Spectrophotometric determination of trace amounts of copper(II) using the homolysis reaction of the copper(II)–N-p-nitrobenzyl-5,10,15,20-tetrakis(4-sulphonatophenyl)porphine complex

Abstract

N-p-Nitrobenzyl-5,10,15,20-tetrakis(4-sulphonatophenyl)porphine[p-NO₂Bz(Htpps)^4–] was synthesized and used for the spectrophotometric determination of trace amounts of copper(II) based on the following reaction: p-NO₂Bz(Htpps)^4–+ Cu^II→p-NO₂Bz[Cu(tpps)]^3–+ H⁺ [graphic omitted] Cu(tpps)^4–+p-NO₂BzOH. The p-nitrobenzyl group was removed to produce the non-substituted porphyrins by cleavage of the C—N bond (homolysis reaction). The reaction caused a large change in the absorption spectra and was specific for copper(II) as this ion fitted well into the porphyrin core. Excess of zinc(II) and palladium(II) did not show the homolysis reaction. The acid dissociation constants of the porphyrin (–log K_a) were found to be 2.51 ± 0.02 and 7.70 ± 0.02 at 25 °C and an ionic strength of 0.1 mol dm^–3(NaNO₃). Optimum conditions for the determination of copper(II) were investigated and the method was applied to the determination of ppb levels of total copper(II) in a river water sample and in some interstitial sea-water samples from the mud at the bottom of the Ariake Sea. The molar absorptivity was 4.22 × 10⁵ dm³ mol^–1 cm^–1 and the Sandell sensitivity was 0.15 ng cm^–2 of copper(II).