Abstract: An association complex was formed between the anionic complex HgI2-4 and methylene blue MB+ in a slightly acidic solution of pH 6.5,and floated by shaking with benzene.The association complex HgI2-4·2MB+ was separated out in a separatory funnel as the 3rd phase between the water phase and benzene phase,and after discarding the water and organic phases,it was dissolved in a definite volume of acetone,and its absorbance was measured at the absorption maximum of 657 nm.Beer′s law was kept in the range of 12.0-22.0 mg of Hg(Ⅱ) in 25 mL of test solution.Molar absorptivity (ε657) was found to be 3.5×105L·mol-1·cm-1.Detection limit of the method was 5.1×10-7mol·L-1.Precision was tested at the concentration level of 12.0 μg Hg(Ⅱ) per 25 mL solution for 11 determinations,giving value of RSD′s of 3.98%.Test for recovery was performed by addition of standard Hg(Ⅱ) solutions to 3 simulated water samples,and values of recovery obtained were in the range of 94.8% to 100.1%.