A stock solution of 10−3 mol L−1 PQ was prepared by dissolving the corresponding amount of paraquat chloride in bidistilled water. A working solution of 10−4 mol L−1 PQ was prepared daily from the stock solution, by dilution in bidistilled water. pH was adjusted by adding H2SO4 or NaOH. All chemicals used in these experiments were extremely pure (used without further purification). They were obtained from Merck, Fluka, and Riedel de Haen Chemical Companies.