Complexation of gold(III)-chloride at the surface of hematite

The acid/base properties of the hematite/water interface and surface complexation with gold has been studied by potentiometric titrations in the four component system H+ - equivalent to FeOH - AuCl4- - Cl-. Equilibrium measurements were performed in NaCl media at 298.2 K. In the evaluation of equilibrium model from experimental data the constant capacitance model was applied. The acid/base properties were investigated in 0.1 M NaCl in the range 2.6 less than or equal to pH less than or equal to 7.4. The resulting intrinsic constants for protonation and deprotonation of hydroxyl groups at the surface were log beta(1,1,0,0(int))(S) = 7.10 +/- 0.06 and log beta(-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, the potentiometric titrations were performed in the range 2.0 less than or equal to pH less than or equal to 10.2. Titration data was supplemented with analysis of Au in the aqueous phase by atomic absorbance spectrometry. The equilibrium model proposed consists of the following monodentate surface complexes: equivalent to FeOHAuCl3 (log beta(0,1,1,-1)(S) = 1.45 +/- 0.03), equivalent to FeOHAuCl2OH (log beta(-1,1,1,-2)(S) = -3.89 +/- 0.02), equivalent to FeOHAu(OH)(3) (log beta(-3,1,1,-4)(S) = -21.94 +/- 0.05). A slightly better fit could be obtained by assuming formation of a bidentate complex with the composition (equivalent to FeO)(2)Au(OH)H2O. However, based on structural arguments this complex was rejected.