Aluminium(III)–pyrocatechol violet equilibria: a potentiometric study

Abstract

The equilibrium reactions between aluminium(III) and the dye pyrocatechol violet {2-[(3,4-dihydroxyphenyl)(3-hydroxy-4-oxocyclohexa-2,5-dien-1-ylidene)methyl]benzenesulfonic acid, H₄L} have been studied by potentiometric titration in aqueous solution, I= 0.10 mol dm^–3 K(Cl), 25.0 °C. Equilibrium constants long β_pqr for the reactions pH⁺+qAl³⁺+rH₃L^–⇌ H⁺_pAl³⁺_q(H₃L^–)_r^p+3q–r were determined, as were the dye deprotonation constants, log β_–n,0,1(n= 1–3). The dye displayed both quinonoid and 1,2-dihydroxyaryl binding. The data were interpreted in terms of a series of complexes [Al(H₂L)_n]^{(3 – 2n)+}(quinonoid binding) forming at –log h < 4.5, where h is the free hydrogen-ion concentration, whereas at high –log h values the 1,2-dihydroxyaryl complexes [Al(HL)_nL_{3 –n}]^(n–9)+(n= 1–3) form. Mixed-site binding occurred at intermediate –log h values. Calculations confirmed that –log h= 5.6–6.5 is optimum for spectrophotometric measurements, with a 1 : 2 complex dominating in this range. Modelling calculations indicated that the presence of gibbsite colloids in natural waters could lead to a significant error in the determination of Al³⁺.