Effect of Unequal Diffusion Coefficients

For an electrochemically reversible wave measured at a microdisc electrode for the process 

The difference of 10 mV is significant since the ferrocene/ferrocenium redox couple is often used as a redox marker in non-aqueous electrochemistry (acting as an internal reference electrode), including that using room temperature ionic hquids as solvents. The correction for the difference in diffusion coefficients is necessary to give a valid reference scale. 

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Often, for convenience, diffusion coefficients of all species in a mechanism are assumed equal, however unrealistic this probably is. If the reader wants to assume this, the d s in what follows can simply be set equal to unity. A recent paper [504] underlines the point, finding significant effects of unequal diffusion coefficients, and Pedersen et al. [438] in fact measured differences of about 20% between the diffusion coefficients of some organic compounds and the radical anions formed by their reduction. Such differences may well be significant in a simulation. 

For the cubic-type solutions, the situation is less simple. Flowever, the observed (minimum) velocity tends to zero as Db/Da tends to zero. For Dfi/Dyi > 1, the dimensionless velocity grows as J Db/Da) and then the actual velocity dx/dt will again depend only on and be independent of Da [20, 21]. We return to the effects of unequal diffusion coefficients in Section 6 to discuss how fronts may become unstable to perturbations. 

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