Polarography and reactions with

The underlying theory may be simplified as follows. Polarography is concerned with electrode reactions at the indicator or micro-electrode, i.e. with reactions involving a transfer of electrons between the electrode and the components of the solution. These components are called oxidants when they can accept electrons, and reductants when they can lose electrons. The electrode is a cathode when a reduction can take place at its surface, and an anode when oxidation occurs at its surface. During the reduction of an oxidant at the cathode, electrons leave the electrode with the formation of an equivalent amount of the reductant in solution ... 

In order to overcome the drawbacks of DC polarography, various new types of polarography and voltammetry were developed. Some new polarographic methods are dealt with in this section. They are useful in chemical analyses as well as in studying electrode reactions. 

As described in Chapter 8, current-potential curves in polarography and voltammetry are useful for obtaining mechanistic information on electrode reactions. However, for complicated electrode processes, the information obtained from the current-potential curves is not conclusive enough. In order to get more conclusive information, it is desirable to confirm the reaction products and/or intermediates by some other technique. In this chapter, we focus our discussion on such techniques. We deal with electrolytic and coulometric techniques in Section 9.1 and the combinations of electrochemical and non-electrochemical techniques in Section 9.2. 

Polarography and cyclic voltammetry26 of [V(bipy)3]z (z =2+, 1+, 0, 1-, 2-, 3-) gave Elt2 values of -0.8, -1.01, -1.11, -1.55 and —2.23 V for these five steps, and smaller polarographic waves at -2.10 and -2.6 V were attributed to the reduction of free bipy. From comparison of the Em values for the bipy complexes with those for 4,4 -dimethyl-2,2 -bipy and 5,5 -dimethyl-2,2 -bipy, the electron added (or removed) in the reactions of [V(L)3]Z (z = 2+, 1+, 0) was at t2g orbitals.26 On the other hand, for V(bipy)3/V(bipy)J and V(bipy)J/V(bipy)2-the added electron would occupy orbitals with a predominant n character. However, [V(bipy)3] is diamagnetic, and an intermediate character has been proposed for it. 

Considerations of convenience and economy have led us to restrict the scope of this work in several ways. We have excluded electrochemical techniques, such as conductometry, high-frequency conductometry, and dielectro-metry,i, in which neither the electrical double layer nor any electrode reaction need be considered. These furnish information so widely different from, and so rarely used in conjunction with, that provided by polarography and its congeners that combining them would be difficult and of little use. 

Polarography is valuable not only for studies of reactions which take place in the bulk of the solution, but also for the determination of both equilibrium and rate constants of fast reactions that occur in the vicinity of the electrode. Nevertheless, the study of kinetics is practically restricted to the study of reversible reactions, whereas in bulk reactions irreversible processes can also be followed. The study of fast reactions is in principle a perturbation method the system is displaced from equilibrium by electrolysis and the re-establishment of equilibrium is followed. Methodologically, the approach is also different for rapidly established equilibria the shift of the half-wave potential is followed to obtain approximate information on the value of the equilibrium constant. The rate constants of reactions in the vicinity of the electrode surface can be determined for such reactions in which the re-establishment of the equilibria is fast and comparable with the drop-time (3 s) but not for extremely fast reactions. For the calculation, it is important to measure the value of the limiting current ( ) under conditions when the reestablishment of the equilibrium is not extremely fast, and to measure the diffusion current (id) under conditions when the chemical reaction is extremely fast finally, it is important to have access to a value of the equilibrium constant measured by an independent method. 

As the polarographic curve, furthermore, can be influenced by certain compounds or inhibitors, adsorption phenomena can complicate the interpretation of the curves, and catalytic waves may suggest further reductions than those found by macroelectrolysis, a certain caution must be exercised in evaluating the voltammetric data. In most cases, however, no complications arise, and with a little experience the differences mentioned above are not serious drawbacks, but are of value as the combination of polarography and macroelectrolysis then throws light on one or more of the steps in the reaction. 

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