Kinetics of Cr(III) ions discharge in solutions based on a deep eutectic solvent (ethaline): Effect of water addition

Abstract The kinetics of electroreduction of Cr(III) ions in a deep eutectic solvent, ethaline (an eutectic mixture of ethylene glycol and choline chloride), and water-ethaline mixtures on a glassy carbon electrode was investigated by means of cyclic voltammetry technique. The electrochemical reduction of chromium (III) ions to chromium (II) soluble ions was shown to proceed irreversibly. Some kinetics parameters (standard electrochemical rate constants, transfer coefficients and activation energy for charge transfer) as well as transport properties (diffusion coefficients and activation energy for diffusion) were calculated and discussed as a function of extra water content (10, 15 and 20 wt%) at the temperatures of 313, 323, 333 and 343 K. An increase in water content in the fluid results in both higher values of diffusion coefficients of Cr(III) ions and a reduction of activation energy of diffusion mass transfer. However, the introduction of water into ethaline causes the deceleration of charge transfer (i.e. a decrease in standard rate constants and an increase in activation energy of electron transfer) which may be due to the changes in Cr(III) speciation in solution (the reduction of the fraction of chlorinated complexes with increasing water content).