Aluminum determination in soil solution. 1. Evaluation of existing colorimetric and separation methods for the determination of inorganic monomeric aluminum in the presence of organic acid ligands

Abstract

Colorimetric methods of analysis and separation procedures were evaluated for their ability to selectively measure the inorganic monomeric forms of aluminium (Al) toxic to plant roots. An Al-fulvate complex of natural origin was prepared and characterized by gel filtration chromatography and 2 7Al NMR to act as a model organic complex of Al. All five colorimetric methods tested failed to adequately discriminate between inorganic monomeric and organically complexed forms of Al. Best discrimination was achieved by the 15 s 8-hydroxyquinoline method, with only 46% of the organically complexed Al being measured as inorganic monomeric Al. All methods with longer reaction times measured >80% of the complexed Al as inorganic monomeric Al. Separation procedures by using cation exchange resins to remove inorganic monomeric Al from aluminium fulvate solutions were found to produce a decrease in sample pH of 0.8, even though the resin was used in the potassium form to eliminate H+ ion exchange. The decrease in sample pH caused the Al complex to dissociate and 34% of the Al was retained on the resin. With buffered Chelex 100 in the potassium form, change in sample pH was reduced to 0.2 pH units, but 23.5% of the Al was still retained on the resin. Separation of organically complexed forms of Al from inorganic monomeric forms by gel filtration chromatography (GFC) was shown to be ineffective when sulfate is present due to co-elution of the SO4-Al complex with the fulvic acid-Al complex.