Conductivities and ion association constants of ferric chloride and chromic chloride in DMF and DMSO as a function of temperature

Abstract

The equivalent conductivities of anhydrous ferric chloride (FeCl3) and anhydrous chromic chloride (CrCl3) were measured in nonaqueous aprotic solvents such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) at temperatures between 278.15 and 318.15 K. In both DMF and DMSO, conductivity values for FeCl3 were found to be higher than those for CrCl3. In addition, the conductivity values for both FeCl3 and CrCl3 in DMF were higher than those in DMSO at all temperatures. The conductivity data were analyzed by the Robinson-Stokes equations. The limiting equivalent ionic conductivities for ferric ion (Fe3+) and chromic ion (Cr3+) and the ion association constants (K (A) ) for FeCl3 and CrCl3 were determined in DMF and DMSO. The K (A) values calculated for both FeCl3 and CrCl3 in DMF were higher than those in DMSO. This can be ascribed to an increase of the ion association constants with a decrease of the relative permittivity of solvents used in this study. The K (A) values increased with the increase in temperature in the studied solvents. Thermodynamic functions (Gibbs' free energy, entropy, and enthalpy of ion association) were estimated from the temperature dependence of the ion association constant. The positive values of entropy and enthalpy found for FeCl3 and CrCl3 at all temperatures indicate that the association process in DMF and DMSO is endothermic.