Interaction of ofloxacin–Al(III) pH dependent complexes with the water|1,2-dichloroethane interface

The transfer of Al(III)–ofloxacin (OFX) complexes across the water|1,2-dichloroethane interface was investigated using cyclic voltammetry and potential steps followed by a negative scan. The electrochemical response was determined by the pH of the solutions analyzed. At pH < 3 the main species in solution is HOFX+ whose diffusion controlled transfer is observed. At pH ≫ pKa2 highly hydroxylated species of Al(III) become important, inhibiting the complex formation and, as a result, a profile similar to that of the base solutions is obtained. In the range 3 < pH < pKa2, different species of Al(III) (with and without OH− in its coordination sphere) can be present which form several complexes with OFX. Transfer of these compounds coupled to adsorption/desorption processes results in very characteristic and different voltammetric profiles in this pH region, pointing to lateral interaction dependent on the nature of the adsorbates. Cyclic voltammetry and potentiometry were used to determine the stoichiometry and stability constant of the complexes at pH 3.55. A comparison with the results corresponding to Fe(III) complexes is carried out. The differences observed are explained in the light of the chemistry of Al(III) and Fe(III).