Local Structure around Cr~(3+) Ions in Dilute Acetate and Perchlorate Aqueous Solutions

摘要

The hydration structure and aqueous acetate complexation of Cr~(3+) ions were studied by X-ray absorption fine structure (XAFS) spectroscopy as a function of pH, concentration, acetate:Cr ratio, and age of the solution. In the perchlorate solutions, we found an octahedral hydration shell around the Cr~(3+) ion at 1.96 A, confirming previous results through an independent analysis. Distinct Cr-Cr correlation was observed in the Cr acetate solutions, indicating that acetate groups bridge between the metal ions in a polymer structure. Modeling of the data confirmed a cyclic trichromium complex in acetate solutions. Similar spectral features in the Fourier transform were observed at 3.0-3.5 A for both hydrated and polynuclear Cr. Comparison of the spectral content of such features in the two different cases showed that the origin of the 3.0-3.5 A structure is multiple scattering within the first O shell alone. Thus, no spectral contribution could be attributed to the outer hydration molecules in data for hydrated Cr~(3+). We also report on differences in first-shell O backscattering (or possible spectral contributions from H atoms) in the aqueous solutions relative to the crystal oxide, determined by systematic analysis of a Cr_2O_3 standard.