All reagents used were of analytical grade unless otherwise stated. Ultra pure water was utilized throughout. 5-Br-PADAP and ascorbic acid were purchased from Shanghai Zhongqin Chemical. Stock standard solution of Fe3+ (1000 mg L1 in 1.0 mol L1 HNO3) was purchased from National Non-ferrous Metal & Electronic Material Analytical and Testing Center Working solutions of Fe3+ were prepared by appropriate stepwise dilution of the stock standard