Kinetics of formation of Fe(III) complexes of gallic acid (3,4,5, trihydroxybenzoic acid) and its methyl ester were monitored spectrophotometrically with stopped flow technique. Acetate and formate buffers covering a rates were monitored under pseudo first order conditions with ligands in at least ten fold excess.
Ligand concentration dependence of Kobs for the formation of iron (III) gallic acid complexes were determined in both these buffers but that of iron (III) gallic acid methyl ester were determined in acetate buffer only. Kobs was found to increase with pH as well as increasing concentration of ligands and gets leveled off at higher concentrations of the ligands. A rate law was derived and the mechanism for the formation of iron (III) gallic acid complex was proposed which complies with the rate law derved.
Temperature dependence of Kobs for the formation of iron (III) gallic acid complexes were also determined and the rate of formation of complex at various temperatures were calculated. Ln k/T v/s 1/T was plotted to find out the activation parameters which are in good agreement to the values reported in literature for the similar systems.
Rates of conversion of (i) monogallate iron (III) specie which dominates at pH 4.0 and (ii) bisgallate iron(III) specie, the dominating speice at pH 5.4 to trisgallate iron (III), dominating specie at pH 8.0 were also determined. These rates were found to be faster in case of mono complex converting to tris complex as compared to the rates of conversion of bis to tris complex.
Rates of substitution of acetohydroxamic acid and benzohydroxamic acid for gallic acid in these iron (III) complexes were also determined. It was observed that these substitutions are biphasic, a faster phase followed by a slower phase. Faster phase seems to get completed in seconds and the slower phase takes few hours to get completed.
Rates of faster phase were found to be fastest for monogallate iron (III) complex, comparatively slower in bisgallate iron (III) complex and slowest in the case of trisgallate iron (III) complex.
Rates of faster substitution of these hydroxamic acids were also studied in various gallic acid methyl ester iron (III) complex species formed at pH 4.0, 5.4 and 8.0 in this case also the rates were found to be fastest with the complex fromed at pH 4.0, slower at pH 5.4 and no faster substitution was observed in the complex formed at pH 8.0