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

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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) n NH 3−n + H 2 O(Et) n NH 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) n NH 3−n molecules and the (Et) n NH 4−n + ions. The diffusion coefficient of the (Et) 3 NH + ion is found to be 14% smaller than that of the (Et) 4 N + 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.

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