was prepared by adding 10 mmol Bi2O3 to 20 ml hydrochloric acid (0.12 mol) and then diluted to 100 ml with distilled water. A stock solution of 3000 mg L1 KH2PO4, 200 mg L1 sodium arsenite and 200 mg L1 potassium dichromate were prepared in a volumetric flask and diluted to the required concentrations (60e1800 mg L1). Bi2O3, KH2PO4, NaAsO2, K2Cr2O7 and other reagents used in this work were all at analytical grade