RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Acid/base properties and phenylphosphonic acid (H₂L) complexation at the water-suspended-and-aged γ-Al₂O₃/water interface have been studied in 0.1 M Na(Cl) medium at 25.0°C in the range 5.0 < -log[H⁺] < 9.5. Equilibrium measurements were performed as a series of potentiometric titrations supplemented with spectrophotometric phosphorous analyses. In the evaluation of the experimental data, the contribution from electrostatic forces was accounted for by using the constant-capacitance model. The adsorption of phenylphosphonic acid onto the studied phase was found to be strong at -log[H⁺] < 7. Desorption was observed when -log[H⁺] was increased toward 9.5. The specific capacitance was evaluated from data collected in the absence of phenylphosphonic acid. The model describing the acid/base reactions and phenylphosphonic acid binding onto the studied alumina surface, (≡AlOH), is given by the following equilibria, intrinsic constants, specific capacitance, and site density:
≡AlOH + H⁺ = ≡AlOH₂⁺   log β_110(int) = 7.51 ± 0.09
≡AlOH = ≡AlO⁻ + H⁺   log β_−110(int) = −8.87 ± 0.11
≡AlOH + H₂L = ≡AlLH + H₂O   log β_011(int) = 9.58 ± 0.18
≡AlOH + H₂L = ≡AlL⁻ + H⁺ + H₂O   log β_−111(int) = 2.94 ± 0.08
≡AlOH + H₂L = ≡AlLOH²⁻ + 2 H⁺   log β_−211(int) = −5.30 ± 0.13
C = 1.40 F m⁻²
Site density = 1.03 sites/nm²
In a series of modeling calculations, the complexation features of phenylphosphonic acid are compared to those of orthophosphoric acid.