ULTRASONIC-ABSORPTION STUDY OF THE COMPLEX-FORMATION OF ZINC(II) CHLOROACETATE AND N-BUTYRATE IN AQUEOUS-SOLUTION

Ultrasonic absorption of aqueous zinc(II) chloroacetate and n-butyrate has been measured with a pulse technique over the frequency range 3-2 55 MHz. The single relaxations observed in the metal-rich concentratio n regions are attributed to the complex formation of zinc(II) carboxyl ates via the Eigen-Tamm mechanism of stepwise association. The reactio n parameters are determined from the concentration dependences of the relaxation frequency and amplitude. The association constant of outer- sphere complexing and the rate constant of ligand substitution are det ermined to be 0.94 and 11.4 x 10(7) s-1 for chloroacetate and 1.8 and 6.4 x 10(7) s-1 for n-butyrate. The volume changes of the outer-sphere complexing and those of the ligand substitution are -0.2 and 5.7 cm3 mol-1 for chloroacetate and 0.2 and 10.8 cm3 mol-1 for n-butyrate. The above reaction parameters of chloroacetate and n-butyrate are very si milar to those of formate and propionate, respectively, reported earli er. The ligand dependences of the reaction parameters are likely to be correlated with the pK(H) value rather than the size of the ligand.