By Sunil Bhardwaj


Take 1N H2SO4 in a beaker and arrange the pt electrodes in it. When small potential of 0.5V is applied across the two electrodes, the ammeter connected in the series shows deflection. As the potential applied from the battery increases to 0.6, 0.7 … and so on, the current must increase. But the ammeter shows more or less constant current. Why?

The answer is because H2SO4, ionised during the electrolysis, liberating H2 and O2 gases at the electrodes and forming the gas H2 and O2 gas electrodes. These two electrodes form electrochemical cell. Which has practical amf value 1.7 volt. As this emf oppose the pot. which is applied from the cell i.e. why there is no electrolysis phenomenon. So no deposition at the electrodes and no Increase of current. But when the applied potential is more than 1.7 volt then, there will be continuous electrolysis phenomenon and current increases rapidly.

If we plot a graph of current vs pot. applied. The nature of the graph is:

Def: The minimum externally applied potential at and above which electrolysis proceed continuously, is called decomposition potential - Ed The decomposition pot. of all aqueous acids and alkali is 1.7volt. Except HCl which is 1.3 Volts.

Experimental Method:

It is consist of a cell. Two electrodes are immersed in the electrolyte. The phenomenon of polarisation is minimised by stirring the electrolyte. Two electrodes are connected with voltmeter through galvanometer. The battery and variable resistance is connected across two electrodes.


Keep the jockey in contact with B. The pot. applied for this is found out in voltmeter and for this applied potential current is recorded in galvanometer which is due to the deposition of electrolysis products at the electrodes. i.e. inverse law of Faraday. Go on moving Jockey from B to A and for each position of Jockey, we will find out the value of current. When the observations are analysed it is found that initially current more or less remain constant for the applied potential. But after certain value i.e. Ed the current increases sharply, indicating now continuous phenomenon of electrolysis take place.


1) Es depends on nature of the electrolyte.
2) Concentration of the electrolyte.
3) Nature of the electrodes.
4) Nature of the products of the electrolysis.
5) Temperature.

1) It is characteristic property, therefore varies from electrolyte to electrolyte.
2) Zn and Fe are more electropositive, they are difficult to deposit on the electrodes because Ed value is high.
3) Cu and Ag are less electropositive and Ed value is low are therefore easy to deposit.
4) As the Ed for Fe is more and for Cu It is less, therefore they can be separated from the mixture.