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 inc lude weakly complexed Al species in their 'reactive' or 'labile' Al fr actions and thus are of limited value for the estimation of free Al3+. Capillary electrophoresis (CE) has the potential for direct measureme nt 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 i on exchange method by analysing artificial solutions containing low mo lecular weight organic acids as well as soil extracts and seepage wate rs and was found to be the only method closely matching the theoretica lly calculated values of free Al3+. In samples from the upper soil hor izons of an acid forest soil less than 14% of total Al was present as foe Al3+, whereas the colorimetric method found more than 65%, and the ion exchange method more than 80% of total Al in a 'reactive' or 'lab ile' form. The latter methods thus would seriously overestimate Al tox icity, whereas using CE Al toxicity is likely to be only slightly unde restimated.