Uncertainties when using dual-wavelength or single-wavelength spectrophotometry to monitor the course of hexavalent chromium reduction mediated by zero-valent iron

Chromium is ubiquitous; whereas in small amounts Cr(III) is vital in the diet of humans, Cr(VI) is highly toxic. The latter is commonly found in contaminated waters and brownfields worldwide. Zero-valent iron (Fe0) can detoxify Cr(VI) efficiently by mediating its reduction to Cr(III). In this study, a Cr(VI)-contaminated soil was treated with a surfactant solution (Tween® 20) and the resulting extract was reacted in acidic medium with Fe0. The disappearance of Cr(VI) with time was determined with the 1,5-diphenylcarbazide colorimetric method using single-wavelength spectrophotometry (SWS) and dual-wavelength spectrophotometry (DWS). The major sources of uncertainty in the measurements were identified and calculated in both cases. Over the entire course of the reaction, the uncertainty associated with the mass concentration of Cr(VI), γ(Cr), was decreased appreciably for the DWS method relative to the conventional (SWS) method of quantitation.