Reductions of Pentacyanoferrate(III) Complexes by Ascorbic Acid

• Main Authors: LuMing Lin, 林律銘
• Other Authors: Andrew Yeh
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/38557564560232789838

碩士 === 東海大學 === 化學系 === 90 === The reductions of ascorbic acid on Fe(CN)5L2-(L=CN-, 4,4'-bpy) complexes undergo outer-sphere electron transfer mechanism. The rates of the reduction depend greatly on the pH of the solution for the reason that ascorbic acid exists in different forms, namely H2A，HA- and A2-, at various pH values. The kinetic studies of the reactions at [H+]= 0.01~0.10 M and pH=4~9 yield specific rate constants of reduction as k0(H2A)=0.42 ±0.02 (L=CN-), 2.5 ±0.2 (L=4,4'-bpy), k1 (HA-)= (5.2 ±0.1)´102 (L=CN-), (9.6 ±0.1)´103 (L=4,4'-bpy) ; k2 (A2-)= (2.03 ±0.07)´107 (L=CN-) M-1s-1，respectively at m=0.50 M HClO4/LiClO4 and T=25℃.Our kinetic results conform with Marcus theory for outer-sphere electron transfer. Moreover, the detailed theoretical analysis indicates that this tremendously large rate difference with k0:k1:k2~1:103:108 (L=CN-) arises predominantly from the equilibrium barrier and the intrinsic reactivity of the different acid forms of the ascorbic acid, and is rather insensitive to the charge effect of the reactants.