ELECTROLYTIC DISSOCIATION 253 



This excess is withdrawn by the action of the bat- 

 tery. This process is evidently the basis of electro- 

 plating, whereby silver is deposited upon a baser metal, 

 which is used as the cathode. Similarly, the electroly- 

 sis of copper sulphate is used commercially either for 

 copperplating or for the refining of copper derived 

 from ores. 



In the third case the products which appear are the 

 results of secondary reactions at the electrodes. In 

 the case of a solution of sodium hydroxide, NaOH, the 



ions are Na and OH. Now, metallic sodium enters 

 into a vigorous reaction with water, forming hydro- 

 gen gas and sodium hydroxide, 2Na+2H 2 O, giving 

 H 2 -f2NaOH. In conduction through sodium hy- 

 droxide, when the sodium ion has neutralized its de- 

 ficiency of one electron, it is in a condition to enter 

 into this reaction. The formation of electrically neu- 

 tral atoms of sodium is followed, then, immediately 

 by the decomposition of water and the liberation of 

 molecules of hydrogen gas. At the anode the neu- 

 tralized hydroxyl ions (OH) form water and liberate 

 oxygen, thus 4(OH)=O 2 +2H 2 O. It was this second- 

 ary reaction which in the earlier days of the investi- 

 gation of electrolysis obscured to some extent the 

 actual process and resulted in the idea that an electric 

 current decomposed l water. The more nearly free 



1 A similar secondary reaction is obtained if a dilute acid is used, 

 as for example sulphuric acid. In that case hydrogen is liberated 

 at the cathode in the primary reaction. At the anode the water is 

 decomposed by the neutralized sulphate radical and oxygen is 

 liberated, thus 



2H 2 0+2S0 4 =2H 2 S0 4 +0, 



