Spectroscopic studies on 3- and 5-formylsalicylic acids and their complexes with Fe(III)

The electronic absorption spectra of 3-formylsalicylic acid (3-fsa) and 5-formylsalicylic acid (5-fsa) have been studied in different pure and mixed solvents. From the pure solvent study, the Einstein transition probabilities (A_if and B_if), dipole moment (D_if), oscillator strength (F_if), life time (τ), and the molar extinction coefficient (?) of the absorption bands were determined. The hydrogen bonding and orientation energies between solute and solvent molecules have been investigated from the mixed solvent spectra. The equilibrium constants, pK_a1 and pK_a2, of the two acids have been determined spectrophotometrically and a new approximate method for pK_a1 determination is suggested. The complex formation between 3-fsa and 5-fsa and Fe³⁺ in solution has been investigated spectrophotometrically and the stoichiometric ratios of the two systems have been determined applying the continuous variation, mole-ratio and the slope ratio methods which all showed a 1:1 type of complexes.