Figure 6.12
Timing sequence for differential pulse polarography (left) and resulting differential
polarogram (right).electrode by cathodic deposition then determined by anodic stripping. The sample solution is
electrolysed for an accurately measured period of time during which reducible species are deposited at a
stationary mercury drop or mercury-coated solid electrode. The polarity of the cell is then reversed and
a linear potential sweep, sometimes pulsed, is applied to produce anodic polarographic waves of the
sample components concentrated in the mercury electrode. The initial pre-concentration or pre-
electrolysis step must take place under carefully controlled and reproducible conditions, as only a small
fraction of the electroactive species is deposited at the micro-electrode. Thus, variations in pre-
electrolysis time, stirring rate, temperature, electrode potential and current density between samples and
standards must be eliminated as far as possible. The ultimate sensitivity of the method is determined by
the length of the pre-electrolysis time which can be up to an hour or more. However, by employing a
differential pulse technique during the subsequent anodic stripping rather than a linear potential sweep,
equivalent sensitivities can be obtained with pre-electrolysis times of a few minutes or less, and with
better reproducibility. A diagram of an anodic stripping instrument and a typical stripping trace are
shown in Figure 6.13. Metals which form amalgams with mercury, e.g. Pb, Cd, Cu, Zn and Sn, are
readily determined by this technique but the ultimate sensitivity is limited by the level of impurities
present in reagents and the background electrolyte solution. Nevertheless, concentrations in the range
10 –^8 – 10 –^9 M (ppb) can be determined under favourable conditions and methods have been devised for
Pb in blood and traces of Cu, Cd, Pb and Zn in drinking water.