Study of the Complexation, Adsorption and Electrode Reaction Mechanisms of Chromium(VI) and (III) with DTPA Under Adsorptive Stripping Voltammetric Conditions

Abstract

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 À 50 mV (vs. Ag j AgCl) (3 mol L À1 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.

Sequential voltammetric determination of chromium, thallium, cadmium, lead, copper and antimony in saline hemodialysis concentrates using electrolyte pH gradient

“…At pH 6.2, the decrease in charging current with respect to the voltammogram of the supporting electrolyte (Figure 2A) indicates the adsorption of DTPA molecules on the electrode surface. The hump in the DTPA curve over the potential range from -1250 to -1450 mV has been observed elsewhere 14 and is attributed to the reorientation of DTPA molecules. This reorientation is due to the change in electrostatic interation between DTPA and the electrode.…”

“…Indeed, by lowering the pH from 6.2 to 4.6, the sensitivity of chromium determination strongly decreases as a consequence of the low solubility of chromium in its "free form" on the mercury electrode. 22 By means of cyclic voltammetry, AC voltammetry and electrocapillary measurements, the adsorption of DTPA has been shown to be reversible 14 and pH dependent, as observed herein. Chromium (III) builds complexes with DTPA adsorbed to the electrode surface.…”

“…DTPA species are electrochemically inactive in the potential range from 100 to -1600 mV. 14 In oxygenated solutions, Cr(VI), the most toxic chromium form, [15][16][17][18] is the predominant species. Thus, it was considered as the chromium form present as a contaminant in DCs.…”

“…98%). 13,14,23 Figure 2A through D show AC voltammograms of the charging currents (phase angle = 90°) as related to the adsorption/desorption process of DTPA, considering the pH gradient used over the entire potential range of the sequential method (-1400 to 0 mV). At pH 6.2, the decrease in charging current with respect to the voltammogram of the supporting electrolyte (Figure 2A) indicates the adsorption of DTPA molecules on the electrode surface.…”

Speciation of Trace Levels of Chromium with Bismuth Modified Pyrolyzed Photoresist Carbon Electrodes

“…Therefore, the analysis of Cr(VI) constitutes an issue of general concern in particular for the difficulties relating to its speciation. A nalyses of Cr(VI) by A dCSV with mercury electrodes have been performed using different complexing agents such as diethylene triamine pentaacetic acid (D TPA ) [18,19], cupferon [20] or pyrocatechol violet (PCV) [21,22]. R ecently, bismuth modified electrodes have been proposed as a valid alternative to H g also for the A dCSV of Cr(III) and Cr(VI), using the above cited ligands [23][24][25].…”