Studies in Outer-Sphere Complexation—Part I :Tris(2-Guanidiniumbenzimidazole)chromium(III) Salts in Aqueous Solutions

Chemical Laboratory, Bhagalpur University, Bhagalpur-812 007

Manuscript received 6 September 1978, accepted 5 June 1979

  A conductometric study of the equilibrium Cr(GBH)\(^3_3{^+}\)+ X^-\(\rightleftharpoons\)Cr(GBH)₃X²⁺,where GBH =2-guanidinobenzimidazole and X^-=Cl^-, Br^-, and NO^-₃ has been carried out in aqueous solutions at 25°, 35° and 45°. The association constants (60.8-24.4 1.mole^-1) of the outer-sphere complexes, Cr(GBH)₃X²⁺, compare well with those of known cobalt(III) systems. Thermodynamic functions, viz, changes in free energy ( - ∆G° =2434 - 2019 cals mole^-1), enthalpy (∆H° =5336- 3268 cals mole^-1 and entropy (∆S°=26.07-16.63 cals mole^-1, deg^-1) have been calculated from the values of equilibrium constants ,at different temperatures. Ionic radii of Cr(GBH)³⁺₃ and Cr(GBH)₃X²⁺ have been evaluated from limiting conductance data using Stokes law and from the association constants of the ion pairs using Bjerrum relationship.