Crossover from molecular to ionic diffusion in dilute aqueous solutions of hydrolysed ethylamine, diethylamine and triethylamine

Abstract

Binary mutual diffusion coefficients (D) are reported for aqueous solutions of ethylamine, diethylamine and triethylamine at concentrations from 0.2 to 100 mmol dm⁻³ at 25 °C. Hydrolysis of each amine [(Et)_nNH_3−n + H₂O(Et)_nNH_4−n⁺ + OH⁻] produces a sharp increase in D with increasing dilution. Guided by equations developed previously for weak-acid diffusion, the results are analysed to determine the limiting diffusion coefficients of the (Et)_nNH_3−n molecules and the (Et)_nNH_4−n⁺ ions. The diffusion coefficient of the (Et)₃NH⁺ ion is found to be 14% smaller than that of the (Et)₄N⁺ ion, despite the smaller size of the former. Taylor dispersion is used to make the diffusion measurements. Below 1 mmol dm⁻³ the sharp upturn in D causes the measured diffusion coefficients to vary with the initial concentration difference. In this region the measured D values are interpolated or extrapolated to zero initial concentration difference to determine the differential mutual diffusion coefficient for each carrier-stream composition.