By Sunil Bhardwaj

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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. $$ { H }_{ 2 }S{ O }_{ 4 } \rightleftharpoons { H }^{ + } + H{ S{ O }_{ 4 } }^{ - } $$ $$ H{ S{ O }_{ 4 } }^{ - } + { H }_{ 2 }O \longrightarrow { H }_{ 2 }S{ O }_{ 4 } + { { OH } }^{ - } $$ At Cathode, $$ 2{ H }^{ + } + 2{ e }^{ - } \longrightarrow { H }_{ 2 } $$ At Anode, $$ 2{ OH }^{ - } \longrightarrow \frac { 1 }{ 2 } { O }_{ 2 } + { H }_{ 2 }O + 2{ e }^{ - } $$ Outside the solution the current flows from cathode (-) to anode ( + ), while inside the solution it flows from anode to cathode. 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 on which constitute the gas electrodes and two will form the electrochemical cell. It has some fixed value of emf. The direction of this emf is opposite to the current from battery. This phenomenon of back emf brought by the products of the electrolysis is known as polarisation. The polarisation is due to concentration changes in the vicinity of electrodes.

At Anode, $$ { H }_{ 2 } \longrightarrow 2{ H }^{ + } + 2{ e }^{ - }$$ At Cathode, $$ \frac { 1 }{ 2 } { O }_{ 2 } + 2{ H }^{ + } + 2{ e }^{ - } \longrightarrow { H }_{ 2 }O$$ Net cell reaction is, $$ \boxed { { H }_{ 2 } + \frac { 1 }{ 2 } { O }_{ 2 } \longrightarrow { H }_{ 2 }O } $$Therefore It is commonly called as concentration polarisation.

The following are the factors which can minimise the polarisation:

1) If the electrolyte is constantly stirred whole the bulk will be having the same concentration. There is no increase in the concentration in the vicinity of electrode and polarisation does not take place,

2) To remove the deposition of gases which form the gas electrode are brushed off time to time.

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 minimise the polarisation are known as depolariser.