Adsorption of organic acids on metal oxides: Application of the surface potential measurements

Simultaneous interpretation of surface potential, electrokinetic and adsorption data for salicylic acid on hematite surface was introduced. The interpretation on the basis of the surface complexation model using both 2-pK and 1-pK models suggested that salicylate ion is bound to one metal ion at the solid surface by two oxygen bonds. The equilibrium parameters describing the interfacial equilibrium were obtained as follows: for the 2-pK model the thermodynamic equilibrium constant for protonation and deprotonation of surface sites are lg   K p 0 = 4.1 ± 0.2 and lg   K d 0 = − 8.2 ± 0.2 , respectively, while the thermodynamic equilibrium constant of adsorption of salicylic acid is lg   K a 1 0 = 1.6 ± 0.1 . For 1-pK model the thermodynamic equilibrium constant of adsorption of salicylic acid was obtained as lg   K a 3 0 = 2.3 ± 0.2 . The inner layer capacitance increases with pH from 1.8 to 3 F m−2. It was shown that the surface potential measurement by single crystal electrodes is a helpful tool in elucidation of the complex equilibrium at the interface.