Polarographic Waves

In some cases two polarographic waves are seen as the redox reaction occurs in steps. Half-wave potentials are given for each wave, with the corresponding change in oxidation states shown in parentheses. 

Consider the pyruvic acid system in Scheme XV. Let HA and A represent pyruvic acid and pyruvate, respectively, and suppose the system is buffered. At a pH well below the pX of HA, a single polarographic wave characteristic of the reduction of HA is observed. At a pH well above the pX, a wave (at a much more reducing potential) is observed that is characteristic of the reduction of A. ... 

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... 

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 ... 

This equation represents the potential as a function of the current at any point on the polarographic wave, it is sometimes termed the equation of the polarographic wave. 

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 information regarding the polarographic behaviour of a substance is not available, cognisance must be taken of the fact that in addition to the normal polarographic wave associated with the reduction and the diffusion of the species... 

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

Again two one-electron polarographic waves are obtained and two different products may be isolated. 

A minimum different from the above can be observed on the polarographic wave of peroxydisulphate at about -1-0.1 Volt in the presence of both copper(II) and arsenic(ril) which does not occur with either constituent alone . The decrease in the current due to peroxydisulphate, 4, is kinetic in nature and caused... 

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. 

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)... 

Taking into account the results obtained by polarography as well as controlled potential electrolysis, the reaction which proceeded in Range A and gave the polarographic wave was estimated to be composed of two-electron oxidation of NADH and one-electron reduction of CQ at the W/DCE interface. The oxidation of NADH is accompanied by the dissociation of one H in W. 

The polarographic wave of TBA or TPenA is observed at potentials where the polarographic current (curve 1) due to the eleetron transfer appears, whieh suggests that the coupling of the transfer of TBA" " or TPenA with the electron transfer occurs when W containing NADH and TBA or TPenA is shaken with DCE containing CQ [44]. Since... 

When F appi was more positive than —0.3 V (this potential range will be called as Range B) where the polarographic wave was not observed, the main oxidation product was also CQ, and CQH2 was oxidized to CQ almost quantitatively at —0.1 V. 

In order to clarify the reason for the coupling of the redox reaction between O2 and CQH2 with the ion transfer at the W/DCE interface in system of Eq. (25), current-scan polarograms for ion transfers at the W/DCE interface (cf. curves 3 to 5 in Fig. 5) were compared with that for the interfacial redox reaction (cf. curve 1 in Fig. 8). From the comparison, it is clear that transfers of TPenA" " and TBA+ from W to DCE proceed at potentials in Range A where the polarographic wave due to the redox reaction... 

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... 

Fig. 3.25. Interference by maxima in polarographic waves of (a) a single analyte, (b) two analytes.

For method K3 and K4 where again Eltart is chosen to be less negative than E , we must consider the pulse technique in relation to the polarographic wave in the same ways as in Figs. 3.41(a) and 3.47 however, the measurement result is different as we sample the current either at the pulse top (K4) versus its base or at the pulse base (K3) versus its top (see Fig. 3.51). 

These results are plausible since according to Sand a two-fold concentration of a component yields a four-fold transition time. Now, these features show, in contrast to the net separation and pure additivity of polarographic waves and their diffusion-limited currents as concentration functions, that in chrono-potentiometry the transition times of components in mixtures are considerably increased by the preceding transition times of any other more reactive component, which complicates considerably the concentration evaluation of chronopotentiograms. 

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