Studies of equilibrium, structure, and dynamics in the aqueous Al(iii)-oxalate-fluoride system by potentiometry, 13C and 19F NMR spectroscopy

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, (delta)H# = 67 ± 5 kJ mol-1, (delta)S# = -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, (delta)H# = 164 ± 17 kJ mol-1, (delta)S# = 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 (log(beta)) 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.