RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The acid/base properties of the hematite/water interface and surfacecomplexation with gold has been studied by potentiometric titrations in thefour component system H⁺ – ≡FeOH –AuCl₄⁻ – Cl^&minus:. Equilibrium measurementswere performed in NaCl media at 298.2 K. In the evaluation of equilibriummodel from experimental data the constant capacitance model was applied. Theacid/base properties were investigated in 0.1 M NaCl in the range 2.6 ≤ pH ≤ 7.4. The resulting intrinsic constants for protonation anddeprotonation of hydroxyl groups at the surface were logβ_1,1,0,0(int)^s = 7.10 ± 0.06 and logβ_{−1,1,0,0(int)}^s = −7.80 ± 0.06. The density of proton active surface sites was 2.85 nm⁻² and the specific capacitance 2.5 C V⁻¹ m⁻². In the investigation of surface complexation of gold, thepotentiometric titrations were performed in the range 2.0 ≤ pH ≤ 10.2.Titration data was supplemented with analysis of Au in the aqueous phase byatomic absorbance spectrometry. The equilibrium model proposed consists ofthe following monodentate surface complexes: ≡FeOHAuCl₃(log β_0,1,1,−1^s = 1.45 ± 0.03), ≡FeOHAuCl₂OH (log β_{−1,1,1,−2}^s = −3.89 ± 0.02), ≡FeOHAu(OH)₃ (log β_{−3,1,1,−4}^s = −21.94 ± 0.05). Aslightly better fit could be obtained by assuming formation of a bidentatecomplex with the composition(≡FeO)₂Au(OH)H₂O. However, based onstructural arguments this complex was rejected.