Abstract
The object of the present work was to investigate the effects of substituents on the spectra and stability of the ferric phenolate series. Such a study requires a knowledge of the thermodynamic dissociation constants of the phenols and the stability constants of the respective complexes. Since the ferric phenolates are coloured their stability was measured spectrophotometrically. For this purpose a new method which allows for the relative instability of the complexes and the limited solubility of the respective phenol had to be designed. As there was some uncertaintity regarding the reliability of the published data for the meta- and para-acetyl phenols the thermodynamic dissociation constants of the latter have been redetermined using the spectrophotometric method of Ernst and Menashi. It has been shown that, in the pH range employed for the meta isomer, a modification of this method permits the determination of the thermodynamic dissociation constant without the explicit use of the activity coefficients of the charged species. The absorption spectra of the ferric phenolate and eighteeen substituted ferric phenolates have been measured in the wavelength range 400-1000mu. It has been found that a rough linear relationship exists between the stability of the complex and the stability of the proton complex. The effect of the substituents on the stability and position of the characteristic absorption bands is discussed in terms of pi-bonding effects.