On the formation of iron(III) hydroxo acetate complexes

The formation of hydroxo acetate complexes of iron (III) ion has been studi ed at 25 degreesC in 3 M (Na)ClO4 ionic medium by measuring with a glass el ectrode the hydrogen ion concentration in Fe(ClO4)(3)-HClO4-NaAc mixtures ( Ac-= acetate ion). The acetate/metal ratio ranged from 0 to 6, the metal co ncentration varied from 0.005 to 0.06 M, whereas [H+] was stepwise decrease d from 0.1 M to initial precipitation of hydroxo-acetates. This occurred, d epending on the acetate/metal ratio, in the -log[H+] range 1.85-2.7. The po tentiometric data are consistent with the presence of Fe-3(OH)(3)AC(3)(3+), Fe-2(OH)(2)(4+), Fe-3(OH)(4)(5+), Fe-3(OH)(5)(4+) and, as mine species, of Fe-3(OH)(2)Ac-6(+), FeAc2+, FeAc2+, FeOH2+ and Fe(OH)(2)(+). Previously pu blished EMF measurements with redox and glass half-cells were recalculated to refine the stability constants of FeAc2+, FeAc2+ and Fe3(OH)2AC6+. Forma tion constants *beta (pqr) for pFe(3+) +(q-r)H2O + rHAc reversible arrow tF e(p)(OH)((q-r))(Ac)r(3p-q) + qH(+) (in parenthesis the infinite dilution va lue): log(*)beta (111)=-1.85 +/-0.02 (-0.67 +/-0.15), log(*)beta (122) = -3 .43 +/-0.02 (-1.45 +/-0.15); log(*)beta (363)= -5.66 +/-0.03 (-2.85 +/-0.40 ), log(*)beta (386)= -8.016 +/-0.006 (-4.06 +/-0.15), log(*)beta (220)= -2. 88 +/-0.02 (-2.84 +/-0.05), log(*)beta (340)= -6.14 +/-0.18 (-6.9 +/-0.4), log(*)beta (350)= -8.44 +/-0.09 (-7.65 +/-0.15).