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

Values of the ionization constant of acetic acid in H₂O and D₂O (K_HAc and K_DAc) and the deuterium isotope effect, ΔpK = 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 ∼ 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, Λ, of the strong electrolytes were used to calculate the molar conductivity at infinite dilution, Λ°, using the Fuoss−Hsia−Fernández−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 the ionization constants of acetic acid. This same procedure was done for the pertinent deuterated solutes in D₂O. Measured values of log K_HAc, log K_DAc, and ΔpK 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.; Herting, D. L. J. Solution Chem. 1978, 7, 901−913). The deuterium isotope effects, ΔpK, become independent of temperature above ∼420 K, at a value approximately 0.1 unit lower than that at 298 K. These values are ΔpK = 0.43 ± 0.01 and ΔpK = 0.51 ± 0.01, respectively. The temperature dependence of the Walden product ratio, (λ°η)_D2O/(λ°η)_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 ∼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.