Ionization of acetic acid in NaCl(aq) media: a potentiometric study to 573 K and 130 bar

The ionization quotients of aqueous acetic acid have been determined precisely in NaCl(aq) media to 5 mol/kg from 50 to 300{degree}C with a potentiometric cell previously developed at Oak Ridge National Laboratory. Pressure coefficients were also determined to 250{degree}C. The cell contains hydrogen electrodes in a concentration cell configuration and is operated in a flow mode. Results have been combined with selected information in the literature and modeled by both the Pitzer ion-interaction treatment and a conventional ionic strength approach. Thermodynamic quantities for the ionization reaction have been derived including the equilibrium constant, activity coefficient quotients, and pressure coefficients, along with the changes in enthalpy, entropy, heat capacity, and volume for the reaction. Dramatic increases in negative values for {Delta}H{degree}, {Delta}S{degree}, {Delta}C{sub p}{degree}, and {Delta}V{degree} are seen that appear to extrapolate to negative infinity at the critical temperature along the saturation vapor pressure curve. However, increases in pressure and salinity diminish this trend until, for example, at a density of 1 g/cm{sup 3} the quantities remain relatively constant as has been shown for other such processes involving ions. These results provide the first measurements of the activity coefficient ratio for the ionization of an organic acid at high temperatures. The results provide a basis for use of acetic acid-acetate as a relatively stable buffer for physical chemical studies and as a protonated ligand for metal complexation measurements of the kind reported recently from this laboratory. A comparison of the ionization constants and enthalpies for ionization of several organic acids shows modest trends with the electron-releasing character of the carboxylate.