Dissolution of Cr₂O₃(s) and the Behavior of Chromium in Concentrated NaOH Solutions

Abstract

The solubility of Cr2O3(s) in (1 and 6) mol kg–1 sodium hydroxide solutions has been investigated at 25 °C. Even after 75 days of solid/solution contact the apparent solubility continued to increase slowly. Analysis of the solution phase by UV–vis spectrophotometry revealed that all of the dissolved chromium was present as Cr(VI), in the form of CrO42–(aq); no Cr(III) could be detected. Consistent with this finding, Cr(III) dissolved in 6 mol kg–1 NaOH was found to be oxidized to CrO42–(aq) over of period of days. No significant changes to the solid phase, Cr2O3(cr), were found, either by X-ray diffraction or by X-ray photoelectron spectroscopy, as a result of its contact with NaOH(aq). Both CrO42– and Cr(III) were found to be unreactive toward a range of organic and inorganic compounds in 6 mol kg–1 NaOH over many days at ambient temperatures. However, CrO42– catalyzed the decomposition of L-ascorbate, and was rapidly reduced by elemental Zn to Cr(III) in NaOH(aq).