Instrumentation
Micro-electrode (dropping mercury carbon or platinum), reference electrode, variable dc source,
electronics and recorder.
Applications
Quantitative and qualitative determination of metals and organic compounds at trace levels (10–^4 to 10–^8
M); relative precision 2–3%. Amperometric titrations are more versatile and more precise than
polarography.
Disadvantages
Measurements very sensitive to solution composition, dissolved oxygen and capillary characteristics.
Impurities in background electrolyte limit sensitivity.
The study of current–potential relations in an electrolysis cell where the current is determined solely by
the rate of diffusion of an electroactive species is called voltammetry. To obtain diffusion-controlled
currents, the solution must be unstirred and the temperature of the cell thermostatically controlled so as
to eliminate mechanical and thermal convection. In addition, a high concentration of an
electrochemically inert background or supporting electrolyte is added to the solution to suppress the
migration of electroactive species towards the electrodes by electrostatic attraction. Typically, the cell
comprises a mercury or platinum micro-electrode, which is readily polarizable, and a calomel or
mercury-pool reference electrode, which is non-polarizable. By using a small polarizable electrode,
conditions can readily be attained wherein the diffusion current is independent of applied potential and
directly proportional to the concentration of electroactive species in the bulk solution. Measurement of
such limiting currents forms the basis of quantitative analysis. The polarizable micro-electrode is
usually made the cathode at which the electroactive species is reduced. The most widely used electrode
is the dropping mercury electrode DME and the technique involving its use is known as polarography.
A plot of current flowing in the cell as a function of the applied potential is called a polarogram or a
polarographic wave (Figure 6.9). At small applied potentials, only a residual current flows in the cell
caused by the reduction of trace impurities in the sample solution and by charging of the mercury drops.
The charging effect is analogous to the behaviour of a condenser. Above the decomposition potential, at
which point reduction of an electroactive species is initiated, the current increases with applied
potential, until it levels off at a limiting value. The difference between the limiting value and the
residual current is known as the diffusion current, id. If, on increasing the applied potential further, other
species in the solution are reduced, additional polarographic waves will be observed. Finally, the
current will increase due to reduction of the supporting electrolyte or of the electrode