Spectrophotometric study of the reactions of proton and copper (II) with arsenazo I in aqueous solution

Abstract

The systems of proton and/or copper(II) ion with Arsenazo I (H₆L) have been characterized at 25 °C and I= 1.0 mol dm^–3(KCl) using u.v.–visible spectrophotometry. The cumulative mixed protonation constants of the ligand L^6– are log β₀₁₁= 12.114(19), log β₀₂₁= 21.871(18), log β₀₃₁= 29.506(16), log β₀₄₁= 32.155(6), and log β₀₅₁= 34.298(6). Complexing reactions and equilibrium constants of the copper(II)–Arsenazo I system are given by (i)–(v) below. Both mixed and mononuclear Cu²⁺+ H⁺+ L^6–⇌[Cu(HL)]^3– log β₁₁₁= 23.529(29)(i), Cu²⁺+ L^6–⇌[CuL]^4– log β₁₀₁= 16.797(58)(ii), 2Cu²⁺+ 2H⁺+ L^6–⇌[Cu₂(H₂L)] log β₂₂₁= 31.878(31)(iii), Cu²⁺+ HL^5–⇌[Cu(HL)]^3– log K₁₁₁⁰¹¹= 11.415(35)(iv), 2Cu²⁺+ H₂L^4–⇌[Cu₂(H₂L)] log K₂₂₁⁰²¹= 10.007(36)(v) complexes have been found, the former includes a diprotonated dinuclear species in solutions at higher metal-to-ligand ratios and hydrogen ion concentrations, the latter a monoprotonated mononuclear species prevailing at lower metal-to-ligand ratios and at higher pH values. Data have been analysed by using two computer programs: the least-squares program LETAGROPVRID, version SPEFO, and the program SQUAD. The experimental spectra are compared with the calculated ones and the constants are discussed in connection with those of similar compounds reported in the literature. Accurate molar absorption coefficients are computed and the resolved spectral bands are discussed and assigned. Tentative structures of the proposed complexes are given.