The acid/base properties of the hematite/water interface and surface comple
xation with gold has been studied by potentiometric titrations in the four
component system H+ - equivalent to FeOH - AuCl4- - Cl-. Equilibrium measur
ements were performed in NaCl media at 298.2 K. In the evaluation of equili
brium model from experimental data the constant capacitance model was appli
ed. 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 intrin
sic constants for protonation and deprotonation of hydroxyl groups at the s
urface 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 investiga
tion of surface complexation of gold, the potentiometric titrations were pe
rformed 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 (l
og 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 as
suming formation of a bidentate complex with the composition (equivalent to
FeO)(2)Au(OH)H2O. However, based on structural arguments this complex was
rejected.