Controlled-current techniques

The difficulties in conventional polarography as mentioned in Section 3.3.1.1, especially the interference due to the charging current, have led to a series of most interesting developments by means of which these problems can be solved in various ways and to different extents. The newer methods concerned can be divided into controlled-potential techniques and controlled-current techniques. A more striking and practical division is the distinction between advanced DC polarography and AC polarography. These divisions and their further classification are illustrated in Table 3.1. In treating the different classes we have not applied a net separation between their principles, theory and practice, because these aspects are far too interrelated within each class. 

I. Controlled-potential techniques II. Controlled-current techniques ... 

The techniques in this field can be divided into two groups, viz., (I) controlled potential and (II) controlled current techniques (cf., Table 3.1). 

Mediation is mentioned in Bott, A. W., Redox properties of electron-transfer metalloproteins . Current Separations, 18, 47-54 (1999). In addition, Bott, A. W., Electrochemical titrations, Current Separations, 19, 128-132 (20(X)), and Bott, A. W., Controlled current techniques, Current Separations, 19, 125-127 (2000), are both worth consulting. 

Controlled-current techniques in stationary solution saw extensive development and application in the 1960s. However, they were largely supplanted by con-trolled-potential techniques, especially cyclic voltammetry, in the 1970s. Today, controlled-current techniques in stationary solutions are used occasionally. 

Figure 4.1 General excitation signals for controlled-current techniques. (A) Current step. (B) Double current step.

Heineman, W. P. Kissinger, P. T., Large-Amplitude Controlled Current Techniques, in Laboratory Techniques in Electroanalytical Chemistry, Kissinger, P. T. Heineman, W. R., eds., Marcel Dekker, New York, 1984, pp. 129-142. 

Chronopotentiometry — is a controlled-current technique (- dynamic technique) in which the - potential variation with time is measured following a current step (also cyclic, or current reversals, or linearly increasing currents are used). For a - nernstian electrode process,... 

Current step— The excitation signal used in controlled current techniques in which the potential is measured at a designated time [i]. See also - chronopotentiometry, -> cyclic chronopotentiometry, - staircase voltammetry. Ref. [i] Heineman WR, Kissinger PT (1984) In Kissinger PT, Heine-man WR (eds) Laboratory techniques in electroanalytical chemistry. Marcel Dekker, New York, pp 129-142... 

Cyclic chronopotentometry — A controlled current technique where the applied - current step is reversed at every transition time between cathodic and anodic to produce a series of steps in the potential vs. time plot - chronopotentiogram. The progression of transition times is characteristic of the mechanism of the electrode reaction. For example, a simple uncomplicated electron transfer reaction with both products soluble and stable shows relative -> transition times in the series 1 0.333 0.588 0.355 0.546 0.366... independent of the electrochemical reversibility of the electrode reaction. 

A fundamental disadvantage of controlled-current techniques is that double-layer charging effects are frequently larger and occur throughout the experiment in such a way that correction for them is not straightforward. Treating data from multicomponent systems and stepwise reactions is also more complicated in controlled-current methods, and the waves observed in E-t transients are usually less well-defined than those of potential sweep i-E curves. 

Figure 8.1.2 Different types of controlled-current techniques.

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