Dissociation Constant (p<i>K</i>_a) and Thermodynamic Properties of Some Tertiary and Cyclic Amines from (298 to 333) K

Abstract

The potentiometric titration method was used to measure pKa values of nine amines [2-(diisopropylamino)ethanol [2-DIPA], N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine [THPEDA], 2-{[2-(dimethylamino) ethyl]methyl amino} ethanol [2-DMAEMAE], tris[2-(2-methoxyethoxy)ethyl]amine [TMEEA], N-(2-hydroxyethyl) aniline [2-HEAN], 1-(2-hydroxyethyl)piperazine [HEP], piperazine [PZ], monoethanolamine [MEA], and N-methyldiethanolamine [MDEA] at (298.15, 303.15, 313.15, 323.15, and 333.15) K. pKa values of the last three amines were compared with published data to validate the procedure and assess the accuracy of the instrument. Thermodynamic quantities, such as standard enthalpy (ΔH°·kJ–1·mol–1) and entropy (ΔSo·kJ–1·mol–1) for the dissociation process, were determined at 298.15 K using the van’t Hoff equation. From the experimental results, the values of the standard state thermodynamic properties were derived and compared to the values of commercially available amines used as absorbents for CO2 capture. Among the studied amines, 2-DMAEMAE was identified as having a high pKa (9.18) and lower heat of dissociation than MEA (27.77 kJ/mol, as compared to 48.59 kJ/mol for MEA) and can therefore be considered a potential candidate for CO2 capture applications.