Autoprotolysis constants of ethylene glycol–water mixtures at different temperatures and related thermodynamic quantities

Abstract

Autoprotolysis constants (K_s) of glycol–water mixtures (containing 10, 30, 50, 70, and 90% by weight of glycol) were measured at different temperatures ranging from 5–45° using a cell of the following type: Pt, H₂(1 atmosphere)|NaS| MBr, AgBr–Ag. where M stands for Na or Li, and S^– stands for lyate ion.

The experimental data can be fit in the form of equations of Harned–Robinson type using the least square method as given below: For 10%, pK_s= [graphic ommitted] + 2·77 + 0·00329T. “ 30%, pK_s= [graphic ommitted] + 1·49 + 0·00708T. “ 50%, pK_s= [graphic ommitted] – 7·11 + 0·02271T. “ 70%, pK_s= [graphic ommitted] + 4·38 + 0·00433T. “ 90%, pK_s= [graphic ommitted] – 2·67 + 0·01711T.

The value of pK_s is found to pass through a minimum at a value of around 50% by weight of glycol. This has been interpreted in the light of the dielectric constant as well as the relative acidity and basicity of glycol and water.

Thermodynamic quantities such as ΔH°, ΔG°, ΔS°, and ΔC_p° for the processes have been evaluated for 25°.