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Polarography is a subclass of Voltametry where the working Electrode is a dropping Mercury Electrode (DME) or a Static Mercury Drop Electrode (SMDE), which are useful for their wide Cathodic Ranges and renewable surfaces. It was invented in 1922 by chemist Jaroslav, for which he won the Nobel prize in 1959.
Polarography is a voltammetric measurement whose response is determined by combined diffusion/convection mass transport.
The simple principle of polarography is the study of solutions or of electrode processes by means of Electrolysis with two Electrodes, one polarizable and one unpolarizable, the former formed by mercury regularly dropping from a Capillary Tube.
Polarography is a specific type of measurement that falls into the general category of linear-sweep voltammetry where the electrode potential is altered in a linear fashion from the initial potential to the final potential.
As a linear sweep method controlled by convection/diffusion mass transport, the current vs. potential response of a polarographic experiment has the typical Sigmoidal Shape.
What makes polarography different from other linear sweep voltammetry measurements is that polarography makes use of the (DME) or the static mercury drop electrode.
A plot of the current vs. potential in a polarography experiment shows the current oscillations corresponding to the drops of Hg falling from the capillary.
If one connected the maximum current of each drop, a sigmoidal shape would result. The limiting current (the plateau on the sigmoid), called the diffusion current because diffusion is the principal contribution to the flux of electroactive material at this point of the Hg drop life.
There are various limitations in particular for the classical polarography experiment for quantitative analytical measurements.
Because the current is continuously measured during the growth of the Hg drop, there is a substantial contribution from capacitive current. As the Hg flows from the capillary end, there is initially a large increase in the surface area. As a consequence, the initial current is dominated by capacitive effects as charging of the rapidly increasing interface occurs. Toward the end of the drop life, there is little change in the surface area which diminishes the contribution of capacitance changes to the total current.
Notes:
In D.C polarography supporting electrolyte is taken in excess, so that all migration current is carried by suporting electrolyte.
Thus electroactive species remain free from migration current:
1. E1/2 does not depend on concentration of electroactive species.
2. DME is a micro electrode.
3. Limiting current is sum of diffusion current and residual current.
il = id + ir
Correct option is (d)
