Aluminum determination in soil solution. 2. Short-term colorimetric procedures for the measurement of inorganic monomeric aluminum in the presence of organic acid ligands

Abstract

Colorimetric procedures were developed for the determination of inorganic monomeric aluminium (Al) in the presence of organic ligands including high molecular weight fulvic acids and short chain carboxylic acids. The procedures take advantage of marked differences in reaction rate between the chromogens aluminon and pyrocatechol violet (PCV) and aluminium in either free inorganic forms or organically complexed forms. The aluminon method uses a 30 s reaction and has a working range for total Al concentrations between 20 and 250 PM. The more sensitive PCV method uses a reaction time of 60 s and provides a linear calibration line in the range 0.2-25 µM. Both methods are calibrated by using standard solutions containing known total A1 concentration prepared with varying ratios of inorganic monomeric and organically complexed Al. The determination of Al in soil solution and its fractionation into inorganic monomeric and organically complexed forms requires the measurement of total Al in filtered soil solutions by inductively coupled plasma atomic emission spectroscopy (ICPAES) and the use of one of the short reaction time colorimetric procedures. Limited testing on soil solutions from acid, Al toxic soils showed better agreement between inorganic monomeric Al concentration determined by the new procedures and relative yield data than that obtained by existing colorimetric methods.