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

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.).