Redox potential characteristics of cupric chloride solutions

The nature of the redox potential in concentrated cupric chloride solutions where [NaCl] = 280 g/L and [Cu2+] = 10–40 g/L was studied at temperatures 25–85 °C for 49 days. The redox potential of the solution was studied as a function of solution storage time, temperature and initial cupric chloride concentration. Measured redox potential values varied in the range 0.56–0.75 V vs. Ag/AgCl, increasing with increasing total copper concentration and temperature. The rate of reduction of cupric to cuprous species was highest in the most concentrated solution and occurred in 5 days, with the concentration of copper species 40 g/L. The solutions having initial copper concentrations of 30, 20 and 10 g/L reached steady state after 20, 26 and 26 days, respectively. The standard electrode potential, Eo, in concentrated cupric chloride solution was determined using the Nernst equation. The form of cuprous complex was proposed on the basis of the value of Eo, since the cupric/cuprous standard electrode potentials divide clearly in two groups. The Eo for redox couples, with cuprous present as a chloro-complex form, is a minimum of 0.3 V higher than the Eo for redox pairs with uncomplexed cuprous ions. Eo value for the redox pair existing in a HydroCopper® type environment was calculated to be − 0.039 (± 0.121) V vs. Ag/AgCl. This suggests that in a very concentrated cupric chloride solution without any additives, not all cuprous ions present are complexed with chlorides.