Thermodynamics of protonation of alkanolamines in aqueous solution to 325° C

• Published in: Thermochimica acta 1989-11, Vol.154 (1), p.119-127
• Main Authors: Oscarson, J.L., Wu, G., Faux, P.W., Izatt, R.M., Christensen, J.J.
• Format: Article
• Language: English
• Subjects: Chemical thermodynamics; Chemistry; Exact sciences and technology; General and physical chemistry
• ISSN: 0040-6031; 1872-762X
• DOI: 10.1016/0040-6031(89)87124-2

Equilibrium quotients ( K') for the protonation of diethanolamine (DEA), 2-(2-aminoethoxy) ethanol or diglycolamine (DGA), and N-methyldiethanolamine (MDEA) were determined potentiometrically over the ionic strength range 0.004–4.0 at 298.2 K. Corresponding Δ H values were determined by a flow calorimetric procedure at six temperatures from 299.9 to 422.1 K. At 299.9 K the protonation reactions become less exothermic, i.e. -Δ H decreases, in the order DGA (primary amine), DEA (secondary amine), MDEA (tertiary amine). From 299.9 to 422.1 K this order remains unchanged but the differences among the Δ H values decrease. The Δ H results are used, together with the 298.2 K lg K values (valid at ionic strength, μ = 0), to calculate the dissociation constants of the corresponding protonated alkanolamines from 298.2 to 423.2 K. The effect of μ on the lg K value (valid at μ = 0) for proton ionization from protonated alkanolamines is large. The change of lg K' with μ is nearly insensitive to substitution of functional groups on the nitrogen atom. The isocoulombic reaction principle was used to extrapolate K and Δ H to 423.2 K. Equations are given describing lg K, Δ H, Δ S and Δ C p at μ = 0 from 298.2 to 423.2 K.