Determination of free Al3+ in soil solutions by capillary electrophoresis

Colorimetric and ion exchange methods are commonly used to distinguish and measure Al species in natural waters. Unfortunately they also include weakly complexed Al species in their ‘reactive' or ‘labile' Al fractions and thus are of limited value for the estimation of free Al³⁺. Capillary electrophoresis (CE) has the potential for direct measurement of Al³⁺, and its performance has been verified experimentally. The method also detected the stable and positively charged AlOx⁺ complex formed with oxalic acid. It was compared with a colorimetric and an ion exchange method by analysing artificial solutions containing low molecular weight organic acids as well as soil extracts and seepage waters and was found to be the only method closely matching the theoretically calculated values of free Al³⁺. In samples from the upper soil horizons of an acid forest soil less than 14% of total Al was present as free Al³⁺, whereas the colorimetric method found more than 65%, and the ion exchange method more than 80% of total Al in a ‘reactive' or ‘labile' form. The latter methods thus would seriously overestimate Al toxicity, whereas using CE Al toxicity is likely to be only slightly underestimated.