Calibrations for As3+ and As5+ were performed from standard solution of both As species prepared by analytical grade of As(NO3)3 and As2O5, obtained from (Merck). Appropriate dilutions were made from these solutions with 1.0 M HCl. The chemical modifiers, stock standard solution (2000 mg/l) was prepared from Mg(NO3)2 (Merck), and Pd stock standard solution, 3000 mg/l was prepared from Pd 99.999% Aldrich (Milwaukee, WI, USA).