Measurement of temperature-dependent stability constants of Cu(I) and Cu(II) chloride complexes by voltammetry at a Pt ultramicroelectrode.

Cu(II)Cl complexes can undergo stepwise electrochemical reduction to Cu(I)Cl and then to Cu. In this paper, the Nernst equation is used to simultaneously calculate the temperature-dependent stability constants of both mononuclear Cu(II)Cl and Cu(I)Cl complexes for the first time. First, voltammograms were recorded at a Pt ultramicroelectrode (UME) with a variable of free chloride concentration at a given temperature. Using the equilibrium of Ag/AgCl redox couple, the free chloride activities were measured on an Ag electrode in different NaCl solutions, adjusting for the influence of ionic strength. This electrochemical technique was proved to be feasible at 25 and 90 °C. With increasing temperature, the Cu(II)Cl complex favored a lower coordination number and the CuCl2 complex prevailed, whereas the CuCl4(2-) was predicted to be unstable. The CuCl2(-) and CuCl3(2-) complexes were still the primary species for the Cu(I)Cl complex.