Abstract
A number of coloured 1:1 complexes formed by the interaction of iron(lll) with ligands containing the phenolic hydroxyl group was studied in aqueous solutions at 25 C. The stability constants of the complexes and the ionisation constants of the ligands were determined by spectrophotometric methods. These methods were devised in such a way that it was possible to calculate the equilibrium hydrogen ion concentrations of the experimental solutions by successive approximations procedures. The direct measurement of the pH's of the solutions was thus avoided, and the only physical quantity measured was the optical density at a selected wavelength. This proved to be particularly valuable in those cases in which complex formation was followed by a fairly rapid reduction-oxidation reaction (ligands I and II). Since a great deal of time and effort was devoted to the selection of the experimental conditions and to the treatment of data, the number of complexes studied was modest. The following ligands were used: [chemical formula]. Aqueous solutions of the ligands were found to be photochemically unstable in alkaline and, to a lesser extent, in acid media. Four different variants of the spectrophotometric method for the determination of ionisation constants were used to determine the indicator constant of bromocresol green (B. C. G. ) and their relative merits compared and discussed. The indicator constant of B. C. G. was used to determine the first ionisation constants of I and II which could not be determined by direct spectrophotometry. In order to assess the accuracy of the indicator method, the ionisation constants of acetic acid and propionic acid were determined with B. C. G. as the indicator. It was shown that whereas o-hydroxyphenylacetic acid forms a chelate complex with iron(lll), coordinating through both the phenolic and the carboxyl oxygen atoms, melilotic acid acts as a monodentate ligand, coordinating through the phenolic oxygen only. The iron(lll) complex of II is thus a complex acid. The spectrophotometric methods devised for the determination of the stability constants of the complexes formed by iron(111) with monobasic phenols (ligands III to VI) took into account the dimer, Fe[2](OH)[2][4+], and allowed the use of excess of either the metal or the ligand. In comparison with existing methods, these methods are particularly advantageous when the solubility of the ligand is low or when the complex has low stability and/or a small extinction coefficient. When the range of concentrations selected for the determination of stability constants is such that only a small fraction of the constituent (metal or ligand) present at the lower concentration is complexed, considerable uncertainties in the determined constant arise, even though it is possible to determine the product i (stability constant) x (extinction coefficient of the complex), with reasonable accuracy. Under certain conditions, these uncertainties are further exacerbated by the neglect of the dimer Fe[2](OH)[2][4+]. The stability constants are discussed in relation to the ionisation constants of the ligands and the stabilities of related complexes.