Mercury stock standard solution (1000 mg L1) was prepared by dissolving 0.0677 g of HgCl2 in the acid mixture HNO3–H2SO4–HClO3 (1:5:1) in a 50 mL digestion flask with heating on a hot plate at a temperature 200 ± 5 C until the solution became clear. The solution was then diluted to 50 mL with water. Mercury standard working solutions were freshly prepared by diluting an appropriate aliquot of the stock solution. Stannous chloride solution (10% v/v) was prepared by dissolving the salt in 1 M HCl.The solution was aerated with nitrogen gas at 50 mL min1 for 30 min to expel any elemental mercury from it. Five molar (5 M) NaOH was prepared by dissolving 20 g NaOH in 100 mL double distilled water. KMnO4 (0.5%) in 0.5 MH2SO4 was prepared by dissolving KMnO4 in 0.5 M H2SO4. Cadmium and lead stock standard solutions of 1000 mg L1 each were prepared from commercial concentrated standard solutions for atomic absorption spectrometry
(E. Merck). Cadmium and lead standard solutions were prepared by diluting the stock solutions for the calibration of the Atomic Absorption Spectrophotometer.