ISSN 1017-2548(Print)
ISSN 2234-8530(Online)
Volume 33, Number 3
JKCSEZ 33(3)
June 20, 1989 

The spectra of the Co(Ⅱ)-CyDTA(CyDTA: cyclohexyldiaminetetraacetic acid) complex have been measured in aqueous solution of pH = 6-13.2. The red shift of the spectrum in the more basic solution was ascribed to the transformation of CoCyDTA2- into CoCyDTA(OH)3-. The equilibrium constant, K_OH = [CoCyDTA(OH)3-]/[CoCyDTA2-][OH-] was 75M-1 at 40℃. The electron transfer reactions of CoCyDTA2- and CoCyDTA(OH)3- with Fe(CN)63- have been studied using spectrophotometric technique in the range of pH applied to the determination of equilibrium constant. The pseudo first-order rate constants observed (kobs) were not changed upto pH = 10.8, but increased with increasing pH in the range of pH = 10.8∼13.0. The rate law reduced in the range of pH = 6-13 was kobs = (k3[CoCyDTA2-] + k4[CoCyDTA(OH)3-])/(1+K1[CoCyDTA2-]). The rate constants of the reactions (3a) and (3b), k3 and k4 respectively have been determined to be 0.529 and 4.500 M-1 sec-1 at 40℃. The activation entropies (147±1.1J K-1 mol-1 at pH = 10.8) and activation volumes (6.25cm3mol-1 at pH = 10.8) increased with increasing pH, while the activation enthalpy (12.44 ± 0.20 kcal/mole) was independent of pH. Using the pH effect on the rate constants, the activation entropies and the activation volumes, the mechanism of the electron transfer reaction for Co(Ⅱ)-Fe(Ⅲ) system was discussed.