Not all chromates show the same pre-edge feature. Implications for the modelling of the speciation of Cr in environmental systems

High-resolution XANES spectra for various chromates show that their pre-edge feature varies dramatically with Cr-polymerization. Ab-initio FEFF calculations reproduces those observed experimental trends. Those variations can be easily explained with Pauling's bond valence rules. As chromates are used to model the fraction of toxic Cr(VI) in samples of environmental importance (among others), a careful choice of the chromate model compound is essential for a reliable model of the Cr-speciation in 'unknown' samples.