Direct-current polarography

The chromous-chromic reaction is highly unstable because of the high instability of the chromous salt. This redox couple is only stable when the concentration of the chromous salt is 5 to 10 times higher than that of the chromic salt. This reaction has been studied60 in 1.0 JV H2S04, and the plots of AFoo/ V versus co-1/2 are given in Fig. 9. The value of a is 0.47 and fc° = 2 x 10-3 cm/s. The results (Table 3) are comparable to those obtained by direct current polarography. 

Studies using a differential-pulse polarographic technique, which is recommended in place of classical direct current polarography, yielded the following results. For... 

DCP Acronym for direct current polarography. See - po-larography, and subentry DC polarography. 

FIGURE 9. Electrochemical behavior of Auranofin, 37, in deoxygenated alkaline 0.06 M K3PO4 ethanol/water (1 1) solution (a) direct current polarography (b) differential pulse polarography. Reproduced by permission of the American Pharmaceutical Association from Reference 48... 

This broad spectrum of analytical problems can be solved by electroanalytical techniques as will be shown with the following results. The use of electroanalytical methods in polymer research is often restricted to direct current polarography and this paper lays emphasis on the demonstration of the application of other methods. Showing the applicability of electroanalytical methods in polymer research might initiate further work in this field to get a widespread use of these methods in polymer science. 

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. 

Direct Current Polarography (Employing a Dropping-Mercury Electrode)... 

Equation (1.2.40) is very frequently used to determine the formal potential of a redox system with the help of cyclic voltammetry however, one should never forget that it holds true only for reversible systems, and provided that the symmetry coefficient a = 0.5 (see Chap. 1.3). To be cautious, it is better to refer to the value determined by Eq. (1.2.40) as the mid-peak potential determined by cyclic voltammetry. The formal potential E has the same meaning as discussed above for direct current polarography. Hence Eqs. (1.2.32) and (1.2.34) can be applied accordingly. 

Reversibility is not an absolute property in electrochemistry. It depends on the mutual ratio of the electrochemical time constant and the charge-transfer reaction rate. In direct current polarography the method s time constant is given by the drop time, the time available for the establishment of equilibrium, which can be varied only over a limited interval. Electrode reactions with charge-transfer control... 

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