Standard, hydrogen electrode rate constant

Soon after the discovery of the absorption spectrum of the solvated electron in pulse radiolysis experiments (Chapter 2), the rates and mechanisms of its reaction with a wide variety of solutes was studied. Although it is a transient species, the solvated electron is a very important reducing agent. Indeed, its reduction potential is very negative the value of E°(H2O/e5 ) for the solvated electron in water is equal to -2.8 V with respect to the standard hydrogen electrode. The reaction rate constant and the probability of encounter with another species decide the so-called lifetime ofthe solvated electron, which therefore depends on the experimental conditions. 

The values of and vary as a function of the selected reference electrode, because E also depends on this parameter. Normally, one indicates the potentials (and therefore also the corresponding rate constants) with respect to the standard hydrogen electrode. 

However, the active dissolution of titanium depends markedly on temperature in acid solution. At lower temperatures, the picture is not so clear. It is necessary to have a quantitative measure of the rate of the hydrogen reaction and the titanium dissolution reaction. The complete set of current-potential and impedance-potential data has been tested against the theory given above. The best strategy seems to be to fit to a single electrode reaction and then to look for deviations from the expected behaviour for a perfect redox reaction. A convenient way of doing this is to represent the electrochemical data as a standard rate constant-potential curve in conjunction with a double layer capacity-potential curve [21]. 

Fig. 7. Analysis of the experimental steady-state current-potential and impedance-potential data from E = - 1300 mV to E = — 600 mV for a titanium rotating-disc electrode (45 Hz) in a solution of 2 M hydrochloric acid, (a) Standard rate constant-potential curve calculated for the hydrogen evolution reaction on titanium assuming that DA = 7.5 x 10 5cm"1s 1 and E° = - 246 mV. The Tafel slope 6C = 211 mV and the measured ohmic resistance was 0.4 ohm cm2. The potentials are the "true potentials, (b) High-frequency double layer capacity-potential curve. The potentials are the measured potentials.

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