Conductometric Study of First and Second Ion Associations of Hexaamminecobalt(III) and Tris(ethylenediamine)cobalt(III) Ions with Sulfate Ions at Temperatures from 0 to 50 °C

Abstract

Abstract Conductivity measurements for aqueous solutions of sulfates of hexaamminecobalt(III) ([Co(NH3)6]3+) and tris(ethylenediamine)cobalt(III) ([Co(en)3]3+) complexes have been made at various temperatures from 0 to 50 °C. The conductivity data were analyzed to obtain the first ion-association constants, but could not be completely explained if the second ion association (triple-ion formation between ion pair and sulfate ion) was not taken into consideration. The ion-association constants had minimum values at particular temperatures and were significantly larger than those predicted by electrostatic ion-association theories. Excess thermodynamic functions for the first ion association over the theoretical prediction were estimated and explained by assuming the ion–ion and ion–water hydrogen bonding. The excess entropy and enthalpy changes were significantly increased with increasing temperature, while the Gibbs free energy change was little dependent on temperature; these results were attributed to the destruction of bulk water structure with temperature. A similar contribution of hydrogen bonding was present in the second ion association besides the electrostatic interaction between the anion and the ion pair having a large dipole moment. The difference between [Co(NH3)6]3+ and [Co(en)3]3+ in hydrogen bonding is also discussed.