Speciation Analysis for Trace Copper in Natural and Tap Water by Catalytic Method

Abstract

A catalytic spectrophotometric method for the determination of reactive copper in fresh water has been developed using the copper-catalyzed aerial oxidation of L-ascorbic acid to dehydroascorbic acid (DAA) and the formation of quinoxaline derivatives (QX) by the reaction of DAA with o-phenylenediamine at pH 6.9 and 25°C. The absorbance of QX was measured at 340 nm for a reaction time of 5 min. The calibration curve was linear up to 7 μg L⁻¹ of copper. The detection limits were 0.1 μg L⁻¹ for non-acidified samples and 0.5 μg L⁻¹ for acidified samples. Reactive copper species were characterized as Cu²⁺ and Cu^IIL_i, where i = 1 or 2 for a unidendate and i = 1 for a bidentate ligand (L). The proposed method was useful to estimate reactive copper species, e.g., 40 - 65% as Cu²⁺ and CuOH⁺ in tap water and 10 or 20% as copper complexed with humic substances like fulvic acid in river water. Unreactive copper species were also characterized.