Page:Text-book of Electrochemistry.djvu/19

2 INTRODUCTION. CHAP. was in the circuit gas evolution had actually taken place, the polarisation current would be stronger, and would last longer than in the case where no bubbles of gas had been produced. Cause of Polarisation.— Let us assume that gas had been evolved at the plates. After the battery has been in action for some time plate P1 has become covered with a film of oxygen and plate P2 has a similar envelope of hydrogen. The plates, originally quite similar, after the passage of the current behave like two different metals. In the same way that a current can be obtained between a copper plate and a zinc plate immersed in sodium sulphate solution, the current is now obtained when the platinum plate covered with oxygen is connected by means of a conducting-wire with the plate covered with hydrogen, both being immersed in the solution. The gases which cling to, or have penetrated into, the plates are used up in giving rise to the current which, consequently, soon stops.

Even in the case where no apparent evolution of gas has taken place when current has been drawn from the battery the plates behave in the same manner. It is, therefore, assumed that here too the gases are really separated, but in such small quantity as not to make themselves evident, and this assumption is supported by various considerations.

The polarisation of the plates presents itself, therefore, in the property which these have of behaving like two different metals which exert an opposing electromotive force against the electromotive force of the battery. Plate P1 is said to be "polarised " with oxygen, plate P2 with hydrogen.

The Electrolytic Decomposition of Water.— Other changes, besides the separation of gases, take place at the plates P1 and P2, which also are of importance for the polarisation. If a few drops of litmus solution be added to the salt solution it is found that the liquid in the neighbourhood of plate P1 becomes red, whilst that near plate P2 remains (or becomes) blue, indicating that the salt solution, originally neutral, has altered near the electrodes, and has become acid at P1 and basic at P2.