Simple notes of dropping mercury electrode PDF

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Detailed notes if dropping mercury electrode first semester...

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DROPPING MERCURY ELECTRODE Dropping mercury electrode is a polarisable electrode and a gradually increasing negative potential can be applied easily Apparatus

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It consists of a mercury reservoir from which mercury drops come down as small drops through a capillary. This acts as the cathode. It is known as the indicator electrode or micro electrode. The height of the mercury reservoir is adjusted to set the drop time ie, 1 to 5 seconds. Drop time is the time required to form every fresh drop of mercury from capillary. Inside the tubing wire contact is made where mercury flows through. The anode consists of mercury pool at the bottom of the reservoir, which acts as the reference electrode and its area is larger, so that it is not polarized. Both cathode and anode are connected across the appropriate ends of a battery The applied voltage can be changed by adjusting the sliding contact along the potentiometer wire EF.

Working 

Consider a polarographic cell containing a solution of cadmium chloride, to which external EMF is applied.

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To the analyte solution supporting electrolyte like KCl is added ie, 50-100 times of the sample concentration to eliminate migration current. Pure nitrogen or alkaline pyrogallol solution is passed through the solution to expel out the dissolved oxygen The positively charged ions present in the solution will be attracted to the dropping mercury electrode by an electrical force, and by a diffusive force resulting from the concentration gradient formed at the surface of the electrode Thus the total current flowing through the cell may be reagraded as the sum of electrical and diffusive forces When the applied voltage is increased and the current is recorded, a graph will be obtained.

In the graph from A to B, a small current flows. This is known as the residual current and is carried by the supporting electrolyte and impurities present in the sample. At point B, the potential of the electrode becomes equal to the decomposition potential of the cadmium ions The current then increases along the curve BC At the point C current no longer increases linearly with applied voltage but reaches a steady limiting value at the point D After this no increase in current is observed at higher cathode potentials Thus the current corresponding to the curve CD is known as the limiting current The difference between the residual current and the limiting current is called the diffusion current (id) The diffusion current is directly proportional to the concentration of analyte and therefore used as an quantitative analysis

Advantages of DME 

Surface area is reproducible

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Constant renewal of electrode surface eliminates poisoning effect. It is useful over the range of + 0.4 to - 1.8V

Disadvantages of DME  The electrode cannot be used above +0.4V (vs SCE), because mercury dissolves and anodic wave is recorded. Lesser than -1.8V , hydrogen is liberated.  The capillary is difficult to maintain since dust or other particulate matter can block the capillary....