| Popis: |
The local environment around Au (III) in aqueous solutions containing 1 M NaCl was determined as a function of pH and Au concentration using X-ray absorption spectroscopy (XAS) at ambient temperature and pressure. The solution Au concentrations studied were 10− to 10−3 M and the pH ranged between 2 and 9.2. No significant changes of Au speciation were detected with increasing Au concentration; however, major speciation changes were caused by variations in pH. At pH = 2, Au is coordinated by four Cl atoms (mean d [AuCl] = 2.28 –2.29 ± 0.01 A), whereas at pH 7.5 and 9.2, Au is coordinated by three Cl and one O (or OH) and by two Cl and two O (or OH), respectively (mean d[AuCl] = 2.28 ± 0.02 A; mean d[AuO or AuOH] = 1.97 ± 0.02 A), indicating replacement of Cl by O (or OH) with increasing pH. In all solutions studied, the number of first-neighbors around Au(III) is close to four. XANES analysis suggests the presence of a square-planar geometry for AuX4 (X = Cl, O) at all pH values studied. These results are in excellent agreement with those from our previous Raman, resonance Raman, and UV/visible spectroscopy study of gold(III)-chloride solutions (PECK et al., 1991), which found that AuCl4−, AuCl3(OH)−, and AuCl2(OH)2− are the majority species in the pH ranges 2–6, 6–8.5, and 8.5–11, respectively. We did not find evidence for Au(I)Cl2− or Au(I)Cl(OH)− complexes in our pH 7.5 and 9.2 solutions, as was recently suggested by Pan and Wood (1991) for acidic gold chloride solutions at temperatures > 100°C, although we can't rule these complexes out as minority species ( |
