ULTRASONIC-ABSORPTION STUDY OF THE COMPLEX-FORMATION OF CADMIUM(II) CARBOXYLATES IN AQUEOUS-SOLUTION

Ultrasonic absorption of aqueous cadmium(II) acetate, propionate, and n-butyrates has been measured with a pulse technique over the frequency range 3-260 MHz. The single relaxations observed in the metal-rich concentration regions are attributed to the complex formation of cadmium(II) carboxylates via the Eigen-Tamm mechanism of stepwise association. The reaction parameters are determined from the concentratio dependences of the relaxation frequency and amplitude. The association constant for outer-sphere complexing and the rate constant for ligand substitution lie in the ranges 0.91-1.14 and (4.1-5.4) x 10(8) s-1, respectively, both being almost independent of the nature of the carboxylate ions, in contrast to the case of zinc(II) carboxylates previously reported. The volume changes of the outer-sphere complexing and those of the ligand substitution are determined to be 1.7-2.4 and 4.6-6.7 cm3 mol-1, respectively; these values are very different from those of cadmium(II) thiocyanate reported earlier.