An infrared and X-ray absorption study of the structure and equilibrium ofchromate, bichromate, and dichromate in high-temperature aqueous solutions

The structure and speciation of bichromate, dichromate, and chromate was st udied to temperatures up to 400 degreesC using the combined information obt ained from infrared (IR) and X-ray absorption fine structure (XAFS) spectro scopy. Actual species concentrations were obtained from the IR measurements . which were then used as fixed input parameters for the XAFS analysis. Wit h this procedure a complete description of the molecular structure of each chromate species to high temperatures was obtained. In agreement with previ ous high temperature studies on the chromate system, the bichromate species was found to become thermodynamically favored at high temperatures under a cidic or near-neutral conditions. For example, only the bichromate species was found to be present at 350 degreesC in a 0.145 m aqueous chromate solut ion having an initial solution pH of 3.3. Because of this, the bichromate m olecular structure could be fully characterized from the measured XAFS data . The first-shell structure around the central chrome atom is extremely sim ilar in the bichromate. and dichromate structures with virtually identical Cr-O bond distances for the three terminal oxygen atoms and with only a sli ghtly different Cr-O bond distance for the bridging oxygen. As a result of this structural similarity, the v(as)(CrO3) asymmetric stretching frequency of the bichromate and dichromate species nearly overlap in the IR, as is e vident from the quantitative analysis of this band. Furthermore, the chroma te structure in the ion-paired species, (Na+)(CrO42-), at high temperature is nearly identical to that in the free chromate ion.