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Effects of Fluoride on Colorimetric Measurement of Solution Aluminum

Department of Agronomy, Univ. of Georgia, Athens, GA 30602

* Corresponding author.

ABSTRACT

Modifications have been reported in the aluminon and 8-hydroxyquinoline methods of measuring Al in solutions in order to exclude polymeric or solid phase Al. The effects of varying F concentrations on Al measured by these techniques have not been previously investigated. The results of this study reveal that, Al measured by aluminon, without acidification and heating (the monomeric Al technique), decreased rapidly in solutions with F/Al molar ratios 10. The addition of F after the addition of aluminon-acetate buffer to Al solutions or acidification and heating (the total Al technique) of Al-F solutions resulted in a rapid decline in aluminon absorbance, thus suggesting an increase in formation of Al-F complexes that are not reactive with aluminon. Predicted speciation of Al in solutions revealed that at F/Al molar ratio > 2, almost 100% of Al added was complexed with F, and a further increase in F/Al molar ratio resulted in a decrease in the predicted concentrations of AlF²⁺ and AlF⁺₂ species with a corresponding increase in AlF⁰₃ species. Aluminum that was reactive with 8-hydroxyquinoline with 15-s reaction time (the labile Al technique), appeared to exclude a large proportion of Al-F complexes. The recovery of Al by the labile Al technique decreased <5% at an F/Al molar ratio of 2. However, 8-hydroxyquinoline reactive Al increased with an increase in the reaction time. The recovery of 8-hydroxyquinoline reactive Al was >65% at 3-h reaction time in solutions containing 40 µM Al and 400 µM F. The concentration of Al reactive with 8-hydroxyquinoline (15-s reaction time) was lower than the sum of predicted concentrations of Al³⁺, Al(OH)²⁺, Al(OH)⁺₂ and Al(OH)⁰₃ species but was slightly higher than the sum of Al³⁺ and Al(OH)²⁺ species. In solutions with potential for Al-F complexation, Al measured by 8-hydroxyquinoline (15-s reaction time) would represent phytotoxic Al more accurately than that measured by the monomeric Al technique (using aluminon; 30-min reaction time), since the former technique largely excludes less phytotoxic Al-F complexes.

Contribution from the Dep. of Agronomy, Univ. of Georgia. Research supported by Florida Inst. of Phosphate Res. and Georgia Agric. Exp. Stn.

Received for publication August 3, 1987.