Pressure-dependent stability of cadmium chloride complexes: Potentiometric measurements at 1–1000 bar and 25°C

Abstract

Potentiometric measurements were performed in the Cd(NO3)2-KCl-H2O system at 25°C and 1–1000 bar using an isothermal cell with a liquid junction and equipped with a solid contact Cd-selective electrode. At 1 bar, the stepwise equilibrium constant of the fourth cadmium chloride complex CdCl42− has been determined (log K40 = −0.88 ± 0.25). The pressure-dependent stability constants for all cadmium chloride complexes have been experimentally established for the first time. As pressure increases from 1 to 1000 bar, the stability constants for the first, third, and fourth complexes change by less than 0.05 logarithmic units, whereas that for the second complex decreases by 0.33 logarithmic units. On the basis of these data, the partial molar volumes of four cadmium chloride complexes have been determined under standard state conditions: V0(CdCl+) = 2.20 ± 3, V0(CdCl2 (aq)) = 42.21 ± 5, V0(CdCl3−) = 63.47 ± 10, and V0(CdCl42−) = 81.35 ± 15 cm3mol−1. The linear correlation between the nonsolvation contributions of molar volumes and the number of ligands corresponds to the change in coordination from octahedral in Cd2+ and CdCl+ to tetrahedral in CdCl2 (aq), CdCl3−, and CdCl42− complexes. Using theoretical correlations, the HKF parameters allowing calculation of the volumetric properties of cadmium chloride complexes in a wide range of temperature and pressure have been obtained. The pressure effect on cadmium concentration in sphalerite in equilibrium with the H2O-NaCl hydrothermal fluid has been estimated. It is shown that the Cd content in sphalerite increases with pressure.