Chronocoulometry applied to the determination of a surface excess

In this section, a brief discussion is given of the chronocoulometric method [46, 47, 138, 139] to determine the surface excess Tq of an electroactive species O at some potential Ei chosen in the region far anodic to its half-wave potential. The initial concentration of O is Cq, and the initial concentration of R is zero (i.e. R is initially absent), so that, in the initial situation, no significant electrolysis takes place. 

At t = 0, the potential is stepped instantaneously to a value Ef far cathodic to the half-wave potential. Then the following events occur. 

The total charge involved in these processes is given by [47] 

This expression is evidently valid if (i) the time elapsed since the step was applied is sufficiently long for qM ( ,) and T0 to attain their new equilibrium values (ii) the parameter / = kfDoV2 (1 + exp f) in eqn. (37b) is sufficiently large to neglect the term 1112 in this equation. 

Equation (171) would allow rD to be determined if Qd) were known. An approximate estimate of Qdl could be obtained from a blank experiment, stepping from Ei to E in the absence of O. However, since qM ( ,) will be a function of T0, this procedure is not rigorous. A much better, or even rigorous, correction for Qdl can be performed by means of the double potential step chronocoulogram [47, 137]. In this method, the potential is stepped back from Ef to E after a period r and the charge Q t r) is measured as a function of time. It needs some algebraic manipulation [137] to derive the fact that a plot of Q (t r) vs. 6 = r1/2 + (t — t)v2 — f)/2 is a straight line and has an intercept equal to 

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