Simultaneous determination of Cr(III), Fe(III), Cu(II) and Pb(II) as UV-absorbing EDTA complexes by capillary zone electrophoresis

Abstract

Capillary zone electrophoresis (CZE) with on-column UV detection was used to separate and determine Cr(III), Fe(III), Cu(II) and Pb(II) complexes with EDTA. Distribution species-pH diagrams showed that, under the experimental conditions chosen (pH 5.5), [Me-EDTA] − and [Me-EDTA] 2− species for trivalent and divalent metals, respectively, were present. By adding a cationic surfactant, such as TTAB (tetradecyltrimethylammonium bromide), to the background electrolyte, an improvement in the peak shapes and shorter migration times were achieved. The chelating complexes showed the following order of mobility in electropherograms: [EDTA] 2− > [Cu-EDTA] 2− >[Pb-EDTA] 2− >[Cr-EDTA] − >[Fe-EDTA] −. At 225 nm under a negative applied voltage of 30 kV, using a capillary of 30.5 cm effective length, in 0.1 M acetate buffer and 0.1 m M TTAB as carrier solution, the complexes were determined within 6 min, but the resolution of the Cu(II) and Pb(II) chelates was poor. However, by using a capillary of 60 cm effective length, simultaneous separation of these chelates the EDTA was achieved. Factorial design was used to investigate the effects of chromium,, excess of EDTA and boiling time of the solution on the formation of the [Cr-EDTA] − complex. A fitting model was found in which the EDTA concentration was a significant factor. The detection limits of all chelates were in the range 6–27 μM.