Facilitated diffusion fast reaction limit

Besides transport through complexation with the carrier, it also occurs nonspecific gas transport through the solvent phase in which the carrier is dissolved. Therefore, the characteristic of facilitated transport is the occurrence of a reversible complexation process in combination with the diffusion process. Two different cases have to be taken in account (i) diffusion is rate limiting (fast reaction), (ii) reaction is rate Hmiting (slow reaction and relatively fast diffusion) [2]. The second case does not occur frequendy and only the first one wiU be considered. The selective transport of gases in facilitated transport systems can be described in analogy to a dual-mode transport mechanism [5, 6, 8]. 

In this equation is the purely transport controlled limiting current, D is the diffusion coefficient assumed to be identical for all species, and k is the first-order rate constant for the chemical reaction step. A plot of the observed limiting currents versus 5 determined from the first one-electrode process is shown in Fig. 10. A theoretical line can be fitted to the experimental points and a rate constant of k = 600 100 s-i is in good agreement with the value reported in the literature, k = 740 200 s [67]. For the reduction of ortho-bromonitrobenzene, a rate constant ofk = 200 50 s was determined, which is also in close agreement with the literature value of k = 250 s [71]. The implication is that ultrasound facilitates the measurement of fast rate constants under steady state conditions at... 

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