Electroactive Form

In the general case, the initial concentration of the oxidized component equals Cqx and that of the reduced component cRed. If the appropriate differential equations are used for transport of the two electroactive forms (see Eqs 2.5.3 and 2.7.16) with the corresponding diffusion coefficients, then the relationship between the concentrations of the oxidized and reduced forms at the surface of the electrode (for linear diffusion and simplified convective diffusion to a growing sphere) is given in the form... 

The variation of the peak current with the electrode kinetic parameter k and chemical kinetic parameter e is shown in Fig. 2.31. When the quasireversible electrode reaction is fast (curves 1 and 2 in Fig. 2.31) the dependence is similar as for the reversible case and characterized by a pronounced minimum If the electrode reaction is rather slow (curves 3-5), the dependence A fJ, vs. log( ) transforms into a sigmoidal curve. Although the backward chemical reaction is sufficiently fast to re-supply the electroactive material on the time scale of the reverse (reduction) potential pulses, the reuse of the electroactive form is prevented due to the very low kinetics of the electrode reaction. This situation corresponds to the lower plateau of curves 3-5 in Fig. 2.31. 

For those cases in which the concentration of B is such that either electroactive form A or electroactive form C is practically predominant in the solution (the corresponding height of polarographic wave will be denoted as id), it is possible to plot i (Ia or ic for a given concentration of B) in the relation log il(ia — i) against log E. This plot is linear (Fig. 3) if the reaction studied follows equation (1), and the intersection for log ij ia — i) = 0 corresponds to log K. The advantage of this treatment is that deviations from linearity, which indicate deviations from the scheme, are easily detected. 

The reduction of the 0-methyl oximes [94] is probably analogous to the reduction of the oximes in that the nitrogen-oxygen bond is cleaved before the saturation of the carbon-nitrogen double bond. The similarity in the reduction in acid solution of the oxime and its alkylated derivatives is understandable when the close resemblance between the electroactive forms is considered ... 

The equilibrium between the electro-inactive form A that is transported from the bulk of the solution towards the surface of the electrode and the electroactive form C is established at a finite rate. To simplify the treatment, the experimental conditions are usually chosen so that the compound B (usually a component of the reaction medium) is present in excess so that its concentration can be considered constant. Equation (20) is then simplified to (21) ... 

In case (i) the current is expected to depend on pH in the shape of a dissociation curve in case (ii) the current would increase linearly with hydrogen ion concentration finally, for (iii), steadily increasing current, dependent on the buffer composition, would be observed. In practice either several dissociations or combinations of these factors are involved. The observed pH-dependence may show a U-shape or bell-shape, or a maximum. Sometimes the above-mentioned reactions are combined with acid-base reactions involving the electroactive form, of the type mentioned in the preceding paragraph. 

The prewave is caused by the adsorption on the surface of the electrode of some of the electroactive material. If the adsorbed form is easier to reduce then it will be reduced at a lower potential than the rest of the electroactive form in the solution. The adsorbed form will thus form a separate prewave at more positive potentials (for a reduction or cathodic process) than the main wave. Since there are only a limited number of sites available for adsorption, they will become fully occupied above a certain concentration. Thus the height of the adsorption prewave will become constant after a certain concentration and any further increase in concentration will only increase the height of the main wave. 

Adsorption postwaves are similar to the prewaves but are caused by adsorption of the electrolysis product, or when the adsorption of the original electroactive form makes it more difficult to reduce. Usually adsorption of a species releases energy which is then available to the electrolysis step, making it easier to reduce the species. 

The different protonated forms of an individual compound can vary considerably in their polarographic behaviour. Often only one protonated form is electroactive. In other cases a second protonated form is also electroactive but at a quite different potential. In some cases the electroinactive form can be rapidly protonated or depro-tonated to the electroactive form in a fast step before the electrolytic step, giving rise to the wave but at a shifted potential. In other cases the electroinactive form remains totally inactive and the wave vanishes when the pH is adjusted to a value where this form is dominant. Sometimes the mechanism of the reduction or oxidation can change with pH. 

Triprolidine contains two nitrogen atoms capable of protonation and therefore exists in three states of protonation with two p/Ca values. The electroactive form is the doubly protonated dicationic form. However the monoprotonated and the unprotonated forms are both capable of undergoing very rapid protonation near the electrode surface. As a result, the wave occurs throughout the pH region and the height is independent of pH. When the monoprotonated or unprotonated forms are present the necessary protonation prior to the electroreduction simply adds to the number of hydrogen ions consumed in the potential-determining step. As a result the shift of half wave potential with pH becomes steeper. 

Generally derivitization is not very selective as other solution components may also be involved producing electroactive forms. The precision of the method will also be reduced. It is a technique of last resort in most cases. 

Often the current falls near the pK value as the electroactive form protonates or deprotonates. This would, of course, cause a small loss in sensitivity. But far more important is the fact that the wave would probably develop a kinetic component and lose its reproducibility due to increased temperature sensitivity. 

The first group of events involves convective or molecular diffusion to the electrode, possibly with some chemical transformation of the dominantly present species into an electroactive form, and the adsorption step. 

On the other hand, the polymerization using sulfonated polystyrene (SPS) as template produced the electroactive form of polyaniline (297-299). The resulting polymer was soluble in water and the conductivity reached 5x 10 S-cm without doping. Besides SPS, a strong acid surfactant, sodium dodecylbenzenesulfonic acid, provided suitable local template environments leading to the formation of conducting polyaniline. Aniline was also polymerized by BOD catalyst to give the polyaniline film, which was electrochemically reversible in its redox properties in acidic solution (300). 

Mechanism 5. Regeneration of the electroactive form of a redox couple at a working electrode can involve a reversible homogeneous chemical reaction. For a reversible electrode reaction, at rapid scan rates, the equations describing the voltammogram are essentially those for mechanism 1. At slow scan rates, the plateau current is given by... 

Mass transport in amperometric systems in which the reagent stream is forced to flow along the surface of the electrode may be described in terms of convective diffusion. Effectively this means that at sufficiently high values of Pg, the Peclet number, the liquid above an electrode may be divided into two distinct zones. In one zone, far away from the electrode surface, convection is important, and the concentration profile is substantially flat. In the other zone, adjacent to the electroactive surface, there is a sharp concentration gradient here diffusion is the predominant mass transport process. The Peclet number is given by v l/D, where is the main stream fluid velocity, and / is the length of the electrode (measured in the direction of fluid flow). Under these conditions, the mass transport limited current z l for a reversible electrode couple (i.e. the concentration of the electroactive form is zero at the electrode surface) is given by... 

Typical examples of systems that follow such a scheme are the reduction of weak acids and the reduction of formaldehyde in aqueous solution, where it is present in both an electroactive form and an inactive hydrated from. The cases where the electron transfer is reversible and where it is irreversible will both be considered here, but schemes where the chemical reaction is of second or higher order will not be discussed. In these cases kinetic data can only be obtained by either forcing the system into a pseudo first order regime or by comparing experimental data with simulated results. 

Step 2 A chemical reaction takes place, which transforms the reactant into an electroactive form 0. 

In some cases, an electroactive form, other than the substance present in the bulk of solution, is formed by a chemical reaction in the neighbourhood of the electrode, and consequently undergoes reduction or oxidation at the mercury dropping electrode. In such instances, when the rate of a chemical process is slow enough to be the determining step of the electrode process, we use the term kinetic or reaction currents ( ). 

Several classes of compounds can be determined when transformed by a chemical reaction into an electroactive form, and for analytical purposes the waves for various substances resulting in such reactions can be followed. In this section a classification is used according to the type of reaction involved in the transformation. The most frequently used methods are nitration, nitrosation, condensation, addition, substitution, oxidation and complex formation. It should be stressed that these types of chemical reactions have so far been relatively unexploited. The analytical chemist, when using polarographic methods, should always bear in mind the possibility of using those types of chemical reactions which give a high yield of an electroactive compound. 

---