STUDIES OF EQUILIBRIUM, STRUCTURE, AND DYNAMICS IN THE AQUEOUS AL(III)-OXALATE-FLUORIDE SYSTEM BY POTENTIOMETRY, 13C AND 19F NMR SPECTROSCOPY

Abstract

The AlOx1-3 (Ox = oxalate) species were identified in 0.6 M aqueous NaCl by 13C nuclear magnetic resonance (NMR). Rate constants and activation parameters for intramolecular cis/trans isomerization of the Werner-type AlOx2- complex (k(298 K) = 5 s-1, ΔH# = 67 +/- 5 kJ mol-1, ΔS# = -6 +/- 6 J mol-1 K-1, the rate determining step could be the breaking of the Al-O(C=O) bond) and a very slow intermolecular ligand exchange reaction of AlOx33- complex and the free ligand (k30(298 K) = 6.6 . 10-5 s-1, ΔH# = 164 +/- 17 kJ mol-1, ΔS# = 225 +/- 51 J mol-1 K-1, D/Id mechanism) were determined by dynamic 1D and 2D 13C NMR measurements. Mixed complexes, AlFOx, AlFOx22-, AlF2Ox-, and AlF2Ox23-, with overall stability (logβ) of 11.53 +/- 0.03, 15.67 +/- 0.03, 15.74 +/- 0.02, and 19.10 +/- 0.04 were measured by potentiometry using pH- and fluoride-selective electrodes and confirmed by 13C and19F NMR. The role of these complexes in gibbsite dissolution was modeled. The mixed Al(III)-Ox2--F- complexes have to be considered as the chemical speciation of Al(III) in natural waters is discussed.