Polarographic wave effect

Filella, M., van Leeuwen, H. P., Buffle, J. and Holub, K. (2000). Voltammetry of chemically heterogeneous metal complex systems. Part II. Simulation of the kinetic effects induced on polarographic waves, J. Electroanal. Chem., 485, 144-153. 

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. 

This explanation for the two polarographic waves seen in Figure 3.2 suggests that the region of transition between the two waves will be sensitive to buffer concentration and composition. Such effects are seen in the polarography-pH profiles of steroid enones, some of which [88] show behaviour like that of cyclohexenone while others show only a linear variation of half-wave potential over the whole pH range of 2 — 11 [89, 90]. 

Phosphorescence, 191, 223 Photoacoustic detector, 177 Photoelectric effect, 238 Photoelectron, 238 Photovoltaic, 176 Plasam, 274 Plate, 10 Polarogram, 361 Polarographic wave, 361 Polyethylene glycols, 32 Polysiloxanes, 31 POPOP, 333 PPO, 333 Precession, 129 Precision, 386 Pulse polarography, 364 Pulsed NMR, 155 Pyroelectric, 175... 

The most common electrochemical effects exerted in bulk solution are related to association (solvation, ion-pairing, complex formation, etc.) with the electroactive substance or electrochemically generated intermediates [4,19]. The importance of solvation can be gauged by comparing calculated and measured values of the parameter AE1/2 (defined as the difference, in volts between the half-wave potentials of the first and second polarographic waves) exhibited by polycyclic aromatic hydrocarbons (PAH) in dipolar aprotic solvents [46,47], It can be shown that AE1/2 is related to the equilibrium constant for disproportionation of the aromatic radical anion into neutral species and dianion, that is,... 

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. 

The catalytic effect of DMP+ can be best demonstrated with an example. Consider the reduction of cyclohexanone 16). This substrate does not exhibit a polarographic wave, nor a CV reduction peak (Fig. la and 2a). On the other hand, DMP+ shows... 

Substituted purines, such as 6-alkylaminopurines and 6-methylpurine, were studied using dc and ac polarography to evaluate the effects of substitution on the electrochemical behaviour of these compounds 155). When the 1,6 and 2,3 double bonds are present, a 4e polarographic wave, due to reduction of these bonds, is generally observed 1S5). 

Fia. 18. Effect of fast chemical reactions on polarographic waves. [ ( =diffusion-controlled limiting current of substance C kinetic current governed by the rate of chemical reaction by which substance C is formed in the vicinity of the electrode.]... 

If the diffusion layer can be renewed, the result is essentially as discussed earlier for NPP at the SMDE. Detection limits are typically poorer than at an SMDE because most solid electrodes are afflicted by background currents from slow faradaic processes associated with the electrode surface itself. If the diffusion layer cannot be effectively renewed, the polarographic wave will not show a plateau, but instead will pass through a peak, then droop at more extreme potentials as cumulative depletion of the electroreactant is manifested. The curve resembles a linear sweep voltammogram for essentially the reasons governing responses in LSV (Section 6.1). 

Polarographic investigations of a number of substituted nitro- and m-dinitrobenzonitriles in DMF solution revealed stable radical anion formation at the first polarographic wave by a reversible 1 -electron transfer. A cyano group at the 5-position of nitrobenzene and 1,3-dinitrobenzene proved to exert an important influence on the half-wave potentials of these benzonitrile derivatives and on the spin densities of the radical anions which were formed. The latter effect was confirmed by ESR measurements. 

This is because n affects the shape of the polarographic wave. The Ilkovic equation tells us that there is a direct effect on the wave height, ie on The Heyrovsky-llkovic equation tells us that the value of n will affect the slope of the rising part of the wave. The greater the value of n the greater the slope, (Fig. 1.6f). 

Fig. 1.6f. Effect of n on the shape of a reversible polarographic wave for the same concentration of analyte...

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