Spectrophotometric determination of trace amounts of copper(I) and reducing agents with neocuproine in the presence of copper(II)

Abstract

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.

Comparative Evaluation of Various Total Antioxidant Capacity Assays Applied to Phenolic Compounds with the CUPRAC Assay

“…, because the standard redox potential of the Cu(II/I)-neocuproine was 0.6 V, much higher that that of the Cu 2+ /Cu + couple (0.17 V) [118]. As a result, polyphenols were oxidized much more rapidly and efficiently with Cu(II)-Nc than with Cu…”

“…OH radicals as a result of its Fenton-type reaction with H 2 O 2 [135]. The stable Cu(I)-chelate was previously shown by us not to react with hydrogen peroxide, but the reverse reaction, i.e., oxidation of H 2 O 2 with Cu(II)-Nc, is possible [118]. Since a cascade of ROS-generating reactions oxidizing lipids is not possible with CUPRAC, there is no negative error of antioxidant determination due to possible prooxidant effect of Cu(I).…”

Hydroxyl radical scavenging assay of phenolics and flavonoids with a modified cupric reducing antioxidant capacity (CUPRAC) method using catalase for hydrogen peroxide degradation

“…This reagent has a standard reduction potential of about 0.6 V [36] which means that reducing agents having redox potentials less than this value would reduce the CUPRAC reagent and generate the cuprous neocuproine chromophore, interfering with the proposed assay. However, by taking sufficiently low amounts of analyte samples for analysis, the proposed method can be effectively used to assay the hydroxyl radical scavenging activity of plant extracts without an interference from the contained antioxidants.…”

The reduction of Cu(II)/neocuproine complexes by some polyphenols: Total polyphenols determination in wine samples

“…The reduction of Cu(II) in the presence of neocuproine, 2,9-dimethyl-1,10-phenanthroline (NC), by a reducing agent, like hydroxylamine hydrochloride, yields a Cu(I) complex with maximum absorption peak at 450 nm (e = 7.5 Â 10 3 L cm À1 mol À1 ) (Tütem, Apak, & Baykut, 1991). Table 1 shows several analytes (reducing agents and copper ion) determined based on the Cu(I)/neocuproine complexes formation.…”