Catalytic spectrofluorimetric determination of copper using aerial oxidation of ascorbic acid in the presence of o-phenylenediamine

Abstract

Copper-catalysed aerial oxidation of L-ascorbic acid to dehydroascorbic acid (DAA) was sensitively monitored by fluorimetric detection (λ_ex= 350 nm; λ_em= 425 nm) of quinoxaline derivatives (QX) formed by the successive reaction of DAA with o-phenylenediamine. The fluorescence intensity (I) was proportional to the square of the reaction time (t) in the initial reaction at pH 6.9 and 25 °C. Using the slope of the I versus t² graph, a calibration graph for copper was constructed with a linear range from 0 to 8 µg l^–1. The detection limit was 0.06 µg l^–1. The interference from Hg^II, Cr^VI, Sn^IV and V^V was tolerable at 3–10 µg l^–1 and that from Fe^II and Fe^III at 50 µg l^–1 in the determination of 4 µg l^–1 of copper. The interference from Fe^III up to 500 µg l^–1 was eliminated by addition of NaNO₂. The proposed method was applied to the determination of copper in river and rain waters.