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. (E_b) and voltage drop (IR_e,) in the electrolyte i.e.

                          V = 

           where     V =  External d.c. voltage

                         E_b = Back e.m.f. of electrolyte

                            I = Circuit current

                         R_e = 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 E_b + IR_e. If the external d.c. voltage is less than this value, electrolysis will not take place.