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Journal of the Korean Chemical Society (JKCS)

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

Mechanism and Activation Parameters (ΔH≠, ΔS≠ and ΔV≠) of Electron Transfer Reaction Between CoⅡCyDTA and FeⅢCN Complex Ions

Co(Ⅱ)-CyDTA와 Fe(Ⅲ)-CN 착이온간의 전자이동반응에서 활성화파라미터 (ΔH≠, ΔS≠ 및 ΔV≠) 와 반응메카니즘
Yu Chul Park, Seong Su Kim

박유철, 김성수
Co(Ⅱ)-CyDTA 착물의 흡수스펙트럼을 pH = 6.0∼13.2의 수용액에서 측정하였다. 흡수에너지는 pH가 증가할수록 낮은 에너지로 이동하였고, 이 현상은 CoCyDTA2-와 CoCyDTA(OH)3-간의 평형상수 KOH = [CoCyDTA(OH)3-]/[CoCyDTA2-][OH-]로 설명할 수 있었고, 그 값은 40℃에서 75M-1이었다. Co(Ⅱ)-CyDTA와 Fe(Ⅲ)-CN 착이온간의 전자이동반응은 KOH 측정과 같은 용액조건에서 분광광도법을 이용하여 고찰하였다. 측정한 kobs는 pH = 10.8까지는 거의 일정하였으나 pH > 10.8에서는 pH의 증가에 따라 증가하였다. pH = 6.0∼13.0에서 적용할 수 있는 속도법칙은 kobs = (k3[CoCyDTA2-] + k4[CoCyDTA(OH)3-])/(1+K1[CoCyDTA2-])이었다. 반응 (3a)와 (3b)의 속도상수 k3와 k4는 40℃에서 각각 0.529M-1sec-1와 4.500M-1sec-1이었다. 활성화엔트로피(147±1.1J K-1 mol-1, pH = 10.8)와 활성화체적(6.25cm3mol-1, pH = 10.8)은 pH가 증가할수록 증가하였지만, 활성화엔탈피(12.44 ± 0.20 kcal/mole)는 pH의 영향을 받지 않았다. 속도상수, 활성화엔트로피, 활성화체적에 대한 pH의 영향을 각각이용하여 Co(Ⅱ)-Fe(Ⅲ)의 전자이동 반응메카니즘을 논의하였다.

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, KOH = [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.

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