An infrared and X-ray absorption study of the equilibria and structures of chromate, bichromate, and dichromate in ambient aqueous solutions

The structure and equilibrium of chromate (CrO₄^2-), bichromate (HCrO₄^-), and dichromate (Cr₂O₇^2-) in ambient aqueous solutions was investigated by both IR and X-ray absorption fine structure (XAFS) spectroscopy. The individual IR bands for each of these species have been identified, and quantitative analysis of the IR bands was used to examine the chemical equilibria. A dilution study at a constant, low solution pH revealed the speciation change from dichromate dominance at high total chrome concentration to bichromate dominance at low total chrome concentration, thereby unequivocally confirming the existence of the bichromate species. The equilibrium constants, obtained from the quantitative analysis of the IR data, agree well with previously reported values. The quantitative IR analysis also revealed that the asymmetric v_as(CrO₃) stretching frequency is virtually coincident for the bi- and dichromate. The complete structural analysis of chromate, bichromate, and dichromate was obtained from an XAFS study of chromate solutions of varying total chrome concentration. The XAFS results confirm the v_as(CrO₃) IR band assignment for both the bi- and dichromate species by showing that the first-shell structure around the central chrome atom is nearly identical for the bi-and dichromate molecular ions. Within experimental uncertainty, the Cr-O bond distance that is associated with the v_as(CrO₃) vibrational mode was found to be identical for the bi- and dichromate structures.