Thermodynamics of protonation of amines in aqueous solutions at elevated temperatures

Highlights: → Effect of ionic strength and temperature on dissociation constants of amines. → Effect of ionic strength of temperature on enthalpies of protonation of amines. → Measured dissociation constants and enthalpies of protonation used for fitting. → Coefficients for thermodynamically consistent correlations given for 5 amines. - Abstract: The dissociation constants, pKa, of monoethanolamine (MEA), N-methyldiethanolamine (MDEA), 2-amino-2-methyl-1-propanol (AMP), 2(2-aminoethyl)etanolamine (AEEA), and piperazine (Pz) were measured by potentiometric titration over the temperature range (298.15 to 363.15) K. Enthalpies of protonation, ΔHp, were measured calorimetrically at temperatures from (298.15 to 393.15) K for MEA, MDEA, and AMP, and from (298.15 to 353.15) K for AEEA and Pz. In addition, the effect of the ionic strength of the solutions on the protonation of MDEA was studied using NaCl as background salt {(0 to 5.5) mol/kg-H2O)}. Correlations for the reaction equilibrium constants for proton dissociation are proposed for the studied amines based on the experimental data from literature and from this work. Both experimental enthalpy data and dissociation constants were used for fitting. The results from this work may be used for thermodynamic modeling of CO2 capture processes using amines.