Electrical Engineering ⇒ Topic : Back e.m.f. or Polarisation Potential
Back e.m.f. or Polarisation Potential
The process of electrolysis is carried out in an apparatus called voltameter or electrolytic cell. When external d.c. voltage is applied across the electrodes, an e.m.f. is set up between each electrode and the electrolyte which opposes the external d.c. voltage. This opposing e.m.f. is called back e.ml (Eh) of the electrolyte and is produced due to the coating of electrodes by the products of electrolysis. This effect is called polarisation and for this reason, back e.m.f. is also called polarisation potential.
The e.m.f set up in the voltameter which opposes the external d.c. voltage is called back e.m.f. of the electrolyte.
The value of back e.m.f. is different for different electrolytes. For acids and alkalies which evolve hydrogen and oxygen, its value is about 1.7 V. For other electrolytes, the value of back e.m.f. depends on the particular salt and generally lies between 0.5 V and 2 V for normal solutions.
Voltage equation for electrolysis. For electrolysis, the applied external d.c. voltage V must overcome the back e.m.f. (Eb) and voltage drop (IRe,) in the electrolyte i.e.
where V = External d.c. voltage
Eb = Back e.m.f. of electrolyte
I = Circuit current
Re = Resistance of electrolyte
Therefore, in order to carry out electrolysis at an appreciable rate, the external d.c. voltage V must be atleast equal to Eb + IRe. If the external d.c. voltage is less than this value, electrolysis will not take place.
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