Polarographic currents, characteristics

The existence of polarographically reducible disulfide bonds in proteins appears to be intimately related to many Brdicka reactions. The reduction of disulfides to sulfhydryls at mercury electrodes, and the reverse process, have also been studied extensively. Insulin and other disulfide-containing proteins give rise to polarographic currents characteristic of this reaction. 

In pulse radiolysis experiments the radicals are produced homogeneously in the solution. The polarographic current, /, is determined by the concentration of radicals at the electrode surface, the rate at which they are oxidized or reduced and the rate at which they are replaced by other radicals diffusing to the surface from the bulk solution account also has to be taken of reactions of the radicals in the bulk solution. By measuring the current, at a fixed time after the pulse, as a function of the potential applied, one can obtain a polarogram which is characteristic of the redox behavior of the radical and so can be used to identify it. Information about the rate of electron transfer can be extracted from measurements of the time dependence of / at a fixed potential. For radicals which undergo self-reaction in the bulk solution, the appropriate relationship is given by Eq. 71, provided the time is shorter than the first half-life of the radical [140],... 

Another important factor which can affect electrode processes and result in changes in polarographic current-voltage curves was recognized in the forties, namely the role of adsorption. Characteristic features on current voltage curves in the case when one component of the electroactive oxidation-reduction couple is adsorbed at the surface of the dropping mercury electrode were recognized by Brdicka (51-53). Effects of adsorbed electroinactive surfactants were first observed by Wiesner (54) and Heyrovsky (55). 

A characteristic source of error is the temperature dependence of voltammetric currents. All equations for the proportionality of polarographic currents to concentration contain the diffusion coefficient. which increases with temperature. The rate constants of preceding and succeeding chemical reactions are also affected by temperature and this must be taken into account when evaluating kinetic and catalytic currents. Electrode reactions that are associated with analyte adsorption processes likewise show a specific temperature dependence. The temperature influence differs and leads to different temperature coefficients for the individual voltammetric methods [48]. 

The potential at the point on the polarographic wave where the current is equal to one-half the diffusion current is termed the half-wave potential and is designated by 1/2. It is quite clear from equation (9) that 1/2 is a characteristic constant for a reversible oxidation-reduction system and that its value is independent of the concentration of the oxidant [Ox] in the bulk of the solution. It follows from equations (8) and (9) that at 25 °C ... 

The individual polarographic characteristic of the analyzed compound is the potential El/2 at which the current strength equals 50% of its maximum value. The binary solvent benzene-methanol or benzene-ethanol is used. The amount of peroxide is proportional to the maximum strength of the electric current at peroxide concentration in solution lower than... 

The rotating disc electrode is constructed from a solid material, usually glassy carbon, platinum or gold. It is rotated at constant speed to maintain the hydrodynamic characteristics of the electrode-solution interface. The counter electrode and reference electrode are both stationary. A slow linear potential sweep is applied and the current response registered. Both oxidation and reduction processes can be examined. The curve of current response versus electrode potential is equivalent to a polarographic wave. The plateau current is proportional to substrate concentration and also depends on the rotation speed, which governs the substrate mass transport coefficient. The current-voltage response for a reversible process follows Equation 1.17. For an irreversible process this follows Equation 1.18 where the mass transfer coefficient is proportional to the square root of the disc rotation speed. 

Although the experimental conditions for diffusion-controlled current may be in effect for a polarographic measurement, the resultant current may not be controlled purely by diffusional processes. A convenient way to test whether this is true is to vary the height of the mercury column. The fluid flow characteristics of a capillary with a hydrostatic head are such that the diffusion current is directly proportional to the square root of the height of the column (small corrections for the surface tension of mercury on glass and for the hydrostatic backpressure of the water-immersed portion of the capillary are necessary for the most precise measurements)... 

The most characteristic parameter of a compound, when examined polarographically by d.m.e. or r.d.e. is the half-wave potential. It is a potential measured in the half height of the wave on the current-voltage curve. In the case of a thermodynamically reversible redox process, the half-wave potential virtually corresponds to the redox potential of the... 

Figure 20.2 A polarographic wave. Polarogram of a solution containing 10 ppm of in KNOj 0.1 M, obtained with a dropping-mercury electrode. The median position of the wave (about —0.35 V) is characteristic of lead while the height of the step, of its concentration. For a better presentation of the graph the oscillations have been damped. Right, a graph shows the measure of i. The residual current is due in part to impurities in the support electrolyte and to traces of oxygen.

The diffusion current constant Ja is used to correct polarographic diffusion currents for differences in capillary characteristics. For average currents... 

DPP (Figure 4) differs from NPP when prior to the pulse apphcation the potential is not constant but is replaced by a ramp voltage with DC polarographic characteristics. The pulse amplitude is constant and the measured current is displayed as the difference between the current sampled closely before pulse application and the current sampled at the pulse end. 

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