Interaction of Iron(III) with 2-Hydroxybenzohydroxamic Acid in Aqueous Solutions.

The complex formation between the Fe3+ ion and 2-hydroxybenzohydroxamic acid (salicylhydroxamic acid, H2L) has been studied at 25 C in 1.0 mol •dm-3 (Na)ClO4 medium by potentiometric titrations with redox Fe3+/Fe2+ and/or glass electrodes. On account of precipitation of Fe(II)-salicylhydroxamates, the redox half-cell was employed only in measuremenrts at [H+] > 3 • 10-3 mol •dm-3. The redox and glass electrode data, collected in the concentration ranges (mol •dm-3) 1• 10-3 e [Fe(III)] e 5 • 10-3, 0 < [H2L]tot e 0.015, and 0.1 g [H+] g 10-2, are consistent with the presence of the species FeHL2+ and Fe(HL)2 +. Glass electrode measurements, in solutions where (10-1.5 g [H+] g 10-4) mol •dm-3, (3• 10-4 e [Fe(III)] e 7 • 10-3) mol •dm-3, and (1 e [H2L]tot/[Fe(III)] e 3) mol •dm-3, are explained by assuming in addition to FeHL2+ and Fe(HL)2 + the complexes FeL2 - and Fe5(HL)5(OH)8 2+. The pentamer is probably a derivative of the hypothetical hydrolytic complex, Fe5(OH)8 7+. Stability constants for their formation in 1 M NaClO4 as well as in dilute solution are reported.