By Sunil Bhardwaj

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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.

In Polarography, we have to pass N2 gas for about 15 - 20 minutes OR in some cases 40 – 45 minutes before the electrolysis in order to remove dissolved O2 if any present in the electro­lyte. In other case O2 will be reduced over DME before the reduction of metal because the reduction potential of O2 on D.M.E. is low. $${ O }_{ 2 } + 2{ H }^{ + } + 2{ e }^{ - }\longrightarrow { H }_{ 2 }{ O }_{ 2 }$$ $${ H }_{ 2 }{ O }_{ 2 } + 2{ H }^{ + } + 2{ e }^{ - }\longrightarrow 2{ H }_{ 2 }O$$ 1. The above reactions are at the cathode will disturb the formation of normal Hg drop.

2. In the reduction of O2 on Hg will result the formation of mercury Oxide which will spoil the smoothness of Hg.

3. Due to formation of H2O the concentration of the electrolyte will decrease.

4. The Hg combines with O2 to form mercury Oxide.

Hence, it is necessary to remove the dissolved oxygen if any present in the electrolyte i.e. why N2 gas is passed through the electrolyte, Freon  12 can also be used for flushing. For alkaline medium O2 is removed by adding the potassium sulphite $${ O }_{ 2 } + { K }_{ 2 }S{ O }_{ 3 }\longrightarrow 2{ K }_{ 2 }S{ O }_{ 4 }$$

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• Chromatography
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