Speciation of metals in different oxidation states by capillary electrophoresis using pre-capillary complexation with complexones

Abstract

Ethylenediaminetetraacetic (EDTA), cyclohexane-1,2-diaminetetraacetic (CDTA) and diethylenetriaminepentaacetic (DTPA) acids were investigated and compared as complexing agents for the speciation of chromium(VI/III), vanadium(V/IV) and iron(III/II) ions by capillary electrophoresis. The chromium(III) was selectively chelated with DTPA in order to impart a negative charge and similar mobility to both the chromium(III) and chromium(VI) species. The best complexing agent for the determination of vanadium species was found to be EDTA. The addition of an excess of EDTA, CDTA or DTPA to the solution containing both iron species causes the rapid and complete oxidation of iron(II) into iron(III) by ambient oxygen. Therefore for the simultaneous determination of both iron species two different complexing agents were used: o-phenanthroline for Fe²⁺ and CDTA for Fe³⁺. Detection was carried out by direct UV absorption depending on the properties of the formed complexes. For all species studied the calibration graphs were linear over at least two orders of magnitude of concentration. The detection limits were in the range from 1 × 10^–6 mol l^–1 for Fe^II to 83 10^–6 mol l^–1 for Cr^VI. Application of the method to the speciation of metal ions in real samples is also demonstrated.