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. The cathode is small in size called micro electrode and is made up of either pt or Au. Anode is large pool of Hg or Reference electrode (SCE) with large area. Now the cathode used is not micro electrode but dropping Hg electrode. (D.M.E.)

The following are the main advantages of D.M.E. over the other electrodes:
The surface of the Hg is smooth, the electrode is reproducible and every time electrode is renewed.
The reduction potential of H+ ions to from H2 gas 2H+ + 2e- = H2 over the Hg is high hydrogen ions from the experimental solution are not allowed to reduce on Hg but the metal ions can be reduced on the surface of DME.
As soon as metallic ions approaches towards the cathode are reduced over DME Mn+ + ne- = M and then metal dissolve in Hg forming amalgam M + Hg = M(Hg) which is falling at the bottom of the container
DME. electrode is working very successfully in the potential range of 2 volt to 0.4 volt. The majority of ions have the reduction potential in this range so can be accurately analyzed by this method.
Since the area of the electrode is small Therefore the amount of the element reduced on the surface of Hg will be small, current produced is small Therefore the change in the concentration of the electrolyte is small and negligible.
As the electrode is renewed every time Therefore No. of the metallic ions can be reduced from the given mixture. Therefore mixture of the metallic ions can be analyzed
The DME. can be used for basic solutions if the [OH-] 0.1 and its reduction potential is beyond 1.4 volt.

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