Sequestering Ability of Oligophosphate Ligands toward Al3+ in Aqueous Solution

Abstract

The interactions between Al3+ and ortho-phosphate (PO43–), pyro-phosphate (P2O74–, PP), tripolyphosphate (P3O105–, TPP) and hexa-metaphosphate (P6O186–, HMP) in aqueous solution have been studied. Formation constants, speciation models, and possible structures for the complexes formed are discussed on the basis of potentiometric, calorimetric, 31P–{1H} NMR, laser desorption mass spectrometric (LD MS) and MS/MS results. 31P–{1H} NMR experiments for Al3+–TPP system are in agreement with the speciation model drawn from analysis of potentiometric titration data, suggesting that TPP binds to Al3+ with two adjacent phosphate oxygen atoms, forming a six-membered ring. LD MS has provided identification and structural information for Al3+–PP, −TPP, and −HMP complexes which supports the reliability of speciation models proposed on the basis of the potentiometric data. Moreover, MS/MS spectra obtained from [ML]+ precursor ion show that the ion species involve linear HMP isomers and that two adjacent terminal phosphate groups probably bind Al3+. Standard enthalpy change values were obtained by titration calorimetry; all are endothermic, as is typical for hard–hard interactions. The dependence of formation constants on ionic strength over the range I = 0.1–1 mol kg–1 is also reported. The sequestering ability of all the ligands under study toward Al3+ was also evaluated by the empirical parameter pL0.5.