ALUMINUM(III)-PYROCATECHOL VIOLET EQUILIBRIA - A POTENTIOMETRIC STUDY

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, H(4)L} have been studied by potentiometric titration in aqueous solution, / = 0.10 mol dm(-3) K(Cl), 25.0 degrees C. Equilibrium constants log beta(pqr) for the reactions pH(+) + qAl(3+) + rH(3)L(-) reversible arrow Hp+Alq3+(H(3)L(-))(r)(p+3q-r) were determined, as were the dye deprotonation constants, log beta(-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(H(2)L)(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)(n)L(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+.