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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 |
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| Authors: | Rodney G.F. Clarke Jenene C. Roberts Richard J. Bartholomew Peter R. Tremaine |
| Affiliation: | 1 Department of Chemistry, Memorial University of Newfoundland, St. John’s, NL, Canada A1B 3X7 2 Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Guelph, Guelph, ON, Canada N1G 2W1 3 University of Ontario Institute of Technology, Oshawa, ON, Canada L1H 7L7 |
| Abstract: | Ionization constants for several simple amino acids have been measured for the first time under hydrothermal conditions, using visible spectroscopy with a high-temperature, high-pressure flow cell and thermally stable colorimetric pH indicators. This method minimizes amino acid decomposition at high temperatures because the data can be collected rapidly with short equilibration times. The first ionization constant for proline and α-alanine, Ka,COOH, and the first and second ionization constants for glycine, Ka,COOH and Ka,NH4+, have been determined at temperatures as high as 250°C. Values for the standard partial molar heat capacity of ionization, ΔrCpo,COOH and ΔrCpo,NH4+, have been determined from the temperature dependence of ln (Ka,COOH) and ln (Ka,NH4+). The methodology has been validated by measuring the ionization constant of acetic acid up to 250°C, with results that agree with literature values obtained by potentiometric measurements to within the combined experimental uncertainty.We dedicate this paper to the memory of Dr. Donald Irish (1932-2002) of the University of Waterloo—friend and former supervisor of two of the authors (R.J.B. and P.R.T.). |
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