Page 121 - Analytical Electrochemistry 2d Ed - Jospeh Wang

106 PRACTICAL CONSIDERATIONS

voltammetric reponse. Many modern instruments, however, automatically subtract
(compensate) the iR drop from the potential signal given to the potentiostat via an
u
appropriate positive feedback.
Bipotentiostats, offering simultaneous control of two working electrodes (e.g.,
ring-disk) are also available. Such instruments consist of a conventional potentiostat
with a second voltage-control circuit. Multipotentiostats, controlling more than two
working electrodes, have also been described (6). The development of ultramicroe-
lectrodes, with their very small currents (and thus negligible iR losses even when R is
large), allows the use of simpli®ed, two-electrode, potential control (see Section
4-5.4). In contrast, ultramicroelectrode work requires ef®cient current-measurement
circuitry to differentiate between the faradaic response and the extraneous electronic
noise and for handling low currents down to the picoamp range. Other considerations
for the noise reduction involve the grounding and shielding of the instrument and cell.
The advent of inexpensive computing power has changed dramatically the way
voltammetric measurements are controlled and data are acquired and manipulated.
Computer-controlled instruments, available from most manufacturers (5), provide
¯exibility and sophistication in the execution of a great variety of modes. In
principle, any potential waveform that can be de®ned mathematically can be applied
with commands given through a keyboard. Such instruments offer various data-
processing options, including autoranging, blank subtraction, noise reduction, curve
smoothing, differentiation, integration, and peak search. The entire voltammogram
can be presented as a plot or printout (of the current-potential values). In addition,
computer control has allowed automation of voltammetric experiments and hence
has greatly improved the speed and precision of the measurement. Since the
electrochemical cell is an analog element, and computers work only in the digital
domain, analog-to-digital (A=D) and digital-to-analog (D=A) converters are used to
interface between the two. Unattended operation has been accomplished through the
coupling of autosamplers and microprocessor-controlled instruments (e.g., Figure

FIGURE 4-4 Microprocessor-controlled voltammetric analyzer, in connection with an
autosampler. (Courtesy of Metrohm Inc.)