Iron(III) complex-catalyzed reduction of tris(oxalato)cobaltate(III) by L-ascorbic acid

Kinetic studies of the reduction of tris(oxalato)cobaltate(III) by L-ascorbic acid (H(2)A) were investigated as a function of pH, buffer composition, iron(III) ion concentration and for differ ent iron(III) complexes (EDTA, Cl-, bipy and phen), The redox process was followed at 600 nm, (25.0 +/- 0.1)degrees C, under pseudo-first-order conditions: [H(2)A] = 3.00 x 10(-2) M and [Co(C2O4)(3)(3-)] = 3.00 x 10(-3) M in the presence of [EDTA] = 3.15 x 10(-3) M and ionic strength 1.0 M (NaCl). A strong pH dependence shows reactivity H(2)A < HA(-) < A(2-). The Fe(III)/EDTA complex catalyzes the reaction with HA(-) but not with A(2-) or H(2)A, showing a maximum catalytic effect around pH 7.5. The pseudo-first-order rate constant is proportional to iron(III) concentration in the range of(0.2-10.0) x 10(-5) M. EDTA has a significant effect on the catalytic ability of the iron(III) ion while bipyridine and phenanthroline completely inhibit its activity. The order of catalytic activities is in agreement with the variation in reduction potential of the iron(III) complexes for the different ligands. When Cu(II) is used as a catalyst, EDTA completely inhibits its catalytic effect; in the presence of Cl-, Cu(II) is a better catalyst than Fe(III).