Direct current polarography half-wave potential

Direct-current (d.c.) polarography is approximately a constant-potential experiment in which the current passed during the lifetime of a single Hg drop (ca. 1-7 s) is measured at a succession of potentials. The potential applied to the dme is scanned slowly (1-5 mV s ), and the resulting current-potential profile is recorded. Figure 1 shows two polarographic scans, one on the residual electrolyte (0.1 M [n-Bu N][PF ] in dimethoxyethane) and one to which cobaltocinium hexafluorophosphate (5 X 10 M) is added. The half-wave potential, E, and the limiting current, i, are derived from this S-shaped curve. 

Each electroanalytical technique has certain characteristic potentials, which can be derived from the measured curves. These are the half-wave potential in direct current polarography (DCP), the peak potentials in cyclic voltammetry (CV), the mid-peak potential in cyclic voltammetry, and the peak potential in differential pulse voltammetry (DPV) and square-wave voltammetry. In the case of electrochemical reversibility (see Chap. 1.3) all these characteristic potentials are interrelated and it is important to know their relationship to the standard and formal potential of the redox system. Here follows a brief summary of the most important characteristic potentials. 

The limiting current is independent of potential and is directly proportional to the bulk concentration of the reducible substance (hence, the importance of polarography in analytical chemistry). The half-wave potential ( 1/2) is that potential for which the condition... 

Direct current polarography is a basic method, from which improved analytical techniques of polarography and voltammetry have been developed. With DCP, inorganic and organic analytes can be analyzed with a sensitivity of about 10" mol/L. For determining several analytes simultaneously, their half-wave potentials must be at least 100 mV apart. 

Direct-current polarography dates back some 60 years to the discovery by Heyrovsky that highly reproducible i-E curves could be obtained using a dropping mercury electrode (DME) (under natural convection) as the working electrode. Thus, in a polarographic analysis, an i-E curve is recorded using a slow linear potential scan and a DME with a droptime of 2-10 s the i-E curve will, ideally, have the S-shape shown in Fig. 12.1(a). The half-wave potential E can be used for qualitative analysis since it is a measure of how readily the electroactive species is reduced or oxidized and therefore is a reflection of its... 

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