The existing spectrophotometric method for the determination of total Cu with neocuproine (Nc) does not allow the differentiation of Cul and Cull. It is shown here that the use of a dilute (3.0 x 10-3 rnol dm-3) Nc solution in weakly acidic or neutral media makes the determination of Cul feasible at the 1 x 10-5 rnol dm-3 level in the presence of up to 0.1 rnol dm-3 Cu". For the determination of trace amounts of Cul in the presence of 1 rnol dm-3 Cull, the latter can be masked by NH3-NH4CI buffer a t pH 10, giving almost the same molar absorptivity for Cul, i.e., 7.5 x l o 3 dm3 mol-1 cm-1. The ability to measure the absorption due to Cul in the presence of an excess of Cull was exploited as the basis of an indirect method for the determination of trace amounts of reductants. Superior to the existing individual methods of determination, which are usually specific for a given reducing agent, the proposed system consists of treating the reductant with the Cull-Nc reagent in ammonium acetate buffered media, followed by measurement of the absorbance of the Cul-Nc chelate at 450 nm. The quantification of a given reductant in the concentration range 1 x 10-6-1 x 10-4 rnol dm-3 usually takes 3 min with a mean relative standard deviation of 3%. The molar absorptivity of an n-electron reductant which has reacted stoichiometrically is approximately 7.5n x lo3 dm3 mol-1 cm-1, i.e., n times that of Cul-Nc. Hence, hydrogen peroxide, ascorbic acid, cysteine, hydroxylamine, hydrazine, thiosulphate, dithionite, mitoxantrone, glutathione, iron(i1) and thiourea were determined with theoretical molar absorptivities; carminic acid, sulphite, tin(ii) chloride, 2,4-dinitrophenylhydrazine, sodium tetrahydroborate(iii) and 2,3-dimercaptopropan-1-ol were determined with the aid of empirical linear absorbanceconcentration plots. The proposed spectrophotometric method is rapid and allows the quantification of biologically important reducing agents.
