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.