Voltammetric response of water in deep eutectic solvent based on choline chloride and urea

This work investigates the electrochemical role of water in the research of deep eutectic solvent (DES) based on urea and choline chloride. The motivation comes from the growing use of this DES, and on reports where a characteristic voltammetric cathodic pre-wave is observed. However, there has been no direct study to verify if such interpretation of the pre-wave is appropriate. In this regard, cyclic voltammetry has been used in this paper to interrogate the reactions inducing the appearance of the pre-wave. These experiments have been carried out using reline (the eutectic mixture of urea and choline chloride) melts containing different amounts of water, compounds which can be incorporated as impurities from air, or as decomposition products of reline. The analyses showed that a pre-wave in voltammograms in reline originates from water electroreduction accompanied with a subsequent choline hydroxide formation. In this sense, reline behaves like a typical mixture of organic solvent + alkali cation salt + water. The voltammetric signal depends on water content in reline. Importantly, this offers a possibility for in-situ electrochemical determination of water content in reline, provided that a calibration curve is constructed.