Page:Text-book of Electrochemistry.djvu/20

I. ELECTROLYSIS 3 This process of passing a current through a salt solution is termed electrolysis. The result of the electrolysis here considered consists partly in the evolution of oxygen at P1 and hydrogen at P2, and partly therein that the solution near P1 contains free acid, and that near P2 free alkali. These phenomena are typical for electrolysis in aqueous solution of oxygen salts of the alkali and alkaline earth metals.

Electrolysis of Salts of the Heavy Metals.— When the solution of a salt of a heavy metal, such as silver or copper, is electrolysed, the phenomenon is somewhat different. Let the plates P1 and P2 be of the same metal as that contained in the salt, e.g. silver electrodes in a solution of silver nitrate. In this case there is no evolution of gas, nor any change in the neutrality, and polarisation does not take place to an appreciable extent. The action of the current passing from P1 to P2 consists in the dissolving of some of the silver of the electrode P1 and deposition of the same amount of silver on P2, and further, the solution near P1 becomes more concentrated, whilst that near P2 becomes more dilute. This concentration change gives rise to a weak electromotive force which corresponds, in a certain respect, to the above-mentioned polarisation electromotive force.

Primary and Secondary Electrolysis.— It may possibly appear strange that the salts of the heavy metals behave differently from the alkali salts on electrolysis. However, if these latter be electrolysed in the fused condition, the metal is separated at the negative electrode; and even from aqueous solution the alkali metal may be separated electrolytically if mercury be used as the negative pole. The formation of alkali at the negative pole in the previous example is not a direct consequence of the electrolysis, but is due to the chemical action of the water on the primarily separated metal. The alkali metal is deposited by "primary electrolysis," and the secondary formation of alkali is termed "secondary electrolysis." When a chloride, e.g. sodium chloride, is electrolysed between platinum plates, chlorine is primarily separated at the positive pole, and this