Polarography.

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Presentation transcript:

Polarography

Polarography is a subclass of voltammetry where the working electrode is a dropping mercury electrode (DME). It is current versus potential are recorded when gradual changing current is take place when current applied to cell.

A cell containing : Solution of interest (electroactive substance) Stable reference electrode. e.g : calomel electrode. Small area of indicator electrode (DME) Auxiliary or counter electrode .

Mercury pool

Hg will dropped in stable flow rate 5- 30 drops/min. Volts applied 50 – 200 mv/min Drop life 2 – 12 sec. Potential range (+ 0.4) – (-2) V

Advantage of Hg electrode As the Hg capillary very narrow so, able to analyse very small volume of analyte. Surface of electrode is very reproducible . Surface area of electrode can be calculated from the weight of drops.

Disadvantage Limited potential (+0.4) – (-2) V. Capillary is very small so easily to bloked .

Qualitative polarogram Polarographic spectrum

Quantitative polarogram Limiting current Quantitative polarogram Diffusion current residual current

Factors affecting diffusion current (Ilkovic equation) ίd = 708 n C D½ m ⅔ t⅙ n : change in valency C : concentration D : diffusion coefficient. m: flow rate of mercury dropping. t : life time of mercury drop.