Complexation of Cd2+ and Cl- ions in aqueous mixtures of 2-butanol

Reactions of chlorocadmium complex formation have been studied in mixtures of 2-butanol + water (w(2-BuOH) 0.05, 0.10, and 0.15) at three ionic strengths and three temperatures by means of direct potentiometry. First, calibration diagrams for cadmium and chloride ions were made, based on e.m.f. (electromotive force) measurements in solution with known concentrations of these ions, followed by e.m.f. measurements in a series of solutions with known concentrations of total cadmium and total chlorides in which complexation took place. Using the latter e.m.f. data and the respective calibration diagrams, concentrations of free cadmium and chloride ions were determined for each ionic strength and temperature. Three complexes were found in each mixed solvent: CdCl+, CdCl2 and CdC3-. Their concentration stability constants (K-n(1)) were calculated by the method of orthogonal polynomials for each ionic strength and temperature. The thermodynamic stability constants (K-n(degrees)) were determined for each temperature in a particular mixed solvent, and were found to increase with increasing temperature, as well as with decreasing relative permittivity of the mixed solvent, for each complex species. By comparison with the K-n(degrees) constants in the 2-butanone + water solvent, the influence of the organic component was established. Thermodynamic quantities for the complex formation reactions, calculated from K-n(degrees), have shown that complexation is spontaneous, endothermic, and accompanied by an entropy increase.