Determination of labile Cu2+ in fresh waters by chemiluminescence: interference by iron and other cations

Analysis of labile Cu2+ in fresh waters using the Cu2+-catalysed oxidation of 1,10-phenanthroline by superoxide anion radical has been investigated. It was found that certain metal ions, notably Fe3+, Fe2+ and Pb2+ enhance the chemiluminescence (CL) intensity of this reaction by up to an order of magnitude when present in the μM concentration range. This enhancement is considered to arise through coordination of the metal ion to 1,10-phenanthroline, which prevents free rotation of the benzene rings in the excited state intermediate thought to be responsible for light emission. This introduces a potential interference when analyzing fresh waters, which usually contain Fe3+ concentrations of this magnitude. However, the enhancement effect saturates at about 4 μM Fe3+, so that reliable results can be obtained if the water sample is supplemented with Fe3+ to reach this level. Application of the enhanced technique to a river water, and a reservoir to which CuSO4 had been added to control algal growth, are described. In both cases, only a small fraction of total dissolved Cu2+ is labile with respect to the chemiluminesence method.