A method for the determination of the total metal ion concentration in samples of unknown composition after separation on a chelating resin was applied to the determination of aluminium(III) concentration in real freshwater samples, using Chelex 100 as a sorbing resin. The method, called chelating resin titration, was previously tested on synthetic samples, and allows also the evaluation of the side reaction coefficient [αM(I)] of the metal ion in the presence of the resin, which is useful for the study of metal speciation. To obtain this information, the sorption equilibria of aluminium(III) on Chelex 100 under the experimental conditions must be exactly known. These equilibria were investigated according to a previously reported method, based on the determination of the sorption curves of aluminium on Chelex 100 from solutions of known composition, sometimes containing chelating substances. It was found that the sorption of aluminium on Chelex 100 takes place by formation of three species, AlL, AlL(OH) and Al(OH)3. The corresponding exchange coefficients were determined. Despite the fact that Chelex 100 sorbs aluminium(III) strongly, it was found that the sorption from fresh waters was not quantitative, which was ascribed to the strong complexation of aluminium in the samples considered. The values of αM(I) of aluminium in some freshwater samples at natural pH and acidified up to pH 3 were determined. They were found to be much higher than those ascribable to hydrolysis or to inorganic and organic ligands present in natural waters, being for instance 1012.1 at pH 7.48 and 107.5 at pH 4.88. A similar determination carried out in a 10–3 mol kg–1 sodium chloride solution containing a known amount of aluminium at pH ≈ 8 gave log αM(I) = 14, completely ascribable to hydrolysis, showing that an accurate evaluation can be obtained by the proposed method.
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