Fading spectrophotometric determination of trace zinc with zinc(II)-ammonium thiocyanate-methylene blue association system

A new fading spectrophotometric method was developed for the determination of trace zinc. In the presence of β-cyclodextrin and polyvinyl alcohol, Zn2+ reacted with ammonium thiocyanate and methylene blue (MB) to form a stable ion-association complex [MB]4[Zn(SCN)6] in NaAc-HAc buffer solution at pH 4.5, which led to the fading of MB. The maximum fading wavelength located at 668 nm. The fading degree of the system was linear with the concentration of Zn2+ in the range of 2.77-160 μg/L. The apparent molar absorption coefficient was 1.85×105 L·mol-1·cm-1. The method has been applied to the determination of zinc in seawater and tap water after enrichment and separation with thiohydroxy cotton fiber, with the RSDs of 1.0 % and 0.8 % and the recoveries of 98 % and 99 %, respectively.