Dual-wavelength Spectrophotometric Determination of Zinc Content in Salt with Zn~(2+)-phen-Congo Red System

In the presence of β-cyclodextrin used as sensitizer,Zn2+ reacted with phen and congo red(CR) to form a stable ion-association complex [Zn(phen)3]CR in NH4Ac buffer solution at pH 7.0,which led to the fading of congo red.As a result,there was a positive absorption peak at 574 nm and a negative absorption peak at 502 nm.A new method for determination of micro zinc by double-wavelength spectrophotometry using 574 nm as the detection wavelength and 502 nm as the reference wavelength was developed.The apparent molar absorption coefficient was 9.8×104 L/mol/cm.The linear range of the determination was 0~0.32 μg/mL,and the limit of detection for zinc was 5.7 μg/L.The method has been applied to the determination of zinc in zinc-fortified nutritious salt and health balance salt.RSDs of 5 determinations was 0.70%~0.83%.Recoveries were 99.37%~100.04%.