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Direct Determination of Aluminum and its Cationic Fluoro-Complexes by Ion Chromatography

CSIRO Div. of Soils, GPO Box 639, Canberra, ACT 2601, Australia

*Corresponding author.

ABSTRACT

An ion chromatographic method was developed for the direct determination of monomeric Al (Al³⁺, and hydroxo-, sulfato-, oxalato-, and fluoro- complexes). Separations were made in one run in 4 min with a 50 mm low-capacity cation exchange column, and detected by postcolumn reaction with pyrocatechol violet. Three peaks corresponding with mono-, di- and trivalent species were obtained. The last peak also included the hydroxo-Al and sulfato-Al species. The AlF²⁺ species could be determined directly, but AlF⁺₂ coeluted with the monovalent oxalato- species. Citrato-Al species were not detected because of the inability of pyrocatechol to decouple the complex in the time frame of the analysis. Concentrations of Al down to 2 µM could be determined with 200 µL injections. The results for 21 Al-F solutions closely matched those predicted from a chemical equilibrium program. The method was applied to extracts of 10 soils and it was found that Al³⁺, including hydroxo-Al species, and AlF²⁺ were dominant. Limitations to the application of the method to soils included coelution of AlF⁺₂ with monovalent organic species that are able to be decoupled by pyrocatechol, and blockage of the column end frits by solutions containing more than 200 mg L^–1 of humic acid. The ion chromatographic method is the first to directly determine AlF²⁺ and other well-defined forms of monomeric Al. It is rapid and can be fully automated.

Received for publication October 26, 1988.