Diffusion coefficients measured by Taylor dispersion in a Fourier ring Aqueous lanthanum chloride at 25 °C

Abstract

A new dispersion experiment is developed by connecting the outlet of a short dispersion tube to the pump inlet, forming a closed-loop flow path. Instead of injecting solution samples into a large excess of carrier solution, one half of the circuit is filled initially with solution at concentration + (Δc/2) and the other half is filled with solution at concentration − (Δc/2). Pumping these solutions around the circuit generates exponentially damped concentration oscillations in a refractometer detector. Fourier analysis of the detector signal gives the mutual diffusion coefficient. The average concentration of diffusing solute is constant, so relatively large concentration differences can be used to measure differential diffusion coefficients. Consequently, baseline drift in the detector signal is negligible. Moreover, recirculation drastically reduces the required volume of solution. Closed-circuit dispersion measurements are reported for binary aqueous solutions of sucrose, glycine, urea, NaCl and KCl. The calibrated circuit is used to measure the binary mutual diffusion coefficient of aqueous LaCl ₃ at concentrations from 0.015 to 2.784 mol dm ⁻³ at 25 °C. The results are used to evaluate the resistance coefficient of a trivalent metal salt.