All chemicals were of analytical grade and double distilled
water was used throughout. A stock solution of Ag(I)
(1000 mg mL1) was prepared by dissolving of 1.5748 g pure
silver nitrate (Merck, Darmstadt, Germany) in water and diluting
to 1000 mL in a volumetric flask. Working standard solutions
were obtained daily by successive dilutions of this stock solution.
Buffer solution at pH 5.5 was prepared from 0.2 mol L1 sodium
acetate (Merck) and 0.2 mol L1 of acetic acid (Merck). A solution
of 2-mercaptobenzimidazole (MBI), 0.026 mol L1, was prepared
by dissolving of 12 mg of MBI (Merck) in 3 mL of acetone
containing 200 mL of carbon tetrachloride.