Local environment around gold(III) in aqueous chloride solutions: An EXAFS spectroscopy study
- Stanford Univ., CA (United States)
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[sup [minus]1] to 10[sup [minus]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[Au-Cl] = 2.28--2.29 [+-] 0.01 [Angstrom]), 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 , 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 AuX[sub 4] (X = Cl, O) at all pH values studied. These results are in excellent agreement with those from the authors' previous Raman, resonance Raman, and UV/visible spectroscopy study of gold(III)-chloride solutions (Peck et al., 1991), which found that AuCl[sub 4][sup [minus]], AuCl[sub 3](OH)[sup [minus]], and AuCl[sub 2](OH)[sub 2][sup [minus]] are the majority species in the pH ranges 2--6, 6--8.5, and 8.5--11, respectively. They did not find evidence for Au(I)Cl[sub 2][sup [minus]] or Au(I)Cl(OH)[sup [minus]] complexes in their pH 7.5 and 9.2 solutions, as was recently suggested by Pan and Wood (1991) for acidic gold chloride solutions at temperatures >100[degrees]C, although the authors can't rule these complexes out as minority species (<10% of the total Au in solution). Their EXAFS results also provide the first direct evidence for Cl second neighbors around AuCl[sub 4][sup [minus]] complexes in the most acidic solutions studied (pH = 2 and 4.5). 73 refs., 5 figs., 1 tab.
- OSTI ID:
- 6486683
- Journal Information:
- Geochimica et Cosmochimica Acta; (United States), Vol. 57:6; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
AQUEOUS SOLUTIONS
PH VALUE
GOLD CHLORIDES
MOLECULAR STRUCTURE
GOLD COMPLEXES
ACTIVATION ENERGY
SODIUM CHLORIDES
X-RAY SPECTROSCOPY
ALKALI METAL COMPOUNDS
CHLORIDES
CHLORINE COMPOUNDS
COMPLEXES
DISPERSIONS
ENERGY
GOLD COMPOUNDS
HALIDES
HALOGEN COMPOUNDS
MIXTURES
SODIUM COMPOUNDS
SOLUTIONS
SPECTROSCOPY
TRANSITION ELEMENT COMPLEXES
TRANSITION ELEMENT COMPOUNDS
400201* - Chemical & Physicochemical Properties