Combining MCR-ALS and EXAFS as tools for speciation of highly chlorinated chromium(iii) in mixtures of deep eutectic solvents and water.

In the last decade, deep eutectic solvents (DES) have risen as promising and cheap alternatives for the often expensive and moisture-sensitive ionic liquids. For the application in metal processing industries such as hard chrome plating, still very little is known of the behavior of metal ions in these types of liquids. Therefore, we use the model-free Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) method to study Cr(iii) as sustainable alternative to the hazardous Cr(vi) and obtain reference UV/Vis spectra of chromium(iii)chlorides in several aqueous solutions and in water-DES mixtures. In addition, the results have been confirmed by EXAFS measurements. We observe that in the DES ethaline, ethylene glycol ligands are coordinating with the chromium(iii) metal ions and hence, different UV/Vis reference spectra are obtained, compared to those in aqueous solutions. Additionally, concentration profiles provide a tool for tuning the coordination chemistry, based on the choice of the appropriate DES mixture or aqueous solutions. Consequently, valuable UV/Vis reference spectra for some known and unknown chromium chloride complexes in several aqueous solutions and DES-water mixtures were obtained, which showed that the coordination chemistry in these liquids can be considerably different and comparison should be done with great care.