Abstract: The effects of halide and mercapto-compound on the redox reaction between silver and Fe (Ⅲ) EDTA were studied with silver rotating disk electrode under constant current.It was found when halide was added in the oxidising solution,the reaction rate in the presence of iodide is much faster than that in the presence of bromide.If there was only chloride in Fe (Ⅲ) EDTA oxidising solution,the reaction can hardly proceed.It was shown that the rate of the redox reaction decreases with increasing pH.Some mercapto-compounds accelerate the reaction.This may be due to the formation of complex between mercaptocompound and silver halide.The mechanism of the accelerating effect of the mercapto-compound was discussed.In the presence of certain amount of bromide the reaction rate increases with increasing Fe (Ⅲ) EDTA concentration.When the concentration of Fe (Ⅲ) EDTA exceeds 0.1 mol,the relation between reaction rate and the concentration of Fe (Ⅲ) EDTA is no longer linear.

Key words: Galvanostat, bleaching mechanism, mercapto-compocnl