J . Phys. Chem. 1986,90,6071-6073 sensitization current at low rotation speeds should also reach a constant value. We observe this behavior for thionine and 2,6DST. To distinguish between these two alternatives, we measure photocurrents at W = 1-4 Hz as a function of light intensity and observe that the photocurrent varies with @1/2 (Figure 2b). This variation of iR with @1/2 means that XGk is dominated by XG,which varies with @'I2. Hence, we conclude that for thionine and 2,6DST the balance is between convective dilution and bleaching. For 4,6-DST, however, the size of is at low W is also partly determined by the rate of the back-reaction kb (Table I, entry 7). In this case, the kinetics are fast enough to make a substantial contribution to XG,k, so that (c,)hv in the stream toward the ring electrode is determined not only by the convective dilution but by kb, and the photocurrent will increase with increasing rotation speed. Comparison of the observed sensitization current at W = 1 (Table I, entry 6) and that calculated from the initial slope of the curve for 4,6-DST on Figure 2c, we estimate for this isomer K = 5 or greater. Substitution in the expression for K~ utilizing the diffusion coefficient of the n-Ti02 particles (obtained from the Stokes-Einstein equation),ls we find that for this isomer kb = 3.84 s-l. For thionine and 2,6-DST, the photoproduct is stable (vide infra), K
