Back EMF in Electrolysis

In this article, we will discuss the back emf or polarization potential in electrolysis.

Electrolysis is an electrochemical process in which an electric current flowing through a solution of an electrolyte results in chemical changes. Electrolysis is carried out in an apparatus called an electrolytic cell or voltameter.

What is Back EMF in Electrolysis?

In an electrolytic cell or voltameter, when an external direct voltage is applied across cell terminals, an electromotive force (EMF) is built up between each electrode and the electrolyte that resists the applied direct voltage. This resisting or opposing emf is called back emf or polarization potential of the electrolyte.

It is generally denoted by the symbol E_b. The back emf in electrolysis is produced because of the coating of electrodes of the voltameter by the products of the electrolysis process. This effect is referred to as polarization, and hence, the back emf is termed as polarization potential as well.

In simple terms, we can define the back emf as under:

The emf developed in the electrolytic cell that opposes the applied external direct voltage is referred to as the back emf of the electrolyte.

The different electrolytes have different values of the back emf. For example, such acids and alkalis that evolve oxygen and hydrogen have the value of back emf equal to about 1.7 volts. For other electrolytes, the value of back emf depends upon the salt. For normal electrolytic solutions, the value of back emf varies from 0.5 to 2 volts.

Voltage Equation for Electrolysis

The voltage equation for electrolysis is given by,

`\V=E_b+IR_e`

Where, V is the applied external direct voltage, E_b is the back emf, and IR_e is the voltage drop in the electrolyte.

From this equation, it is clear that in order to carry out the process of electrolysis at an appreciable rate, the external direct voltage (V) must be equal to E_b­ + IR_e. In other words, if

`\V<E_b+IR_e`

Then, the process of electrolysis will not take place.

Hence, this is all about the back emf in electrolysis and voltage equation for the electrolytic process.