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. The cathode is dropping Hg electrode. (D.M.E.) and Anode is large pool of Hg or Reference electrode (SCE) with large area.

Polarography

It is consist of reservoir containing Hg. Rubber tubing is attached with reservoir and at the end of rubber tubing capillary is fixed. The drop of Hg is falling through the electrolyte. This drop of Hg acts as cathode (D.M.E.). The flask or the beaker contains the experimental solution. The large amount of Hg at the bottom acts as anode. Anode is connected to the variable resistance and then to +ve terminal of battery. A sensitive current recorder is connected in series and a voltmeter is connected between cathode and anode.

The height of Hg reservoir is adjusted that Hg falls at the rate of 20-30 drops per minute Close the circuit and keep the jockey S on X. For this position of Jockey, find out the reading of voltmeter and galvanometer. The jockey was moved from X to Y slowly and for each position of jockey, takes the readings of voltmeter and galvanometer. Now plot a graph of current V/S Volt applied. The graph thus obtained is known as Polarogram. From the graph we can have qualitative as well as quantitative analysis.

Polarography

As half wave potential is characteristic of each ion present in solution, we can have qualitative analysis. And with the help of diffusion current we can have quantitative analysis

D. Ilkovic performed the number of the experiments on Polarography. In each case, he observed the applied potential and current obtained in galvanometer. In each case, he plotted graph of current V/S applied potential and from graph he calculated id He said if id is known to us we can find out the concentration of experimental solution i.e. quantitative analysis is possible. $$ { i }_{ d } = 607n{ D }^{ 1/2 }{ m }^{ 2/3 }{ t }^{ 1/6 } C $$ \(n =\) Valency of metal
\(D =\) Diffusion coefficient which is constant.
\(m =\) rate of fall of Hg in \(g/sec\).
\(C =\) conc. of electrolyte in \(milli\ moles/{ dm }^{ 3 }\)
\(t =\) average time of fall of Hg drop.
\({ i }_{ d } =\) Diffusion current in \(\mu A\) (micro ampere)
As D depends on temp therefore id depends on temperature.

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