First Ionization Constant of Phosphoric Acid and of Acetic Acid in H₂O and D₂O from T = 373 K to 573 K at p = 11.5 and 20 MPa by AC Conductivity Methods

The first ionization constants of phosphoric acid and acetic acid have been measured in H₂O and D₂O from T = 373 K to T = 573 K and p = 11.5 and 20 MPa to yield accurate values of the deuterium isotope effect. Sequential conductivity measurements using a unique high-precision flow-through AC conductance instrument were made on dilute (m ≤ 10^–2 mol·kg⁻¹) aqueous solutions of phosphoric acid, acetic acid, potassium dihydrogenphosphate, sodium acetate, potassium hydroxide, sodium hydroxide, hydrochloric acid, potassium chloride and sodium chloride in light and heavy water under the same experimental conditions (temperature, pressure, flow-rate), so that systematic experimental errors between the two solvents would cancel. The experimental molar conductivities of potassium dihydrogenphosphate, sodium acetate, hydrochloric acid, and the corresponding chloride salts were used to calculate the molar conductivities for the fully dissociated acids [λ(D⁺), λ(D₂PO₄⁻) and λ(CH₃COO⁻)]. Together with the molar conductivities measured for partially ionized acids, Λ(D₃PO₄) and Λ(CH₃COOD), these yielded values for the degree of dissociation, α, and the ionization constants, pK_a1. The iterative process was repeated at each temperature in both H₂O and D₂O where the Fuoss-Hsia-Fernández-Prini (“FHFP”) and the Quint-Viallard (“QV”) equations were used to correct for ionic strength. The resulting values of pK_a1 for phosphoric acid in H₂O agree with those reported from conductivity studies by previous works over the entire temperature range and with low temperature potentiometric studies to within the combined experimental uncertainties. The results for pK_a1 above 298.15 K in D₂O are the first to be reported in the literature. The new values for pK_a(CH₃COOD) yield more accurate values for the deuterium isotope effect on the ionization constant of acetic acid than those reported in our previous work (Erickson et al. in J. Phys. Chem. B. 123:9503–9506, 2019). The single-ion limiting conductivities for dihydrogenphosphate and acetate in D₂O, λ(D₂PO₄⁻) and λ(CH₃COO⁻), were found to be the same as those in H₂O once corrected for viscosity effects, confirming previous observations for other ions.