Thermodynamics of hydrogen-isotope-exchange reactions. Proton solvation in n-hexanol and water

Results are reported for the direct experimental determination of the equilibrium constant for the hydrogen-isotope-exchange reaction, 1/2D2(g) + HCl(hexOH) = 1/2H2(g) + DCl(hexOD), where hexOH is n-hexanol and hexOD is n-hexanol with deuterium substitution in the alcohol function. The reaction was studied in electrochemical double cells without liquid junction for which the net cell reaction is the above isotope-exchange reaction. The experimentally determined value of epsilon0 (296.00K) for this cell is 4.03 +- 0.95 mV (strong electrolyte standard states, mole-fraction composition scale); the value of the equilibrium constant for the reaction is 1.17 +- 0.05. The contributions of isotope-exchange and transfer effects to the magnitude of the standard Gibbs energy change for the above reaction and for the analogous reaction 1/2D2(g) + HCl(aq) = DCl(daq) + 1/2H2(g) are considered. These results support the conclusion of Heinzinger and Weston that the formulation of the solvated proton in water as H3O+, as opposed to H9O4+, is sufficient for the interpretation of the thermodynamics of hydrogen--isotope--exchange reactions in water. The formulation of the solvated proton in n-hexanol as ROH2+ was found to be sufficient for the interpretation of the results on the thermodynamics of hydrogen-isotope-exchange in n-hexanol