«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}^{−}]. For the condition of equilibrium between the complex and its components, the relation [Ag^{+}] × [CN^{−}]^2 / [Ag(CN)_{2}^{−}] = K_{Instability} would hold. Bodlaender[448] determined the value of this constant by measuring the concentrations of the three components under varying conditions. The value found is 1E−21. The value of the instability constant for [Ag(NH_{3})_{2}^{+}], of analogous composition, is 6.8E−8, a very much larger value than the constant of the [Ag(CN)_{2}^{−}] complex. The latter is, therefore, by far the more stable. It must, consequently, be much more difficult to obtain reactions, such as precipitations, of silver-ion in cyanide than in ammoniacal solutions. In fact, it is impossible to precipitate silver chloride by the addition of sodium chloride to KAg(CN)_{2} solution (‹exp.›).[449] Silver sulphide was found to be a much less [p227] soluble salt than the chloride (p. 224), and ammonium or sodium sulphide solution, when added to the cyanide solution, readily precipitates silver sulphide (‹exp.›). (The sulphide is ‹capable› of ‹existence› in the solid phase, therefore, under these conditions.) In view of the extremely small concentrations of silver-ion in the cyanide solution, we have here a striking illustration of the extreme insolubility of the sulphide.