Sequestering Ability of Oligophosphate Ligands toward Al3+in Aqueous Solution

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-1is also reported. The sequestering ability of all the ligands under study toward Al3+was also evaluated by the empirical parameter pL0.5.