Cu2+ stock solution (1.0×10−3 mol L−1) was prepared by dissolving pure CuSO4⋅5H2O (obtained from Hunan reagent factory) with redistilled water, and then it could be diluted to different concentration with water. The Britton–Robinson buffer solutions were used to adjust the pH value
of the reaction system. Na2S2O3 stock solution with the different concentrations were prepared freshly by dissolving and diluting pure Na2S2O3⋅5H2O (obtained from Changsha the sub-intersection reagent chemical plant) with redistilled water. All other chemicals were analytical reagents and the water was redistilled water.