Zinc and hydrogen ion activities in aqueous solutions of zinc chloride in the presence of potassium or barium chloride

by H. W. Crozier

Institution: University of Canterbury
Department: Chemistry
Year: 1930
Record ID: 1301853
Full text PDF: http://hdl.handle.net/10092/7761


When two salts of different electro affinities are present in the same solution, complex salts my be formed, the stronger salt appearing to force the weaker into the anion. The suggestion that the abnormal behaviour of certain electrolytes might be accounted for by assuming the formation of complex ions was first put forward by Hittorf, who, in the course of his study of migration, discovered that the migration ratio for the anion in solution of many double salts and certain single ones increased rapidly with increase in the concentration of the solution, and at high concentrations became greater than unity. Hitterof suggested that this was due to the formation of a double salt in the solution, which gradually dissociated in dilution. By the action of potassium cyanide upon an aqueous solution of silver nitrate, there is first formed a white precipitate of silver cyanide. K.C N. + Ag NO 3 → Ag. CN + K NO 3. This precipitate dissolves in an excess of potassium cyanide, forming potassium silver cyanide. Ag. CN + K CN → K Ag (CN) 2. This is a typical salt of potassium, giving all the reactions of this metal. A precipitate of silver sulphide is formed on passing H2 S through the solution but no precipitate of silver chloride is formed on the addition of a chloride showing that the concentration of silver ion in the solution is extremely small. Moreover, it can be shown by quantitative migration experiments that the silver migrates to the anode as part of the anion i.e. the salt is ionised thus K Ag. (CN) 2 ⇌ Kâ º + Ag (CN) 2 Many examples of such complex cyanides are known. Hittorf showed that in a concentrated solution of equivalent amounts of cadmium iodide and potassium iodide, the cadmium formed part of a complex anion. McBain and Van Rysselberge (J.A.C.S. 50.3009.1928) have shown that "when sufficient excess of chlorine ions or sulphate ions is added to .05 M solutions of salts contained a divalent ion, the migration of the anion is suppressed, or often reversed, showing that the sulphate or chlorine ions have combined with the undissociated molecules to form complex ions". Zinc forms salts of comparatively weak electro affinity, and consequently would be expected to form part of a complex anion in the presence of stronger salts such as those of the alkali and alkaline earth metals.