Nuclear magnetic resonance and Raman studies of the aluminium complexes formed in aqueous solutions of aluminium salts containing phosphoric acid and fluoride ions

Abstract

²⁷ Al and ³¹P n.m.r. have been used to measure quantitatively the formation of complexes between aluminium ions and the ionic and molecular species present in phosphoric acid. The results are consistent with the presence of both monomeric and polymeric species in aqueous phosphoric acid. Five ³¹P lines were observed at low temperatures in solutions of an aluminium salt in aqueous phosphoric acid and these are interpreted as arising, respectively, from the complex Al,H₃PO₄³⁺, from exchanging Al,H₂PO₄²⁺ and Al(H₂PO₄)₂⁺, from two binuclear aluminium species, and from a series of polymeric phosphoric acid ligands Al(H₃PO₄)_n, n 2, although in this latter case the state of protonation of the ligand is not known. The protons in the system undergo fast exchange at all temperatures where the samples are liquid. Addition of fluoride ion to these solutions results in the formation of eight new fluoro-phosphato-aluminium complexes containing direct Al–F bonds, in addition to the known binary complexes AlF_n^(3–n)+. These complexes were revealed by ¹⁹F n.m.r. spectroscopy.