Study of the Interaction between Chromium(III) and Hydroxamic Acids

The complexes of the chromium(III) ion formed by acetohydroxamic acid (Aha, HL) and benzohydroxamic acid (Bha, HL) were investigated by means of potentiometry and spectroscopy, at (25.0 +/- 0.1) degrees C and an ionic strength of 0.1 M KNO(3) and 0.1 M KCl, respectively. Over the acidic pH range, the coordination of the Cr(III) ion to acetohydroxamic acid and benzohydroxamic acid occurs in a 1:1 mole ratio, and the CrL(2+) type complex is formed. At near neutral pH, the CrL(OH) type hydroxo complex is formed. The acid dissociation constants of the ligands and the formation constants of the chromium(III) complexes were determined in 0.1 M KNO(3) ionic medium by potentiometric titration using the BEST computer program. In terms of the ligands, the stability of the complexes ranks in an order such as acetohydroxamic acid > benzohydroxamic acid. The stabilities of hydroxamic acid complexes for V(IV), Cr(III), and Fe(III) ions that have similar ionic radii increase in the order CrL < VOL < FeL.