Figure 6.11
Oscillographic polarography.
(a) Variation of drop area and applied potential with time.
(b) Oscillographic display of a polarogram.from that of the sample solution before display on the oscilloscope screen. Alternatively, by having the
same sample solution in both cells and using sophisticated electronic circuitry, first and second
derivative curves can be displayed. This enables components with E1/2 values as little as 25 mV apart to
be separated, but some sensitivity is sacrificed. Twin-cell operation suffers from the difficulty of
maintaining a pair of exactly matched capillaries.
Differential Pulse Polarography
Pulse polarography seeks to minimize the contribution to measured cell currents arising from charging
of the mercury drops (p. 248). This is achieved by superimposing a 10–100 mV pulse of short duration
(40–60 ms) onto the usual increasing dc potential. The pulse is applied during the last quarter of the
growth of each drop and the diffusion current is measured just before the pulse and again during the last
20 ms. The difference between the two measured currents for each drop is plotted as a function of the
dc potential. The timing sequence of operations and the resulting differential polarogram are shown in
Figure 6.12. Synchronization of the pulse with each drop is achieved in a similar manner to that used in
linear sweep oscillographic polarography. The height of the peak maximum is directly proportional to
the concentration of the electroactive species and it is possible to discriminate between species with
half-wave potentials differing by only 0.05 V. Apart from suppression of charging current, sensitivity in
pulse polarography is enhanced relative to conventional dc polarography because the application of a
voltage pulse to each drop produces higher currents than are obtained when diffusion alone controls the
electrochemical reaction.
Stripping Voltammetry
This is a modification of the polarographic technique whereby extremely low levels of certain metals
are first pre-concentrated at a stationary