28o ELECTRO-ANALYSIS. chap.
refining of copper also belongs to this category of pro- cesses.
If the electromotive force is sufficient to precipitate both metals, both are generally deposited simultaneously. Very oft«n, however, after the primary deposition a secondary reaction takes place between the metal of higher solution pressure and the salt of the other metal. Thus, e.g.y if a solution containing copper and zinc sulphates be electrolysed, l)Oth metals are deposited, but a secondary reaction then takes place, in which zinc dissolves and an equivalent amount of copper is separated. This sort of action occurs particularly when the deposition is carried out very slowly, Le, when the current density is small. If the solution contains much zinc and little copper, it may easily happen that all the deposited zinc cannot re-dissolve, for the copper ions only diffuse slowly to the cathode, and in this way a mixed metal is obtained. It is worthy of note that brass can be prepared in this electrolytic manner. The preparation is more suc- cessful if potassium cyanide solutions of the two metals be used, because then their positions in the electromotive series are close together. A sufficiently high current density (about 0-6 amp,/sq. dm.) must, however, be used if equal quantities of the two metals (zinc and copper) are dissolved in the bath, so that the copper may not be deposited in too large an amount. A piece of brass is used as anode, and this dissolves to replace the metals deposited from the solution.
Position of Hydrogen in Deposition.— Since aqueous solutions are used almost exclusively, a secondary .separation of hydrogen occurs so soon as a metal is deposited whose solution pressure exceeds that of hydrogen (a primary deposition also occurs, provided that no acid is present, but on account of the low conductivity of water this is ver)' small).
It is, therefore, impossible, without some particular device, to deposit the alkali metals, magnesium, or aluminium, from aqueous solution, and these are consequently prepared from their fused salts. By collecting the alkali metals at a
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