Thermodynamic study of the second-stage dissociation of N,N-bis(2-hydroxyethyl)-2-aminoethane sulphonic acid (BES) in water + 10, + 30 and + 50 wt % t-butyl alcohol from 278.15 to 308.15 K

Abstract

The second dissociation constants of N,N-bis(2-hydroxyethyl)-2-aminoethane sulphonic acid (hereafter known as BES) have been determined in 10, 30 and 50 wt% t-butyl alcohol + water mixtures at seven different temperatures from 278.15 to 308.15 K at intervals of 5 K. Precise e.m.f. measurements of cells (without liquid junction) of the type: Pt, H₂(g, 1 atm)∥BES;(m₁),NaBES(m₂),KBr(m₃) in (Bu^tOH + H₂O)|AgBr,Ag, have been made for the process BES^±(s)→ BES^–(s)+ H⁺(s). The thermodynamic quantities (ΔG^⊖, ΔS^⊖, ΔH^⊖ and ΔC^⊖_p) were derived from the values of pK₂ as a function of T using the following equations for: 10 wt %, pK₂=–2823.2/T+ 32.003 – 0.051426T 30 wt %, pK₂= 2140.2/T– 3.0071 + 0.0099986T 50 wt %, pK₂= 3303.0/T– 11.801 + 0.026813T. The results are discussed in terms of standard Gibbs free energies, enthalpies, entropies and heat capacities for the dissociation process, as well as for the transfer of BES from water to 50 wt% t-butyl alcohol + water mixtures. The implications of the results with regard to specific solute–solvent interactions (particularly stabilization of zwitterionic species) are also discussed.