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.»—Quite a number of metal ions are capable of forming more or less stable complex ions with ammonia. For analytical purposes, the most interesting of such complex ions, aside from the silver-ammonium-ion, are those formed by cupric, cadmium, zinc and nickel ions, the most important of which represent bivalent complex ions of the composition[441] Me(NH_{3})_{4}^{2+}. The following instability constants have been determined:[442] At 21°, [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.