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

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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 L6– 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++ L6– [Cu(HL)]3– log β111= 23.529(29)(i), Cu2++ L6– [CuL]4– log β101= 16.797(58)(ii), 2Cu2++ 2H++ L6– [Cu2(H2L)] log β221= 31.878(31)(iii), Cu2++ HL5– [Cu(HL)]3– log K111 011= 11.415(35)(iv), 2Cu2++ H2L4– [Cu2(H2L)] log K221 021= 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.

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