Potentiometric and solubility studies of association quotients of aluminum malonate complexation in NaCl media to 75°C

Abstract

A potentiometric method was used to determine the formation quotients for aluminum-malonate (Al(Ma) y 3−2y, Ma ≡ CH 2(CO 2) 2 2−) complexes from 5 to 75°C at four ionic strengths from 0.1 to 1.0 molal in aqueous NaCl media. Two mononuclear aluminum-malonate species, Al(Ma) + and Al(Ma) 2 −, were identified, and the formation quotients for these species were modeled by empirical equations to describe their temperature and ionic strength dependencies. Differentiation of the two empirical equations with respect to temperature provided thermodynamic quantities for the Al-malonate complexes. The thermodynamic quantities obtained for Al(Ma) + at 25°C and infinite dilution are: log K 1 = 7.49 ± 0.18, ΔH ° 1 = 19 ± 5 kJ · mol −1, ΔS ° 1 = 208 ± 18 J · K −1 · mol −1 and ΔC ° p1 = 331 ± 120 J · K −1 · mol −1; whereas the values for Al(Ma) 2 − are: log K 2 = 12.62 ± 0.40, ΔH ° 2 = 29 ± 10 kJ · mol −1, ΔS ° 2 = 340 ± 36 J · K −1 mol −1 and ΔC ° p2 = 575 ± 230 J · K −1 mol −1. These thermodynamic values indicate that Al(Ma) +, a chelate complex, is much more stable than the equivalent monodentate Al-diacetate complex (Al(Ac) 2 +) (Palmer and Bell, 1994). A solubility study, which was undertaken to verify the 50°C potentiometric data, was performed by reacting powdered gibbsite (Al(OH) 3) with malonic acid solutions at 0.1 molal ionic strength in aqueous NaCl media. The results of the solubility study are in excellent agreement with the potentiometric data.