Stability constants have been measured spectrophotometricallyas well as potentiometrically for iron (III) gallic acid complex. B values determined by both the methods were compared and found to be in good agreement with each other. For spectrophotometric study molar absorptivity of the complex at each pH was measured and used for further calculations.
In potentiometric titrations experimental results were treated in two different ways. Classical Bjerrum method was applied and these data were further used by well known computer program “Best” and compared. The results were also refined by this program.
The stoichiometric rations of different species formed during this complex formation were determined. Concentrations of the species present in aqueous solution at different pH were also studied by these methods.
For thermodynamic study the changes in B values at different temperature were examined and enthalpy and entropy values were determined.
Iron (III) complex with gallic acid methyl ester was also prepared.
The above results were compared with this new complex and the role of carboxylate group on the complex was studied.
The redox potential of these complexes were found to be less positive than the redox potential of Fe(III)/Fe(II) in form of aquo complex which is 0.77v vs NHE. The Eo value of iron (III) gallic acid methyl ester was found to be lower than that of Fe(III)-galilic acid complex at corresponding pH,s.
The rate constants for this redox reaction were studied at pH 4, 4.2, 4.4, 4.6, 4.8, 5.0, 5.3 and 5.4. These values were found to be greater in case of iron gallic acid when compared with the rate constants of iron gallic acid methyl ester. It was also observed that these values had inverse relation with pH. When pH was increased these values became smaller.
Activation parameters were studied for the redox reactions of both the complexes. These values were obtained for gallic acid at pH 4 and 5 and for gallic acid methyl ester at pH 4 only. In both the cases G were found to be positive.