On the reaction of iron(III) with chromium(III)-DTPA chelating agent

The reaction of iron(III) with chromium(III)-DTPA 'chelating agent' has been investigated, in aqueous solution, by measuring both hydrogen ion concentration and emf of the iron(III)/iron(II) couple. In acidic medium (pH < 2.3) and with ligand excess, a 1:1 (metal-ligand molar ratio) fully deprotonated complex has been evidenced. In the same experimental conditions the iron(III) reacts with iminodiacetic acid forming a similar 1:1 deprotonated species. The stability constants of the above equilibria have been calculated at 20<degrees>C in NaClO4 1.0 mol dm(-3). The comparison of these constants, considering the basicity of the ligands, proved the coordination of iron(III) occurs by one iminodiacetic group. The iron(III)-chromium(III)-DTPA complex has been precipitated and the solid compound characterized by IR and thermoanalytical techniques.