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    ABSTRACT: Aluminum-27 (27Al) nuclear magnetic resonance (NMR) spectroscopy was used to characterize Al(III)-hydroxo complexes, in aqueous solutions with total Al(III) concentrations of 1.0–10 μM, using a custom-built NMR probe, coil, and sample bottle with low background aluminum impurities. Published 27Al NMR spectroscopy studies have traditionally used total Al(III) concentrations that are generally outside the range of geochemical interest (total [ Al(III) l ≥ 1000 μM). In this study, lower Al(III) concentrations (≤ 10 μM) were used to more closely approximate natural conditions, while allowing the measurement of mononuclear Al(III) species by 27Al NMR spectroscopy.The sensitivity of the 27Al NMR spectroscopy system, as measured by the signal-to-noise ratio (), is for 1.0 μM Al(III) at pH 2.00 and for 10 μM Al(III) at pH 5.20. This level of sensitivity is within the range of geochemically relevant Al(III) concentrations found in slightly acidic natural waters.Quantitative models are developed which link observations of NMR chemical shifts and linewidth ratios to the calculated equilibrium speciation of mononuclear Al(III) for 10 μM Al(III) solutions at pH values 2.00 to 5.20 (prepared by titrating acidic AI(III) solutions with pyridine). Linear-regression best fits of the models to the NMR data are used to determine the intrinsic chemical shifts and linewidths of individual mononuclear Al(III) species. The intrinsic chemical shift of each Al(III) species “i”, δi (ppm), is (1) δAl3+ ≡ 0 for Al3+ (defined by convention), (2) δAl(OH)2+ = 3.5 (SE = 1.3, N = 10) for Al(OH)2+, (3) δAl(OH)2+ ≅ 3.7 (SE = 1.4, N = 10) for Al(OH)2+ , and (4) δAl(OH)4− = 79.9 (SE = 0.03, N = 4) for Al(OH)4−; where positive chemical shifts are “downfield,” SE = standard error, and N = number of samples.A convention is delineated in which the linewidth of the Al(III) species/peak of interest is normalized with respect to that of a reference species (Al3+) under the same conditions. Such linewidth ratios are independent of investigation-specific variables such as solution viscosity and temperature. Due to the large sample volume (18.8 mL) used here to achieve increased sensitivity, there is some line broadening caused by magnetic field inhomogeneities; however, this line broadening is constant and reproducible both during and between experimental runs, and it was corrected for in the determination of linewidths of individual Al(III) species. For an absolute linewidth of LWAl3+ = 1.6 Hz for Al3+, the linewidth ratio (ppm/ppm) of each species “i”, (), is: (1) (by definition), (2) for Al(OH)2+, and (3) for Al(OH)2+.The increased sensitivity of this system, and the knowledge of intrinsic 27Al NMR spectroscopic parameters for Al3+, Al(OH)2+, Al(OH)2+, and Al(OH)4−, sets the stage for use of 27Al NMR spectroscopy to characterize these species in natural waters and to study other Al(III) species of geochemical interest.
    Geochimica et Cosmochimica Acta 01/1997; · 3.88 Impact Factor
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    ABSTRACT: To summarize, it is important to recognize that (1) the major objective of our GCA paper (Faust et al., 1995) was on the development of 27Al NMR spectroscopy for characterizing mononuclear Al (III) species at concentrations of geochemical relevance; (2) Öhman has only minor comments related to this major objective; and (3) these comments are rebutted above in section 1.We appreciate Öhman's suggestion to consider adsorption of Al(III) onto the Teflon NMR sample bottle. Indeed, a small but significant percentage of Al(III) was adsorbed to the surfaces of the Teflon NMR sample bottle under the conditions of our experiments, partially accounting for the decrease in total concentration of mononuclear Al(III) species that was detected by 27Al NMR spectroscopy. As noted above in section 1, since the 27Al NMR spectroscopic parameters are not affected by decreases in total concentration of mononuclear Al(III) species, this does not affect the interpretation of the 27Al NMR chemical shifts or linewidths.
    Geochimica et Cosmochimica Acta 08/1997; 61(15):3261-3264. · 3.88 Impact Factor

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