms a part of a ‹negative argenticyanide-ion› (Ag(CN)_{ 2}^{ −}).[445] If a current of electricity is passed through such a solution, the silver (all but traces), together with the cyanide groups, moves towards the positive electrode.[446] The complex has been formed, then, by the combination of a positive silver-ion with two negative cyanide ions,[447] which produce a univalent negative argenticyanide-ion, Ag(CN)_{ 2}^{ −}. Recalling the fact that the complex silver-ammonium-ion is not perfectly stable, one might suspect that the complex cyanide-ion, in turn, is not absolutely stable, and that the action, by which it is formed, is balanced, when equilibrium is reached, by a reverse action of decomposition. We would have, then, [p226] K^{ +} + Ag^{ +} + 2 (CN)^{ −} ⇄ K^{ +} + [Ag(CN)_{ 2}^{ −}] or, more simply, Ag^{ +} + 2(CN)^{ −} ⇄ [Ag(CN)_{ 2}^{ −}].