CONDUCTOMETRIC STUDY OF FIRST-ION AND 2ND-ION ASSOCIATIONS OF HEXAAMMINECOBALT(III) AND TRIS(ETHYLENEDIAMINE)COBALT(III) IONS WITH SULFATE-IONS AT TEMPERATURES FROM 0-DEGREES-C TO 50-DEGREES-C

Conductivity measurements for aqueous solutions of sulfates of hexaamm inecobalt(III) ([Co(NH3)(6)](3+)) and tris(ethylenediamine)cobalt(III) ([Co(en)(3)](3+)) complexes have been made at various temperatures fr om 0 to 50 degrees C. The conductivity data were analyzed to obtain th e first ion-association constants, but could not be completely explain ed if the second ion association (triple-ion formation between ion pai r and sulfate ion) was not taken into consideration. The ion-associati on constants had minimum values at particular temperatures and were si gnificantly larger than those predicted by electrostatic ion-associati on theories. Excess thermodynamic functions for the first ion associat ion over the theoretical prediction were estimated and explained by as suming the ion-ion and ion-water hydrogen bonding. The excess entropy and enthalpy changes were significantly increased with increasing temp erature, while the Gibbs free energy change was little dependent on te mperature; these results were attributed to the destruction of bulk wa ter structure with temperature. A similar contribution of hydrogen bon ding was present in the second ion association besides the electrostat ic interaction between the anion and the ion pair having a large dipol e moment. The difference between [Co(NH3)(6)](3+) and [Co(en)(3)](3+) in hydrogen bonding is also discussed.