Potentiometric Water Sensor For Eutectic Solvents Containing Choline Chloride and Ethylene Glycol

Electrochemical reprocessing of metals using eutectic solvents (ESs) is a promising approach due to its availability, cost-effectiveness, ease of preparation, and environmental benefits. However, a significant challenge associated with ESs is their tendency to absorb moisture from the atmosphere, which can impede and alter electrochemical recovery processes. Here, we examine a novel electrochemical method for quantifying the amount of water content absorbed by a commonly used ES formed from choline chloride and ethylene glycol (ChCl:2EG). ChCl:2EG is exposed to the open atmosphere for over 8 days with Cu²⁺ ions initially present at 10 mM. By simply analyzing the relative reduction potentials of the Cu^2+/+ redox couple against the Cu^+/0 redox couple, formed in situ in the voltammetry sweep, the water content of the ES adsorption from the atmosphere was determined via a calibration plot obtained using external reference samples. An upper maximum of ca. 35% water was observed over the 8 days of atmospheric exposure. Instead of depending on the absolute potential measured against a reference electrode, the analysis has measured the potential difference between the two redox couples in situ in solution. As a result, it is more user-friendly, as the measurement is independent of the choice of the reference electrode. We foresee that this rapid in situ potentiometric approach in determining water content in ChCl:2EG could be generalized similarly for other ESs.