Extração e determinação do alumínio trocável do solo

Abstract

This paper describes the determination of exchangeable aluminum by the colorimetric method using aluminon (ammonium salt of the aurintricarboxylic acid) and by the sodium hydroxide titration method, using phenolphtalein or brom thymol blue and the potentiometer in order to indicate the end point. Aluminum was extracted from several soil samples with N KC1 solution varying the proportion of weight to volume of solution, and time of extraction. Aluminum was also extracted with N calcium acetate solutions with different pH (7.0, 6.0, 5.5 and 5.0). The data obtained allow to conclude that the two methods (colorimetric and sodium hydroxide titration method, using phenolphtalein to indicate the end point) afforded a good precision. Nevertheless the titration method can give higher values than the colorimetric method, especially when the exchangeable aluminum is low. When the relationship between soil weight and volume of N KC1 solution varied from 1:100 to 10:100, exchangeable aluminum, determined by titration, decreased. Normal calcium acetate solution with pH 7.0 did not extract exchangeable aluminum. When the pH of calcium acetate decreased the amount of extracted aluminum increased.