Silver iodide is so insoluble, that ammonia[439] may be used to separate it, with a considerable degree of accuracy, from silver chloride, and this separation forms the basis of a method to detect chloride-ion in the presence of an iodide. If a solution with a limited concentration of ammonia is used, the method may be extended also to the separation of chlorides from bromides (see Chap. XVI).[440]
Complex Metal-Ammonium Ions of Copper, Cadmium, etc.
[Cd2+] × [NH3]4 / [Cd(NH3)42+] = 1E−7
[Zn2+] × [NH3]4 / [Zn(NH3)42+] = 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.
The Argenticyanide-Ion.
For the condition of equilibrium between the complex and its components, the relation
[Ag+] × [CN−]2 / [Ag(CN)2−] = KInstability
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(NH3)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.
According to the equilibrium equation given above, the larger the excess of cyanide-ion in the solution, the smaller must be the concentration of silver-ion which is capable of existence in its presence. In agreement with this conclusion, we find that the addition of an excess of potassium cyanide readily redissolves the precipitated silver sulphide (exp.).[450] In other words, even the minute concentration of silver-ion, that must be present in the supernatant liquid above a precipitate of silver sulphide ([Ag+]2 × [S2−] = KSol. Prod., for a saturated liquid), cannot be permanently present with an excess of potassium cyanide. Consequently, the solid phase is incapable of existence in the system.