Deuterium Isotope Effects on the Ionization Constant of Acetic Acid in H2O and D2O by AC Conductance from 368 to 548 K at 20 MPa

作者：Erickson K M; Arcis H; Raffa D; Zimmerman G H^*; Tremaine P R

来源：Journal of Physical Chemistry B, 2011, 115(12): 3038-3051.

DOI：10.1021/jp110337j

Values of the ionization constant of acetic acid in H2O and D2O (K-HAc and K-DAc) and the deuterium isotope effect, Delta pK = pK(DAc) - pK(HAc), have been determined from T = 368 K to T = 548 K at p = 20 MPa, using a flow-through ac conductance cell built at the University of Delaware. Measurements were made on dilute (ionic strength similar to 10(-4) mol.kg(-1)) solutions of acetic acid, sodium acetate, hydrochloric acid, and sodium chloride in H2O and D2O, injected in sequence at each temperature and pressure, so that systematic errors in the measured conductance of each solution, would cancel. Experimental values for the molar conductivity, Lambda, of the strong electrolytes were used to calculate the molar conductivity at infinite dilution, Lambda degrees, using the Fuoss-Hsia-Fernandez-Prini (FHFP) equation. These were used to calculate the molar conductivity at infinite dilution for acetic acid which was in turn used to calculate the degree of dissociation and finally was done for the pertinent deuterated solutes in D2O. Measured values of log K-HAc, log K-DAc, and Delta pK were obtained to a precision of +/- 0.008. The present results are in agreement with the only other accurate study at high temperatures and pressures (Mesmer, R E.; Hefting, D. L. J. Solution Chem. 1978, 7, 901-913). The deuterium isotope effects, Delta pK, become independent of temperature above similar to 420 K, at a value approximately 0.1 unit lower than that at 298 K. These values are Delta pK=0.43 +/- 0.01 and Delta pK=0.51 +/- 0.01, respectively. The temperature dependence of the Walden product ratio, (lambda degrees eta)(D2O)/(lambda degrees eta)(H2O)) indicates a change in the relative hydration behavior of ions, whereby the effective Stokes radii of the sodium, chloride, and acetate ions in D2O relative to H2O reverse above similar to 423 K. The results also suggest that the greater efficiency of the well-established proton-hopping transport mechanisms for OH- and H3O+ at 298 K, relative to OD- and D3O+, is significantly reduced as the temperature increases toward 548 K.

• 出版日期2011-3-31