Let us consider the case of two platinum electrodes dipped in dilute sulphuric acid solution. When a small potential difference is applied across the electrodes, no current is found to flow. When, however, the applied voltage is increased, a time comes when a temporary flow of current takes place. The H+ ions move towards the cathodes and O- ions move towards the anode and are absorbed there. These adsorbed ions have a tendency to go back into the electrolytic solution, thereby leaving them as oppositely-charged electrodes. This tendency produces an emf that is in opposition to the applied voltage which is consequently reduced.
"This opposing emf which is produced in an electrolyte due to the absorption of gaseous ions by the electrolyte from the two electrodes is known as the back emf of electrolysis or polarization."
The value of this back emf is different from different electrolytes. The minimum voltage required to decompose an electrolyte is called the decomposition voltage for that electrolyte.