Page 117 - Analytical Electrochemistry 2d Ed - Jospeh Wang

102 PRACTICAL CONSIDERATIONS

through an intermediate bridge. An inert conducting material, such as platinum wire
or graphite rod, is usually used as the current-carrying auxiliary electrode. Attention
should be given to the relative position of these electrodes, and to their proper
connection to the electrochemical analyzer (see Section 4-4). The three electrodes, as
well as the tube used for bubbling the deoxygenating gas (see Section 4-3), are
supported in ®ve holes in the cell cover. Complete systems, integrating the three-
electrode cell, built-in gas control, and magnetic stirrer, along with proper cover, are
available commercially (e.g., Figure 4-2).
The exact cell design and the material used for its construction are selected
according to the experiment at hand and the nature of the sample. The various
designs differ with respect to size, temperature-control capability, stirring require-
ment, shape, or number of cell compartments. Various micro cells with 20±500 mL
volumes can be used when the sample volume is limited. Particularly attractive are
thin-layer cells in which the entire sample is con®ned within a thin layer (less than
10 mm thick) at the electrode surface (1). Smaller sample volumes can be accom-
modated in connection with ultramicroelectrodes (discussed in Section 4-4) and
advanced microfabrication processes (discussed in Section 6-3). In particular,
lithographically fabricated picoliter microvials (2) hold great promise for assays of
ultrasmall environments (e.g., single-cell systems). Specially designed ¯ow cells
(discussed in Section 3-6) are used for on-line applications. Glass is commonly used
as the cell material because of its low cost, transparency, chemical inertness, and
impermeability. Te¯on and quartz represent other possible cell materials. The cell
cover can be constructed of any suitable material that is inert to the sample. Accurate
temperature control is readily achieved by immersing or jacketing the cell in a
constant-temperature bath.

4-2 SOLVENTS AND SUPPORTING ELECTROLYTES

Electrochemical measurements are commonly carried out in a medium that consists
of solvent containing a supporting electrolyte. The choice of the solvent is dictated
primarily by the solubility of the analyte and its redox activity, and by solvent
properties such as the electrical conductivity, electrochemical activity, and chemical
reactivity. The solvent should not react with the analyte (or products) and should not
undergo electrochemical reactions over a wide potential range.
While water has been used as a solvent more than any other media, nonaqueous
solvents [e.g., acetonitrile, propylene carbonate, dimethylformamide (DMF),
dimethyl sulfoxide (DMSO), or methanol] have also frequently been used. Mixed
solvents may also be considered for certain applications. Double-distilled water is
adequate for most work in aqueous media. Triple-distilled water is often required
when trace (stripping) analysis is concerned. Organic solvents often require drying
or puri®cation procedures. These and other solvent-related considerations have been
reviewed by Mann (3).
Supporting electrolytes are required in controlled-potential experiments to
decrease the resistance of the solution, to eliminate electromigration effects, and
to maintain a constant ionic strength (i.e., ``swamping out'' the effect of variable