FORMATION OF THE AL13 TRIDECAMERIC POLYCATION UNDER DIVERSE SYNTHESIS CONDITIONS

Recent studies using Al-27-NMR spectroscopy indicate that the polynuclear hydroxo-Al fraction that exhibits a moderate reaction rate with the ferron reagent can be equated to the Al13 tridecamer, and we utilized this correspondence to examine the formation of Al13 over a broad range of synthesis conditions, including some pertinent to natural waters. Ferron and NMR analyses revealed that solutions ([Al]T = 2 x 10(-5)-5 x 10(-3) mol L-1) partially neutralized with aqueous NaHCO3 contained from 26 to 87% of Al(T) as the tridecamer. Solutions neutralized with solid CaCO3 or MgO, or with NH3 vapor, also contained substantial quantities (14-46% of Al(T)) of Al13. Bi-carbonate-neutralized solutions were also prepared by injecting base into an acidic Al solution (forward method), by injecting the Al solution into the base (reverse method), and by simultaneous injection of the two solutions. The latter two methods resulted in less Al13 formation than the forward method, and our results thus suggest that Al13 can, but does not always, form in a variety of settings characterized by inhomogeneous conditions at the interface between solutions of differing pH and alkalinity/acidity. Scrutiny of these and previously published results revealed that the "yield" of the tridecamer diminishes with decreasing Al(T), but that it can still be a significant component of solutions with Al(T) concentrations as low as 10(-5) mol L-1.