Polarographic wave maxima

The sensitivity and selectivity can be raised when recording as a function of potential not the current but its derivative with respect to potential. In this case a curve with maximum is obtained (Fig. 23.4) instead of the polarographic wave. The potential of the maximum corresponds to the half-wave potential in an ordinary polarographic curve, and the height of the maximum is proportional to the concentration of the substance being examined. A signal proportional to the derivative can be formed in polarographs with the aid of relatively simple electric circuitry. 

Polarographic waves often show a peak followed by a sharp fall to the limiting current plateau, the cause of which is related to streaming of the solution past the mercury drop. Known as a current maximum, it can be eliminated by adding a surfactant such as gelatin or methyl-red to the sample solution. 

A current maximum of the first kind has the form of a sharp, straight line which starts to form just before the main polarographic wave (curve a in Figure 6.32). Such a maximum can be considerably larger than the wave itself, although it will usually drop suddenly back to the normal wave. Maxima of the first kind are caused by convective effects, as electrolyte flows past the surface of the mercury drop, resulting from surface tension differences at various points on the surface of the drop. 

The application of (2.17)-(2.20) is shown in Fig. 2.18. The response depends on the sphericity parameter p = VD/rov7 [27]. Under the influence of increasing parameter p, the minimum of the forward component and the maximum of the backward component gradually vanish and both components acquire the form of a polarographic wave. At potentials much lower than the half-wave potential, both currents tend to the limiting value which is equal to —p. The net peak potential is equal to the reversible half-wave potential and independent of the sphericity parameter, but the dimensionless net peak current is a linear function of the parameter p. If n sw = 50 mV and uAE = -5 mV, this relationship is ... 

In dilute supporting electrolyte, the maximum on the polarographic waves was observed, which is connected with accumulation of insoluble reduction product on the electrode surface. The apparent rate constant for cathodic reduction process of ammonia complexes of Zn(II) was obtained. 

In a few instances, the polarographic wave is accompanied by a large peak (where the current rises to a maximum before returning to the expected diffusion current plateau). Such an undesired peak, known as the polarographic maximum, is attributed to a hydrodynamic flow of the solution around the expanding mercury drop, can be suppressed by adding a small amount of a surface-active material (such as Triton X-100). 

Maxima are least likely to be encountered if one uses a low rate of mercury flow (jn < 0.5 mg/sec), a concentrated supporting electrolyte, and a low concentration of the electroactive species ( < 10 Af). Maxima can sometimes be avoided by altering the chemical nature of the supporting electrolyte. If need be, the maxima can usually be suppressed by adding to the solution a small amount of a surface-active substance, such as Triton X-100. One should add just enough to eliminate the maximum (determining the amount by trial and error), since even a moderate excess can distort the polarographic wave. 

During the reduction of xanthine oxidase the impulse polarographic wave displays two maxima at potentials 0.08 and -0.01 The position of the first maximum coincides with the enzyme redox potential. In a solution of denatured xanthine oxidase, the first maximum is absent. Accordingly, the view of the authors is that xanthine oxidase in this potential region is reduced in the native state. As for the other maximum, it is possibly associated with the reduction of sulfur-containing groups. 

The curve in Figure 17-13a is called a polarographic wave. The potential at which half the maximum current is reached is called the half-wave potential ( 1/2) in Figure 17-13a. The constant current in the plateau region is called the diffusion current because it is limited by the rate of diffusion of analyte to the electrode. For quantitative analysis, diffusion current is proportional to the concentration of analyte. Diffusion current is measured from the baseline recorded without analyte in... 

In a polarographic wave for a reversible process, the ratio of activities of R and Ox species at the electrode surface progressively increases from zero at the foot of the wave, through unity at the half-wave potential to infinity as the limiting current is approached. The potentials throughout the wave are determined by the Nernst equation in (RT/F) logaR/aox and the current densities by Eq. (71) with both forward and backward component terms comparable. With increasing potential, the rate of the reaction and therefore the current increases and eventually becomes limited by the maximum rate of diffusion determined by the bulk solution concentration, the solution viscosity, and the diffusion constant. 

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