Study of the complexation, adsorption and electrode reaction mechanisms of chromium(VI) and (III) with DTPA under adsorptive stripping voltammetric conditions

The complexation of Cr(III) and Cr(VI) with diethylenetriaminepentaacetic acid (DTPA), the redox behavior of these complexes and their adsorption on the mercury electrode surface were investigated by a combination of electrochemical techniques and UV/vis spectroscopy. A homogenous two-step reaction was observed when mixing Cr(III), present as hexaquo complex, with DTPA. The first reaction product, the electroactive 1: 1 complex, turns into an electroinactive form in the second step. The results indicate that the second reaction product is presumably a 1: 2 Cr(III)/DTPA complex. The electroreduction of the DTPA-Cr(III) complex to Cr(II) was found to be diffusion rather than adsorption controlled. The Cr(III) ion, generated in-situ from Cr(VI) at the mercury electrode at about -50 mV (vs. Ag \ AgCl) (3 mol L-1 KCl), was found to form instantly an electroactive and adsorbable complex with DTPA. By means of electrocapillary measurements its surface activity was shown to be 30 times higher than that of the complex built by homogenous reaction of DTPA with the hydrated Cr(III). Both components, DTPA and the in-situ built complex Cr(III) ion were found to adsorb on the mercury electrode. The effect of nitrate, used as catalytic oxidant in the voltammetric determination method, on the complexation reaction and on the adsorption processes was found to be negligible. The proposed complex structures and an overall reaction scheme are shown.