Association quotients of aluminum sulphate complexes in NaCl media from 50to 125 degrees C: Results of a potentiometric and solubility study

The speciation and metal formation quotients for the complexation of alumin um with sulphate were measured based on potentiometric and solubility exper iments. Potentiometric titrations, utilizing a hydrogen-electrode concentra tion cell, were performed from 50 to 125 degrees C at ionic strengths of 0. 1, 0.3 and 1.0 molal in aqueous NaCl media. Two aluminum-sulphate species, AlSO4+ and Al(SO4)(2), were identified from the titration data and the form ation quotients for these species were modeled by empirical equations to de scribe their temperature and ionic strength dependencies. Thermodynamic par ameters for the complexation reactions were obtained by differentiating the empirical equations with respect to temperature. The thermodynamic quantit ies obtained for the formation of AlSO4+ at 50 degrees C and infinite dilut ion are: logK(1) = 3.7 +/- 0.4, Delta H(1)degrees = -10 +/- 30 kJ . mol(-1) , Delta S(1)degrees = 40 +/- 100 J . K-1. mol(-1) and Delta C degrees(p1) = 1900 +/- 800 J K-1. mol-1; whereas the values for Al(SO4)(2)(-) are: logK( 2) = 5.6 +/- 0.7, Delta H(2)degrees = 10 +/- 50 kJ . mol(-1), Delta S(2)deg rees = 100 +/- 100 J . K-1. mol(-1) and Delta C degrees(p2) = 2800 +/- 800 J . K-1. mol(-1). A solubility study, which was undertaken to verify the 50 degrees C polentiometric data, was performed by reacting powdered gibbsite (Al(OH)(3)) with sulphate solutions at 10(-3.5) and 10(-)4 molal H+, total sulphate concentrations from 0.005 to 0.080 molal, and 0.1 and 1.0 molal i onic strength in aqueous NaCl media. The results of the solubility study ar e in good agreement with the potentiometric data and establish that Al-sulp hate complexation substantially enhances the equilibrium solubility of gibb site. Copyright (C) 1999 Elsevier Science Ltd.