Polarography kinetic currents

Brdicka, R., V. Hanus, and J. Koutecky, General theoretical treatment of polarographic kinetic currents, in Progress in Polarography (Eds P. Zuman and I. M. Kolthoff), Vol. 1, p. 145, Interscience, New York, 1962. 

Refs. [i] Koutecky J (1953) Coll Czech Chem Commun 18 183 [ii] Koutecky J (1954) Nature 174 233 [Hi] Koutecky ], Ct ek J (1956) Coll Czech Chem Commun 21 836, 1063 [iv] Koutecky J, Koryta J (1961) Electrochim Acta 3 318 [v] Brdicka R, Hanus V, Koutecky J (1962) General theoretical treatment of the polarographic kinetic currents. In Zuman P, KolthoffIM (eds) Progress in polarography, vol. 1. Interscience, New York London, pp 145-199 [vi] Cizek / (1987) Prog Surf Sc 25 17... 

Single-sweep i-E curves are step-like in shape for kinetic currents whereas, for diffusion currents, they show peaks. Similarly the incisions on the dEjdt = f E) curves recorded in oscillographic polarography are L-shaped for kinetic currents, whereas they are V-shaped for diffusion currents. 

The mercury dropping electrode was first introduced to electrolysis by J. Heyrovsky.< > The most important milestones in the theoretical development of polarography were the exact deduction of the equation for the limiting diffusion current by D. Ilkovi(5,< > of the equation for the shape of polarographic wave by J. Heyrov-sky and D. Ilkovic, the introduction of the conception of halfwave potentials by the same authors, the recognition of catalytic< > and adsorption currents by R. Brdicka and the development of the theory of kinetic currents by K. Wiesner, R. Brdicka, J. 

Fig. 4.6. Method for determining the concentration of residual impurities, [Impjj jjj., in a reaction mixture if the impurity is a catalyst or co-catalyst. The observed variable x can be peak-height h for a GLC method, absorbance A for spectroscopy, conductivity k for conductimetry, current i for polarography, or rate constant k for kinetics, etc.

However, the peak current in AC polarography markedly depends on the reversibility of the electrode process, being very small for an irreversible process. We can apply this dependence to study the kinetics of the electrode reactions. 

Polarography is valuable not only for studies of reactions which take place in the bulk of the solution, but also for the determination of both equilibrium and rate constants of fast reactions that occur in the vicinity of the electrode. Nevertheless, the study of kinetics is practically restricted to the study of reversible reactions, whereas in bulk reactions irreversible processes can also be followed. The study of fast reactions is in principle a perturbation method the system is displaced from equilibrium by electrolysis and the re-establishment of equilibrium is followed. Methodologically, the approach is also different for rapidly established equilibria the shift of the half-wave potential is followed to obtain approximate information on the value of the equilibrium constant. The rate constants of reactions in the vicinity of the electrode surface can be determined for such reactions in which the re-establishment of the equilibria is fast and comparable with the drop-time (3 s) but not for extremely fast reactions. For the calculation, it is important to measure the value of the limiting current ( ) under conditions when the reestablishment of the equilibrium is not extremely fast, and to measure the diffusion current (id) under conditions when the chemical reaction is extremely fast finally, it is important to have access to a value of the equilibrium constant measured by an independent method. 

Recently a series of dialkylpyrrolidinium (Pyr+) cations have been studied in our laboratory 7-9). These cations are reduced at relatively positive potentials and could be investigated electrochemically as low concentration reactants in the presence of (C4H9)4N+ electrolytes. Using cyclic voltammetry, polarography and coulometry, it was shown that Pyr+ react by a reversible le transfer. The products are insoluble solids which deposit on the cathode and incorporate Pyr+ and mercury from the cathode. Both the cation and the metal can be regenerated by oxidation. Quantitative analysis of current-time transients, from potential step experiments, showed that the kinetics of the process involve nucleation and growth and resemble metal deposition. 

The effects of solution acidity on the polarography of organic compounds have been reviewed, principally in aqueous solution. A thorough discussion of kinetic and catalytic currents that involve hydronium ion has been presented,52 and the irreversible polarographic and voltammetric curves that involve proton transfer in unbuffered and poorly buffered solutions have been discussed.59... 

Kinetic parameters of dimerization can be determined by polarography, -> chronoamperometry, -> linear potential scan and -> convolution voltammetry, -> rotating disc voltammetry, and alternating current sinusoidal polarography. See also -> association. 

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