Ionization of acetic acid in aq. sodium chloride media: a potentiometric study to 573K and 130 bar

Abstract

The ionization quotients of aqueous acetic acid have been determined precisely in NaCl(aq) media to 5 mol/kg from 50 to 300 °C with a potentiometric cell previously developed at Oak Ridge National Laboratory. Pressure coefficients were also determined to 250 °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 AH0, AS0, ACP°, and AV°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/cm3 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.

Ionization constants of aqueous amino acids at temperatures up to 250°C using hydrothermal pH indicators and UV-visible spectroscopy: Glycine, α-alanine, and proline

“…The excellent agreement with the previous work on acetic acid by Mesmer et al (1989) confirms findings by Johnston (1994, 1997), Ryan et al (1997), and Chlistunoff et al (1999) that accurate ionization constants can be obtained with colorimetric indicators under hydrothermal conditions (see also Méndez De Leo et al, 2002). The combination of thermally stable colorimetric indicators with high-pressure flow cells and UV-visible spectrophotometry provides a fast and accurate method for determining dissociation constants for weak acids and bases at extremes of temperature and pressure.…”

“…The results, which are tabulated in Table A1, reproduced those reported by Mesmer et al (1989), with a maximum discrepancy of 0.14 log K a,COOH units and an average discrepancy of Ϯ 0.05 log K a,COOH . These lie within the combined experimental uncertainties, thus demonstrating that our methodology for measuring the ionization constants of weak acids and bases can yield accurate results.…”

“…The method was tested by determining Q a,COOH for acetic acid from 25 to 225°C for comparison with accurate hightemperature potentiometric measurements by Mesmer et al (1989). The results, which are tabulated in Table A1, reproduced those reported by Mesmer et al (1989), with a maximum discrepancy of 0.14 log K a,COOH units and an average discrepancy of Ϯ 0.05 log K a,COOH .…”

“…A1. Comparison of log K a,COOH for acetic acid with the predicted indicator range of acridine:; ______ , log K a,COOH of acetic acid (Mesmer et al, 1989); --, log K HAcrϩ Ϯ 1 log unit. …”

“…The results are summarized in Table A1. The Q a,COOH values calculated from Mesmer et al (1989) are included for comparison. …”