Phosphorus-31 nuclear magnetic resonance study of aluminium(III) ortho-phosphate complexes in aqueous solution

Previously, a model for aluminium(III) phosphate complexation in 3 M (Na)ClO4 aqueous solution at 25 degrees C has been proposed, on the basis of potentiometric titration data, in which there are eleven phosphorus-containing species. Support for this model is provided by the assignment to these species of eight peaks in P-31 NMR spectra obtained, under the same conditions of temperature and ionic strength, from a set of Al(ClO4)(3)-H3PO4 NaH2PO4 solutions. The assignment was made by correlating fractional NMR peak areas with fractional complex concentrations calculated using the potentiometric data model. The chemical shift in the range 0.3-0.91 ppm is assigned to H3PO4, H3PO4- H-6(PO4)(2)(-), H-5(PO4)(2)(-), and in part to Al3H-8(H3PO4)(5)(+) and Al3H-6(H(3)pO(4))(4)(3+), in fast proton exchange. The peaks at -6.8, -7.4 and -12.4 ppm are.assigned to isomers of Al(H2PO4)(2)(+). The signal at -7.4 is also due to AlH2PO42+. Polynuclear complexes have been assigned as follows. Al3H-5(H3PO4)(2)(4+) -11.5, Al3H-6(H3PO4)(4)(3+) -11.7, Al3H-8,(H3PO4)(5)(+) -12.3 and Al3H-6(H3PO4)(4)(3+) -16.8 ppm. A further four peaks were observed but could not be assigned. All chemical shifts are relative to 85% phosphoric acid as external reference.