Explain the role of supporting electrolyte in Polarography #Polarography | Analytical Chemistry

In 1922, Heyrovsky, discovered a technique with the help of which, qualitative, quantitative and even mixture can be analyzed qualitatively and quantitatively, the technique is known as Polarography. In this case electrolysis of experimental solution is done. The cathode is polarized electrode (where deposition takes place) and anode is non polarized electrode.
In Polarography we plot a graph of current in 𝛍A V/S potential applied in volts. The graph obtained is known as Polarogram. It is S shaped wave. The Polarogram is divided into three portions.
The principle of Polarography is concentration gradient and not potential gradient. When the experiments are performed it is found that ions are moving from solution to the electrode surface due to concentration change and also due to electrical force (potential gradient).
The positive ions from the electrolyte go to the surface of DME and get reduced. It is due to the electrostatic attraction because of potential applied across the cathode anode. Because of these ions reduced some current is produced. This current is called migration current.
This migration current depends on transport number of metal ions. It means, if the transport number of the metal ions is zero. The migration current will be zero.
To make migration current zero a salt KC1 is added to the solution. This KC1 will lower the transport number of metal ions. As more and more KCI is added, the migration current is further decreased.
When large amount of KC1 about 35 to 100 times than the reducible metal ions is added, then transport number of metal ions is reduced to zero and so the migration current reduced to zero. Such electrolyte is called supporting electrolyte i.e. KC1 is supporting electrolyte.
As the reduction potential of K+ ions on the surface of D.M.E. is more than 0.4 volt so K+ ions cannot be reduced on D.M.E. Since the reducible ions migrate to the cathode under concentration gradient only. Therefore, experimental solution must not be stirred.