Abstract
The object of this work was to investigate the effect of substituents on the spectra and stability of the ferric salicylate series. In this connexion it was necessary to derive a new spectrophotometric method for the determination of the dissociation constants of diprotic acids. The method is applicable even to the case of extremely weak acids where the direct determination of the extinction coefficient of the doubly charged anion is impracticable. The dissociation constants of salicylic acid, 3-methyl-, 5-chloro-, 5-bromo-, 5-nitro-, and 3-nitro-salicylic acids have been determined by this method. Salicylic acid and its methyl-, chloro-, and bromo-derivatives have been found to be sensitive to the action of light. In the present work complex formation was studied spectrophotometrically. The stability constants of the ferric salicylate complexes were determined by a new method based upon the continuous variation procedure. This method enables the continuous variation data to be used directly to determine not only the equilibrium concentration of the complexes in the solution but also to calculate the hydrogen ion-concentration of the solution. The absorption spectra of the ferric salicylate and substituted ferric salicylates, in the region 240-640 mmu, have also been recorded. Since a satisfactory discussion of the relative stability of complexes is not possible without considering the enthalpies and entropies of complex formation, a calorimetric method of determining these quantities is described. The method has been used to determine the heat of the two ionisation stages of salicylic acid and the heat of complex formation of ferric salicylate. It was found that the standard free energy change of formation of the ferric complexes varies linearly with the standard free energy change of formation of the respective proton complexes. The theoretical significance of this correlation is discussed in terms of pi-bonding effects.