Determination of Traces of Zn~(2+) in Zinc Electroplating Wastewater by Double-Wavelength Spectrophotometry

In the presence of arabic gum,Zn2-reacts with potassium thiocyanate and neutral red(NR) to form a stable ion-association complex [NR]4 [Zn(SCN)6] in the buffer solution of NH4Ac-HAc with a pH value of 5.5,leading to the fading of NR in association with emerge of a positive absorption peak at 462 nm and a negative absorption peak at 575 nm.Thus a new double-wavelength spectrophotometric method was established to determine traces of Zn2+ in the wastewater of zinc electroplating by setting detection wavelength as 462 nm and 575 nm.It was found that Beer's law was obeyed in a range of 0～1.0 μg/mL for Zn2+ concentration,and the apparent molar absorptivity was 5.72×104 L/(mol·cm).When the method was adopted to determine traces of Zn2+ in zinc electroplating wastewater,results were in good agreement with those obtained by 5-Br-DMPAP spetrophotometric method,with a relative standard deviation of less than 4 % and a recovery of 98.7 %～101.8 %.