Mk. Ridley et al., POTENTIOMETRIC AND SOLUBILITY STUDIES OF ASSOCIATION QUOTIENTS OF ALUMINUM MALONATE COMPLEXATION IN NACL MEDIA TO 75-DEGREES-C, Geochimica et cosmochimica acta, 62(13), 1998, pp. 2279-2291
A potentiometric method was used to determine the formation quotients
for aluminum-malonate (Al(Ma)(y)(3-2y), Ma = CH2(CO2)(2)(2-)) complexe
s from 5 to 75 degrees 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 thei
r temperature and ionic strength dependencies. Differentiation of the
two empirical equations with respect to temperature provided thermodyn
amic quantities for the Al-malonate complexes. The thermodynamic quant
ities obtained for Al(Ma)(+) at 25 degrees C and infinite dilution are
: log K-1 = 7.49 +/- 0.18, Delta H(1)degrees = 19 +/- 5 kJ . mol(-1) D
elta S(1)degrees = 208 +/- 18 J . K-1 . mol(-1) and Delta C(p1)degrees
= 331 +/- 120 J . K-1 . mol(-1): whereas the values for Al(Ma)(2)(-)
are: log K-2 = 12.62 +/- 0.40, Delta H(2)degrees = 29 +/- 10 kJ . mol(
-1), Delta S(2)degrees = 340 +/- 36 J . K-1 . mol(-1) and Delta C(p2)d
egrees = 575 +/- 230 J . K-1 . mol(-1). These thermodynamic values ind
icate 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 5
0 degrees C potentiometric data, was performed by reacting powdered gi
bbsite (Al(OH)(3)) with malonic acid solutions at 0.1 molal ionic stre
ngth in aqueous NaCl media. The results of the solubility study are in
excellent agreement with the potentiometric data. Copyright (C) 1998
Elsevier Science Ltd.