Page 82 - Analytical Electrochemistry 2d Ed - Jospeh Wang

3-3 PULSE VOLTAMMETRY 67

limit of classical polarography to the 5 10 6 to 1 10 5 M region. Lower
detection limits are obtained for analytes with redox potentials closer to E pzc ; i.e.,
when i approaches its smaller value, (equation 3-11). Advanced (pulse) polaro-
c
graphic techniques, discussed in Section 3-3, offer lower detection limits by taking
advantage of the different time dependences of the analytical and charging currents
(equation 3-13). Such developments have led to a decrease in the utility of DC
polarography.

3-3 PULSE VOLTAMMETRY

Pulse voltammetric techniques, introduced by Barker and Jenkin (3), are aimed at
lowering the detection limits of voltammetric measurements. By substantially
increasing the ratio between the faradaic and nonfaradaic currents, such techniques
permit convenient quantitation down to the 10 8 M concentration level. Because of
their greatly improved performance, modern pulse techniques have largely
supplanted classical polarography in the analytical laboratory. The various pulse
techniques are all based on a sampled current potential-step (chronoamperometric)
experiment. A sequence of such potential steps, each with a duration of about 50 ms,
is applied to the working electrode. After the potential is stepped, the charging
current decays rapidly (exponentially) to a negligible value (equation 1-49), while
the faradaic current decays more slowly. Thus, by sampling the current late in the
pulse life, an effective discrimination against the charging current is achieved.
The difference between the various pulse voltammetric techniques is the excita-
tion waveform and the current sampling regime. With both normal-pulse and
differential-pulse voltammetry, one potential pulse is applied for each drop of
mercury when the DME is used. (Both techniques can also be used at solid
electrodes.) By controlling the drop time (with a mechanical knocker), the pulse
is synchronized with the maximum growth of the mercury drop. At this point, near
the end of the drop lifetime, the faradaic current reaches its maximum value, while
the contribution of the charging current is minimal (based on the time dependence of
the components).

3-3.1 Normal-Pulse Voltammetry
Normal-pulse voltammetry consists of a series of pulses of increasing amplitude
applied to successive drops at a preselected time near the end of each drop lifetime
(4). Such a normal-pulse train is shown in Figure 3-4. Between the pulses, the
electrode is kept at a constant (base) potential at which no reaction of the analyte
occurs. The amplitude of the pulse increases linearly with each drop. The current is
measured about 40 ms after the pulse is applied, at which time the contribution of the
charging current is nearly zero. In addition, because of the short pulse duration, the
diffusion layer is thinner than that in DC polarography (i.e., there is larger ¯ux of