Polarographic Waves shape

For reversible systems (with fast electron-transfer kinetics), the shape of the polarographic wave can be described by the Heyrovsky—Ilkovic equation ... 

In polarography, we obtained the half-wave potential E// by analysing the shapes of the polarographic wave. E1/2 is a useful characteristic of the analyte in the same way as E . In cyclic voltammetry, the position o/both peaks (both forward and back in Figure 6.13 cathodic and anodic, respectively, in this example) gives us thermodynamic information. Provided that the couple is fully reversible, in the thermodynamic sense defined in Table 6.3, the two peaks are positioned on either side of the formal electrode potential E of the analyte redox couple, as follows ... 

The shape of this wave and the variation with pH are both consistent with fast equ-librium reactions In the pH region lower than the value of pK, for the hydroxyl radical, the reactions of this hydroxyl radical dominate the electrochemical process. Controlled potential reduction at the potential of this first wave indicates a IF process and the principal products are dimers of the hydroxyl radical. The second wave in this acidic region is due to addition of an electron and a proton to the neutral radical. This process competes with dimerization in the mid-pH range where the two polarographic waves merge. Over the pH range 7-9, monohydric alcohol is the principal product. At pH <3 or >12, pinacols are the main products. Unsymmet-rical carbonyl compounds afford mixtures of ( )- and meso-pinacols. Data (Table 10.3) for the ( ) / meso isomer ratio for pinacols from acetophenone and propio-phenone indicate different dimerization mechanisms in acid and in alkaline solutions. 

Here, A is the electrode area, C and D are the concentration and the diffusion coefficient of the electroactive species, AE and co(=2nfj are the amplitude and the angular frequency of the AC applied voltage, t is the time, and j=nF (Edc-Ei/2) / RT. For reversible processes, the AC polarographic wave has a symmetrical bell shape and corresponds to the derivative curve of the DC polarographic wave (Fig. 5.14(b)). The peak current ip, expressed by Eq. (5.24), is proportional to the concentration of electroactive species and the peak potential is almost equal to the half-wave potential in DC polarography ... 

To follow the kinetics in a solution in which two or more components are polarographically active, the waves of these components should be either well separated or they should show a considerable difference in the wave-height (at equal molar concentrations). To obtain sufficiently separated waves, the half-wave potentials of the electroactive species must differ by some 0.1V or 0.2 V (depending on the wave-shape) or more. 

When dehydration occurs as a consecutive reaction, its effect on polarographic curves can be observed only, if the electrode process is reversible. In such cases, the consecutive reaction affects neither the wave-height nor the wave-shape, but causes a shift in the half-wave potentials. Such systems, apart from the oxidation of -aminophenol mentioned above, probably play a role in the oxidation of enediols, e.g. of ascorbic acid. It is assumed that the oxidation of ascorbic acid gives in a reversible step an unstable electroactive product, which is then transformed to electroinactive dehydroascorbic acid in a fast chemical reaction. Theoretical treatment predicted a dependence of the half-wave potential on drop-time, and this was confirmed, but the rate constant of the deactivation reaction cannot be determined from the shift of the half-wave potential, because the value of the true standard potential (at t — 0) is not accessible to measurement. 

The shape of the polarographic wave is further influenced by the nature of the electrode process occurring at the drop surface. Polarographic waves may be reversible, irreversible, or quasireversible. The overall electrode process comprises the diffusion, electron transfer, and electrochemical reaction steps. 

Waves. Deviations from the classical S shape of polarographic waves usually result... 

Derive equation 10.5.13, describing the shape of a reversible polarographic wave, from equation 10.5.11. 

The exact method of measurement depends on the shape of the polarographic wave. Some examples are shown in Figure 3.12. 

This is because n affects the shape of the polarographic wave. The Ilkovic equation tells us that there is a direct effect on the wave height, ie on The Heyrovsky-llkovic equation tells us that the value of n will affect the slope of the rising part of the wave. The greater the value of n the greater the slope, (Fig. 1.6f). 

The significance of reversibilty in the analytical context lies in its effect on the wave shape (or peak shape in some of the more advanced polarographic techniques). Irreversible processes give rise to broader less steeply rising waves or broader peaks. While very many irreversible processes do give rise to well formed waves suitable for analytical application, with others the poor wave shape lowers their precision and hence analytical usefulness. A good example would... 

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