Complexation of Gold(III)-Chloride at the Surface of Hematite

Abstract

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 –AuCl ^-₄ – Cl^-. 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 ^s_1,1,0,0(int) = 7.10 ± 0.06 andlog β ^s_{-1,1,0,0(int)} = - 7.80 ±0.06. The density of proton active surface sites was 2.85nm^-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 ^s_0,1,1,-1 = 1.45 ± 0.03),≡FeOHAuCl₂OH (logβ ^s_-1,1,1,-2 = -3.89 ± 0.02), ≡FeOHAu(OH)₃ (logβ ^s_-3,1,1,-4 = -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.