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anion substantially enhances the production of MV,+ sensitized by Eosin Y.31 Eosin Y is a heavy-atom-substituted xanthene dye (tetrabromofluorescein), for which the internal heavy-atom effect would be expected to reduce the cage escape yield by increasing the singlet -* triplet or back-electron-transfer rates within the solvent cage. Similarly, Edel et al. report a very large enhancement of MV*+ when anthracenecarboxylate acts as a shuttle for a copper(I) complex.9 Their data indicate that the anthracene shuttle causes an enhancement of greater than 60-fold in MV'+ production, indicating that the cage escape yield for the [Cu complex, MV+] pair is <0.02. This suggests a rapid rate of back-electron-transfer within the solvent cage, which in turn can (31) Mau, A. W.-H.; Johansen, O.; Sasse, W. H. F. Photochem. Photobiol. 1985, 41, 503. be attributed to paramagnetism and rapid spin interconversion at the copper complex. A similar explanation was also proposed for the very low yields of charge pairs in the CuTPP-benzoquinone system.13Acknowledgment. This research was supported by a grant from the National Science Foundation, Grant No. CHE-8503092. J.O. thanks the California State University, Fullerton, for providing a sabbatical leave and the University of North Carolina at Chapel Hill for a visiting appointment for the time during which this work was accomplished.

A conductometrlc technique has been used to measure precise blnary diffusion coeff lcients for aqueous benzoic acid solutions at concentrations from 0.0003 to 0.0153 mol dm-a at 25 OC. As the concentratlon drops, dlssociatlon of benzoic acid molecules Increases the overall rate of diffusion of the solute. The concentration dependence of the measured diffusion coefficients is In good agreement with predlcted behavior and yields 0.900 (k0.006) X lo-' m2 8-' for the limiting dlffuslon coefficient of the undlssoclated aqueous benzoic acid molecule. The new diffusion data can be used to model dissolution of the acld, a standard procedure for determlnation of mass-transfer coefflcients.

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Diffusion coefficients of Na2CO3 + H2O and NaHCO3 + H2O have been measured by a conductimetric technique at 25 °C. Above 0.1 mol L−1, diffusion of aqueous Na2CO3 is typical of a simple binary electrolyte. At lower concentrations a significant proportion of the Na2CO3 component diffuses as NaHCO3 and NaOH produced by hydrolysis: [Formula: see text]. Because OH− is more mobile than HCO3−, binary Na2CO3 + H2O solutions undergoing diffusion separate spontaneously into two ternary solutions: Na2CO3 + NaOH + H2O ahead of the diffusion boundary and Na2CO3 + NaHCO3 + H2O left behind the boundary. A procedure is developed for measuring multicomponent diffusion coefficients of hydrolyzing salts. Results for Na2CO3 are in satisfactory agreement with predicted ternary values. As a consequence of the disproportionation reaction: [Formula: see text], diffusion of aqueous NaHCO3 is formally a ternary process. However, because reactants and products have similar mobilities and because only about 1% of the salt disporportionates, diffusion of aqueous NaHCO3 is effectively binary.