Effects of citrate concentration and pH on the electrochemical reduction process of Co（Ⅱ） were investigated by cyclic voltammetry（CV） and electrochemical impedance spectroscopy（EIS）. The results show that Co（Ⅱ） is reduced into two species which are free Co2＋ and [Co（C6H607）] in the solution composed of 0.05 mol/L CoS04·5H2O, 0.20 mol/L Na2SO4 and 0-0.40 mol/L C6H5O7Na3·2H2O in the pH range of 3-9. The reduction behavior depends on the pH of the solution. Co（H） is mainly reduced into the form of free Co^2＋ at pH 3 and into the form of [Co（C6H6O7）] at the pH range of 4-6 in citrate solution. The [Co（C6H6O7）] is first reduced to an intermediate state and then to Co°. Adsorption of the intermediate state exists on the surface of the electrode. Co（Ⅱ） is difficult to be reduced in the solution with the pH above 7, because the existing Co（Ⅱ）-citrate complex species [Co（C6H5O7）]- and [Co（C6H4O7）]2- are more difficult to be reduced than the hydrogen ion.
