Ultrasonic Absorption in Aqueous Salts of the Lanthanides. IV. Sulfates in D2O

The rates of formation of the monosulfate complexes of the trivalent rare earth ions of Pr, Sm, Gd, and Dy have been measured at 25 degrees C in D2O and additional measurements made on Pr, Sin, Gd, Tb, and Ho in water. Using formation constants recently calculated from conductance measurements in D2O, the rate data were compared to our reevaluated data in water to establish if a solvent isotope effect was present, which may contribute to our understanding of the mechanism of complexation. Contrary to previous reports, none is observed, the rate constants being in agreement within experimental error and within the limits of the assumptions used in the mathematical interpretation. Evidence therefore points to a dissociative mechanism in which the rate of exchange of the solvent from the primary hydration sphere of the cation is rate-determining.