In situ Detection of Chromate Using Photoacoustic Spectroscopy

Pulsed-laser photoacoustic spectroscopy (LPAS) can be used to non-destructively assay aqueous Cr(VI) concentrations at trace levels. The technique involves the absorption of light pulses at 371 nm by Cr(VI) species (predominately chromate and bichromate). The absorbed energy is subsequently released in the form of heat generating a local pressure wave that is detected by an ultrasonic transducer. Because the total absorptivity of a Cr(VI) solution depends on the mixture of Cr(VI) species present and, thus, is strongly correlated with pH, a simple working expression relating solution absorbance to pH was developed, and molar absorptivities for several Cr(VI) species are reported. The detection limit for Cr(VI), which is effectively determined by the absorptivity of water, is reported to be 1 ng mL−1 at pH 6. Detection limits near 0.3 ng mL−1 would be expected for solutions at pH 8 and above, where the absorptivities of Cr(VI) species are greater. The nondestructive nature of this technique makes in situ studies of trace-level Cr(VI) chemistry in small volumes possible. We show how the technique can be used to study, in essentially real time, the thermodynamics and kinetics of Cr(VI) sorption by hematite (α-Fe2O3) powder, and the effect of competing ions, such as phosphate, on these properties.