An X-ray absorption (EXAFS) spectroscopic study of aquated Ag+ in hydrothermal solutions to 350 degrees C

The first direct measurements over a range of temperatures from 25-350 degrees C are reported for cation-oxygen (water) distances in aqueous solutions at equilibrium saturated vapour pressures. The silver-oxygen (first shell water) bond lengths in AgNO3 and AgClO4 solutions were obtained using extended X-ray absorption fine structure (EXAFS) measurements on the silver K-edge. At 25 degrees C and 1 bar, the Ag+ ion is coordinated by four inner sphere water ligands with an average silver-oxygen distance of 2.32 (+/-0.01) Angstrom. With increasing temperature to 350 degrees C, the silver-oxygen (H2O) bond length decreases by about 0.10 (+/-0.01) Angstrom and the coordination number decreases from 4 to similar to 3. The Debye-Waller factors remain approximately constant with increasing temperature. Silver-nitrate ion pairing (as AgNO30) was observed only at 200 and 300 degrees C in an 0.10 AgNO3/3.00 m HNO3 solution for which the silver-nitrogen (nitrate) distances of 3.24 and 3.27 Angstrom, respectively, were obtained.