ABSTRACT
DA kelec = y 8 (4)
Note that it may be surprising that a chemical term is considered in Eq. (3), whereas we have supposed that no chemical reaction term was involved within the stagnant layer. As explained in Chapter 1, this stems from the fact that the effect of any chemical reaction within the diffusion layer depends on the relative magnitude of k82 /D versus unity. For usual laboratory conditions, 8 ~ 10-3 cm and D ~ 5 x 10-6 cm2 s-1 and then 82 /D is of the order of0.2 s. Thus, provided k is less than approximately 1 s-1 it has no tangible effect on the concentration profiles of the species, whereas it has a definite effect in the bulk solution due to the long reaction times (usually longer or comparable to half an hour). In practice it is important to decide when the simplification in Eq. (2) or (3) applies to a given experimental situation. The discussion just presented affords a simple answer to the problem. Indeed, consider the electron transfer reaction in Eq. (5), possibly followed by a chemical step in Eq. (6).
