By Sunil Bhardwaj


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 \(\mu A\) 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.


Residual current \((i_R)\): The current which is associated with the impurities is known as Residual current. AB portion of the graph corresponds to residual current \(i_R\)

Diffusion current \((i_d)\): At the point B, the potential applied is equal to reduction potential of metal ions. The metal gets reduced over DME and current is produced.$$ { M }^{ n+ }+{ ne }^{ - }\longrightarrow { M } $$ $$ { M }+{ Hg }\longrightarrow { M(Hg) } $$ As large amount of metal ions are reduced, so the current will increase rapidly. At the point C reduction is complete.$$ i_d = i_L - i_R $$

Limiting Current \((i_L)\): At the point C, all the metallic ions are reduced. As no more metallic ions, so no further increase of current and now the current remain constant. This constant current is known as Limiting current.

Half wave potential \(({ E }_{ 1/2 })\): The halfwave potential is defined as that potential at which half of the total metal ions present in the electrolyte are reduced. OR when \(i= id/2\) then, \(E = { E }_{ 1/2 }\)

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