At first, an appropriate volume Cu2+ solution or sample solution were added to a 2.0 mL eppendorf tube, then, it was mixed with 100.0 µL Na2S2O3 (1.0 × 10-4 M) and 100.0 µL Britton–Robinson buffer solutions (pH=6.8), finally, it was diluted by the appropriate volume of water to keep the total volume of 2.0 mL.