When the electric current is passed between Pt electrodes which are placed in dil. H2SO4. H2 and O2 gases are liberated at cathode and anode respectively.
If now battery is removed and the two electrodes are connected with galvanometer a small current will be seen to flow between two electrodes. The direction of the current is opposite to that during the electrolysis.
It so happens because during the electrolysis Pt electrodes gets covered with the bubbles of H2 and O2 and so these electrodes become gas electrodes and both will form the electrochemical cell. This electrochemical cell has some fixed value of emf. The direction of this emf is opposite to the current from battery. This phenomenon of back emf due to the product of the electrolysis is known as polarisation.
The following are the factors which can minimise the polarization:
1) If the electrolyte is constantly stirred the concentration change in vicinity of electrode can be avoided and if there is no increase in the concentration in the vicinity of electrode and polarisation does not take place.
2) Electrodes can be brushed off to remove the deposited gases like H2 and O2 so they don’t form the gas electrode therefore avoiding polarization.
3) The deposition of the gases on the surface of electrodes is minimised by applying black platinic chloride PtCl4 on the surface of Pt electrodes and the electrodes are known as platinised Pt electrodes.
4) Polarisation phenomenon can be eliminated by adding strong oxidising agent for e.g HNO3, chromic acid, MnO2 etc. so that as soon as H2 is formed it must be oxidised to water. Such substances which minimize the polarisation are known as depolariser.

свернуть