Page:Text-book of Electrochemistry.djvu/299

 284 ELECTRO-ANALYSIS. chap.

gelatine, improve certain properties (density, lustre, and elasticity) of the deposited metal. The influence exerted by these has not yet been satisfactorily explained. Possibly they are connected, like the cases previously mentioned, with surface phenomena (see p. 279).

Difference of the Temperature Influence in Primary and Secondary Processes. — As has been repeatedly mentioned, the velocity of a chemical reaction increases considerably with rise of temperature. As we have seen above, the secondary processes are of a purely chemical nature, and an increase of temperature therefore promotes their influence. It is true that an exception is known to this, namely, the evolution of hydrogen from an acid in very dilute solution (01-normal and weaker) by zinc, particularly at high temperature. However, so dilute solutions are seldom used in practice, and we may therefore disregard this deviation (see p. 106).

In contradistinction to the secondary processes, primary electrolytic deposition depends solely on the current strength, which varies with the temperature only in so far as the resist- ance in the bath diminishes on heating. If a primary pro- cess is disturbed by a secondary one, the disturbance can be increased or diminished by raising or lowering the tempera- ture. In the electrolysis of potassium sulphate with a mercury cathode potassium is primarily deposited at the mercury with formation of potassium amalgam, from which hydrogen is afterwards evolved secondarily. The higher the temperature is, the sooner does this latter process occur. When a normal solution of potassium sulphate was electro- lysed by using 0053 ampere and a circular mercury cathode 3*7 mm. in diameter, hydrogen was evolved after 25 seconds at 20", but after 7-6 seconds at 83°.

These temperature relationships aie of importance in practice. Thus, in the deposition of bronze (copper and zinc), where the deposited zinc seeks to dissolve and precipitate copper, the temperature must not, according to Fontaine {10), exceed 36^

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