mercury electrode, a small amount of the metal may be obtained as amalgam ; but as soon as a fair quantity of it has separated, a secondary decomposition of the solvent-water takes place, and hydroxide is formed — a process which is used in Kellner's method of preparing hydroxides of the alkali metala
If the solution pressure of the deposited metal {e.g. zinc and nickel) is not so high as that of the metals mentioned, it can be separated from aqueous solution (say, solution of the sulphate) without any appreciable disturbance by secondary processes. In technical work, however, the metal obtained in these two cases is frequently spoiled on account of the forma- tion of a small quantity of oxide, and the nickel appears yellowish and the zinc spongy. This formation of oxide can be prevented by addition of acid, which, however, must not give rise to a strong primary separation of hydrogen. In the electrolysis of nickel salts, a weakly dissociated acid is added, such as citric, lactic, or boric acid (or even phosphoric acid), and the nickel obtained has then a pure white colour. For the deposition of zinc a small amount of sulphuric acid ia added to the solution, about 001 per cent., or of aluminium sulphate, which is highly hydrolysed, and a high current density is used (over 1 amp./sq. dm.) in order to avoid a secondary evolution of hydrogen. If too much hydrogen does separate in these cases, the nickel appears leafy, and the zinc is not compact.
Anal]rtical Separation of the Metals. — It has already been mentioned (p. 276) that the precipitation of copper from an acid solution of its sulphate cannot be made com- plete if other metals, particularly of the iron group or zinc, are present. This process cannot, therefore, be used for the quantitative separation of copper from more positive metals. For similar reasons several of the processes referred to above, which are quite good for the deposition of the metal from a pure solution, cannot be used for the separation from other metals. With the electro-analysis we therefore have, as a rule, to combine the ordinary analytical methods. For
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