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have used water-in-toluene microemulsions. The extraction properties of C"V+•, where = 8-18, into the continuous phase of toluene are poor. Other organic solvents such as ethyl acetate or anisóle appear to extract effectively these radical ions. Recently,21 we have demonstrated that C8V+• undergoes an induced disproportionation reaction (eq 7) in a two-phase system of 2C8V+-^C8V: + C8V2+ (7)water-ethyl acetate and have utilized the two-electron reductant, C8V+•, in reduction of dibromostilbenes. This disproportionation equilibrium is unfavored in a homogeneous phase. It is forced toward the two-electron reductant, C8V: due to the opposite solubility properties of C8V2+ and C8V: in the water-ethyl acetate phases. Thus, application of a water-in-oil microemulsion media for the production of two-electron reductants via single-electron-transfer reactions, followed by an induced disproportionation, seems feasible. Consequently, the utilization of the doubly reduced product in the fixation of substrates being solubilized in the organic phase is conceivable. Such experiments utilizing this principle in the fixation of valuable products, i.e., reduction of ketones and C02 reduction, are now under way in our laboratory.

Stopped-flow kinetics studies of the formation and decomposition of 12-molybdophosphate (12-MPA) in HN03 and HCI04 solutions have been performed at 25 °C and 3.0 M ionic strength as a function of solution pH and Mo(VI) concentration. The formation kinetics profile follows a sum-of-exponentlals equation indicative of a consecutive reactions mechanism. The maximum 12-MPA formation rate varies with H+ and Mo(VI) concentrations in a complex manner, even though the dependence of the rate on the phosphate concentration Is a simple first-order dependence. In addition the acidic and basic hydrolysis of 12-MPA and the reaction of 12-MPA with additional phosphate to form the dimeric 9molybdophosphate are reported here. Implications for reaction-rate methods for determining phosphate are discussed.

Sneddon, J.; Hunter, J. K.; Ottaway, J. M.; Fell, S. G. Analyst (London) 1981, 106, 288. Harnly, J. M.; Mlller-Ihll, N. J.; O'Haver, T. C. J. Aotom. Chem., in press. " Y Iare measured at only three wavelengths, the dynamic range will fall short of that which can be achieved using a bi-Gaussian wave. However, the construction of sectored wheels with more than fclur quadrants would eliminate this limitation.The equllibrlum constants for 9-, 11-, and 12-molybdophosphates were determined spectrophotometrically in strong acid solutions wlth the continuous variation method and with computer slmulatlons. For 12-moiybdophosphate, log K,, =:EXPERIMENTAL SECTION Reagents. A 0.5 M stock solution of Mo(V1) was prepared from reagent grade Na2Mo04-2Hz0 (Baker Chemical Co.). Stock solutions of HNO,, HC104, and H2S04 were prepared from the concentrated acid reagents and standardized with NaOH previously standardized with KHP. The stock phosphate solution was prepared from reagent-grade KH2P04, and the 5 M NaC10, stock solution used to control ionic strength was prepared from the reagent grade chemical (G. F. Smith Chemical Co.). This Mo(OH)S(HzO)", Mo20(0H)&O)+, M O~O ( O H )~( H~O )~~' ,

Phosphate and silicate were determined slmultaneously by stopped-flow reactlon-rate measurements at two different times durlng the formation of the 12-heteropolymolybdates. I n solutlons containing 0.40-0.50 M acld, the formatlon of 12-molybdophosphate reaches equlllbrlum rapidly, whlle p-