Encyclopedia of Environmental Science and Engineering, Volume I and II

INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 571

In polarography the dropping mercury electrode, DME, is

utilized and other voltammetric methods employ a variety

of solid and paste electrodes. Polarography includes linear-

scan (classic), current-sampled, and normal- and differ-

ential-pulse, and alternating-current techniques. Other

voltammetric methods comprise fast linear-sweep, cyclic,

and stripping voltammetry.

As described in Section III,B, 2, a, the working elec-

trode is the sensor in these electrochemical methods. The

cell may consist of two or three electrodes. There are two

basic types of cells, electrolytic and galvanic. In an elec-

trolytic cell the redox reaction occurs due to the imposi-

tion of a potential from an outside source. The galvanic cell

is based on a spontaneous reaction occurring through the

proper choice of anode and cathode materials and the elec-

troactive analyte. Electrolytic and galvanic cells are used

in amperometry, while electrolytic cells are employed in

voltammetry.

Voltammetric and amperometric currents are measured

using working microelectrodes (surface areas of a few square

millimeters) under conditions leading to polarization of the

working electrode. The condition of polarization of an elec-

trode exists when a change of potential of the electrode will

not result in a change in the current generated.

Polarization occurs when the analyte is impeded in its elec-

trochemical reaction with the electrode surface. The analyte–

electrode reaction rate is dependent on the rates of a number

of rate processes giving rise to several types of polarization.

These are as follows: Concentration polarization is the limited

access of the analyte to the electrode surface due to the rate

of mass transfer of analyte by diffusion. Charge or electron

transfer polarization arises through the slow, heterogeneous

transfer of electrons between the analyte and the electrode sur-

face. Physical or surface polarization occurs when adsorption,

desorption, or crystallization (electrodeposition) processes

occur. Reaction polarization takes place when the analyte or

intermediates undergo chemical reactions before or after elec-

tron transfer.

(1) Voltammetry 50,75

(a) Polarography 76,77,78

Polarography is a voltammetric method where the drop-

ping mercury electrode, DME, is the indicating electrode.

The DME is a length of capillary tube connected by tubing to

a mercury reservoir situated above the capillary. The polaro-

graphic cell contains the DME and a reference electrode for

a two electrode system and a third electrode, usually a plati-

num wire, in a three electrode system (see Figure 25). Drops

Potentiostat

Input voltage

Ramp voltage

generator

Auxiliary

electrode

Reference

electrode

Solid electrode

Drop

timer

DME

Three electrode potentiostatic

control system

Recorder

Current

amplifier

+

+

FIGURE 25 Three-electrode polarographic and voltammetric analyzer. A DME equipped

with a drop timer or solid electrodes (gold, platinum, glassy carbon, wax impregnated car-

bon, etc.) are employed for polarographic or voltammetric analysis, respectively.

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