Abstract: The kinetics of aerobic oxidation of ascorbic acid(H_2A) catalyzed by Cu(Ⅱ) and its polyamine chelates, of which chelate number vary from 2 to 5, was stu- died,The chelating agents used were ethylenediamine(en), diethylenetriamine(dien), triethylenetetramine(trien) and tetraethylenepentamine(tetren). The pH value was maintained constant by an automatic titrator without using buffer.
The results showed that Cu(Ⅱ) could form a complex with ascorbate, however they did not react directly. Cu(Ⅱ) and Cu(Ⅱ)(en) did not react directly. Cu(Ⅱ) and Cu(Ⅱ)(en) did not change their valence during the catalytic processes. The kinetic determination showed that Cu(Ⅱ)(dien), Cu(Ⅱ)(trien), Cu(Ⅱ)(tetren) and Cu(Ⅱ)(en)_2, of which chelate number are larger than 2 were catalytically inactive. For Cu(Ⅱ)(en) catalyzed reaction, the rate was first order with respect to Cu(Ⅱ)(en) and conformed to the Michaelis-Menten equation with respect to ascorbate. For free Cu(Ⅱ) catalyzed reaction, the rate was first order with respect to both Cu(Ⅱ) and ascorbate.
A mechanism via a ternary complex, Cu(Ⅱ)HAO_2, was proposed. It was suggested that Cu(Ⅱ) and Cu(Ⅱ)(en) undergo the same initial step in the catalytic processes.