Limiting current polarographic

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... 

The role of the pH of the medium in the electrode reactions of organic compounds in aqueous solutions is well understood and has been recently reviewed in detail (Zuman, 1969). In particular, our understanding of this parameter is due to the large number of polarographic investigations where it has been found that the half-wave potential, the limiting current and the shape of the wave for an oxidation or reduction process may all be dependent on the acidity of the medium. 

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... 

Again returning to the diffusion-controlled limiting current, we often meet a considerable influence on its height by catalysis, adsorption or other surface phenomena, so that we have to deal with irreversible electrode processes. For instance, when to a polarographic system with a diffusion-controlled limiting... 

Majer65 in 1936 proposed measuring, instead of the entire polarographic curve, only the limiting current at a potential sufficiently high for that purpose if under these conditions one titrates metal ions such as Zn2+, Cd2+, Pb2+, Ni2+, Fe3+ and Bi3+ with EDTA66, one obtains a titration as depicted in Fig. 3.55 i, decreases to a very low value, in agreement with the stability constant of the EDTA-metal complex and the titration end-point is established by the intersection of the ij curves before and after that point correction of the i values for alteration of the solution volume by the titrant increments as in conductometric titration is recommended. 

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. 

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. 

Hence, the reduction of arenesulfenates at the mercury electrode can proceed by the formation of intermediate arenethiomercuric derivatives. Such derivatives are reduced just after their formation and more easily than the initial arenesulfenates. In line with this argument, it logically follows that the limiting currents of the polarographic waves would depend solely on the diffusion of substances to the electrode. In fact, diffusion currents have been observed experimentally. Experiments of electrolysis on mercury (a preparative scale) confirmed the general conclusion (Todres 1988). 

A perfect prototype of an ideally cation-permselective interface is a cathode upon which the cations of a dissolved salt are reduced. Experimental polarization curves measured on metal electrodes fit the theory very closely. Since in dimensional units the limiting current is proportional to the bulk concentration, the polarization measurements on electrodes may serve for determining the former. This is the essence of the electrochemical analytical method named polarography. (For the theory of polarographical methods see [28]—[30].)... 

The limiting current is proportional to the concentration of the electroactive species, whereas the half-wave potential is specific to the electroactive species, being close to the standard potential of the electrode reaction. Thus, by measuring polarographic waves, we can run qualitative and quantitative analyses. In DC polarography, many inorganic and organic substances (ions, complexes and mole-... 

There is, as yet, no complete theory to explain the phenomenon of polarographic maxima which is manifested by the observed current overshooting the limiting current. The causes are convective mass transport in the solution and adsorption. Three types of maximum have been identified [61] ... 

To apply equation (3) for calculation of the equilibrium constant K waves Ia and ic must both be limited by diffusion. To prove this the current is measured under conditions when it is 15% or less of the total limiting current and its dependence on the mercury pressure is followed. A diffusion current must, under these conditions, show a linear dependence on the square root of the height of the mercury column. Whenever possible, polarographic dissociation curves should be compared with data on dissociation obtained by other methods, e.g. potentiometry, N.M.R. or spectrophotometry. In the latter case it is important to show that the species responsible for a given polarographic wave is identical with that responsible for the observed absorption peak. 

In solutions of histidine and histamine containing pyruvic acid, another reaction takes place (9) in addition to the formation of Schiff base. With increasing concentration of these amino acids apart from the formation of the wave of the Schiff base at a more positive potential, a decrease of the total limiting current can be observed (Fig. 5). The decrease corresponds to the formation of a polarographically inactive compound this may be interpreted by reaction (6) ... 

For the determination of rate constants of the reactions that take place in the bulk of the solution, the change in the limiting current with time is measured. For this purpose, at least one of the components of the reaction mixture (either a reactant, intermediate, or product) must be electroactive and give a measurable polarographic wave. It is advantageous, if this wave (as most polarographic waves are) is a linear function of concentration1. 

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. 

Clearly, the HMRDE curve gives a cleaner polarographic wave than does the RDE, the latter showing, in particular, a larger slope in the region of the limiting current, an effect directly connected to the double layer charging. 

Another important phenomenon that occurs at the dropping mercury electrode is the polarographic maximum15, which occurs when, on reaching the limiting current plateau, the observed current exceeds /L (Fig. 8.10). The causes are mass transport within the electrode and surface adsorption. Three types of maximum have been identified ... 

Proving the existence of a kinetic current is the best possible under polarographic conditions when the studied kinetic current is lower than about 20% of the theoretical diffusion-controlled limiting current. Such currents are independent of mercury pressure, i.e., height of mercury column. Furthermore, such currents have much higher temperature coefficient (5-10% K-1) than diffusion currents (1.8% K-1). 

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