Preferential or Selective Discharge of Ions during Electrolysis

When two or more ions of the same charge are present in a solution of an electrolyte, under identical conditions, and are competing for discharge at the same electrode, one of them gets preferentially discharged. This is known as the selective discharge of ions.

Factors that Determine Which Particular Ion is Discharged:

Below are factors that determine which particular ion is discharged at the cathodeA cathode is the electrode from which a conventional current leaves the electrolyte. It is the negative part of the cell where reduction takes place. More and which particular ion is discharged at the anodeAn anode is an electrode of a polarized electrical device through which conventional current enters the device. It is the positive part of electrolytes where oxidation takes place. More.

1. Electrochemical Series:

The electrochemical series is used to identify the ion that will be discharged preferentially at a particular electrode. The table below shows the electrochemical series for cationsCations are positively charged ions (atoms or groups of atoms that have more protons than electrons due to having lost one or more electrons). Cations are attracted to negatively charged electrodes... More and anions with increasing ease of discharge at the cathode and anode respectively.

Relative Position of Ions in the Electrochemical Series:

Usually, the ions lower in the electrochemical series will be discharged in preference to the ions higher in the series.

For example, in the electrolysis of calcium trioxonitrate(V), Ca(NO₃), solution, apart from Ca²⁺ and NO₃^– ions, the solution of Ca(NO₃) also contains H⁺ and OH^– ions due to the ionization of water.