Residual current, polarographic wave

Here Ee is the standard potential of the reaction against the reference electrode used to measure the potential of the dropping electrode, and the potential E refers to the average value during the life of a mercury drop. Before the commencement of the polarographic wave only a small residual current flows, and the concentration of any electro-active substance must be the same at the electrode interface as in the bulk of the solution. As soon as the decomposition potential is exceeded, some of the reducible substance (oxidant) at the interface is reduced, and must be replenished from the body of the solution by means of diffusion. The reduction product (reductant) does not accumulate at the interface, but diffuses away from it into the solution or into the electrode material. If the applied potential is increased to a value at which all the oxidant reaching the interface is reduced, only the newly formed reductant will be present the current then flowing will be the diffusion current. The current / at any point... 

With a well-defined polarographic wave where the limiting current plateau is parallel to the residual current curve, the measurement of the diffusion current is relatively simple. In the exact procedure, illustrated in Fig. 16.6(a), the actual... 

When toluquinone (TQ) was employed instead of CQ, any special currents other than the residual current were not observed as shown by curve 2 in Fig. 5. The difference in polarographic behaviors between TQ and CQ is attributable to the difference between the standard redox potential of the TQ/TQ couple, iiTQ/TQ- > and that of the CQ/CQ couple, E cq/cq- > in DCE since the potential range available for the appearance of polarographic wave due to the redox reaction at the W/O interface as in Eq. (11) depends strongly on the difference between the standard redox potential of the 01(W)/R1(W)... 

In agreement with the theory of electrolysis, treated in Sections 3.1 and 3.2, the parts of the residual current and the limiting current are clearly shown by the nature of the polarographic waves because for the cathodic reduction of Cd2+ and Zn2+ at the dme we have to deal with rapid electron transfer and limited diffusion of the cations from the solution towards the electrode surface and of the metal amalgam formed thereon towards the inside of the Hg drop, we may conclude that the half-wave potential, Eh, is constant [cf., Fig. 3.13 (a ] and agrees with the redox potential of the amalgam, i.e., -0.3521V for Cd2+ + 2e - Cd(Hg) and -0.7628 V for Zn2+ + 2e -> Zn(Hg) (ref. 10). The Nernst equation is... 

We saw above that the polarographic current rises from zero to a current plateau. The plateau may be horizontal, or it might be gently sloping upwards we called this rise a residual current. Occasionally, there is also a current peak superimposed on the wave (see Figure 6.32). Such peaks are of two types, i.e. maxima of the first kind and maxima of the second kind. Both are caused by enhanced rates of mass transport at the Hg solution interface, as described in the following. 

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.

This capacitive current does not usually form discrete waves of its own but is the major cause of the residual current. This capacitive current increases with increasing potential and so creates the slope of the upper and lower plateaux of polarographic waves. It is a background signal, a source of noise, and as such it is a major limiting factor on the sensitivity of dc polarography. 

Another system in which ring-formation has been considered to be manifested on polarographic curves is the reduction of pyridoxal (77, 80). The reduction wave of this compound changes with pH and the observed plot is similar to that shown in Fig. 22. This dependence can be explained either by hydration (as for other pyridine carboxaldehydes), or by hemiacetal formation. The same two interpretations can be applied to electronic spectra. A comparison with the behaviour of pyridoxal-5-phosphate can contribute to the solution of this problem. With this ester the formation of the hemiacetal form is impossible and practically no current decrease in acidic solutions can be observed. Hence it can be concluded that the decrease in the limiting current of pyridoxal is due to ring formation. Nevertheless, the possibility of some participation by a dehydration reaction cannot be completely excluded, for it is possible to assume that the introduction of a phosphoric acid residue into position 5 either shifts the equilibrium towards the dehydrated form or increases the rate of dehydration. 

Direct-current (d.c.) polarography is approximately a constant-potential experiment in which the current passed during the lifetime of a single Hg drop (ca. 1-7 s) is measured at a succession of potentials. The potential applied to the dme is scanned slowly (1-5 mV s ), and the resulting current-potential profile is recorded. Figure 1 shows two polarographic scans, one on the residual electrolyte (0.1 M [n-Bu N][PF ] in dimethoxyethane) and one to which cobaltocinium hexafluorophosphate (5 X 10 M) is added. The half-wave potential, E, and the limiting current, i, are derived from this S-shaped curve. 

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