Experimental determinations of diffusion coefficients in dilute aqueous solution using the method of hydrodynamic stability.

Abstract

Diffusion coefficients were experimentally determined in dilute aqueous solution at 25 +/- 0.1 degrees C, ionic strength 0.5 M, using Taylor's method of hydrodynamic stability. The methodology described is accurate enough to show significant differences in diffusion coefficients between the various ionic forms of the same species as a function of degree of ionization. In Taylor's method, diffusion coefficients were measured by allowing two solutions of differing solute concentration to contact in a capillary tube, forming a stable, measurable concentration gradient. The solute diffusion coefficient is a function of the gradient, the solution viscosity, the solution density, and some capillary dimensions. Viscosity was maintained constant across experiments and values of sufficient accuracy were available in the literature. Solution densities were measured with a tuning fork densimeter. Compounds studied were o-aminobenzoic acid, benzoate anion, the four forms of phosphate and citrate, and the zwitterionic forms of glycine, diglycine, and triglycine. Based on the results for the four forms of phosphate and citrate, experimental diffusivity values vary with the ionic state of the diffusant, presumably because of the altered state of hydration as charge varies. For the glycine series, the diffusivity showed an unexpected dependency on molecular weight (size).