Rate constants, chronocoulometry

Determinations of rate constants for the catalytic process were carried out by double potential step chronocoulometry... 

There is ample evidence that the plots of the logarithm of the apparent rate constant Tc against the potential difference (Tafel plots) for univalent ions have reciprocal slopes of about 118 mV per decade [42, 61, 124, 132, 142-144, 146]. This behavior is illustrated for several cations and anions in Fig. 13, which displays data obtained from equilibrium impedance measurements [132]. Tafel plots derived from dc or ac voltammetric measurements in a sufficiently broad potential range are usually curved [144, 145, 163]. Chronocoulometry has been claimed [124] to provide independently the rate constants for the forward and the backward ion transfer in Eq. (21) cf. also Girault s review [14]. However, this is impossible in principle, because these rate constants should always be related to each other by Eq. (22). The origin of the value of the apparent charge transfer coefficient and its variation with the potential has been always the key issue. 

In suitable cases, pulse techniques such as chronocoulometry or rapid linear-sweep voltammetry also can be employed to monitor the electrode kinetics within the precursor state "i.e., to evaluate directly the first-order rate constant, k, [Eq. (a) in 12.3.7.2] rather than k. Such measurements are analogous to the determination of rate parameters for intramolecular electron transfer within homeogeneous binuclear complexes ( 12.2.2.3.2). Evaluation of k is of particular fundamental interest because it yields direct information on the energetics of the elementary electron-transfer step (also see 12.3.7.5). 

The application of chronocoulometry is advantageous also for the evaluation of rate constants because the extrapolation from a later time domain is more accurate. The integration preserves the information of the kinetic effect that appears at the early period of the step experiment. 

In 1990, Shao and Girault started a series of investigations based on the kinetic study of the transfer of acetylcholine Ac = CH3C02CH2CH2N (CH3)3 in which the physical properties of one of the solvents were varied. The experimental approach for the measurement of the kinetic parameters was chronocoulometry, a technique which, like convolution linear sweep voltammetry, does not impose any prerequisites on the potential dependence of the rate constants. To verify the suitability of the experimental method, they studied the potential dependence of the rate constant for Ac transfer from water to oil and from oil to water. As illustrated in Fig. 7, the results obtained show that the apparent rate constants obey the Butler-Volmer relationship, expressed by Eq. 10. Note that Fig. 7 has been obtained from two independent experiments. In the first experiment, acetylcholine was only present in the aqueous phase as a chloride salt and forced to cross to the organic phase, whereas in the second, acetylcholine was only present in the organic phase as a tetraphenylborate salt and forced to transfer to the aqueous phase. 

The standard apparent rate constant, by chronocoulometry The results obtained can be summarized as follows ... 

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