A Simple Refraction Experiment for Probing Diffusion in Ternary Mixtures

Abstract

Diffusion is a fundamental phenomenon that is vital in many chemical processes such as mass transport in living cells, corrosion, and separations. We describe a simple undergraduate-level experiment based on Weiner’s Method to probe diffusion in a ternary aqueous mixture of small molecular-weight molecules. As an illustration, the experiment described here uses potassium chloride salt (KCl) and sucrose in water. The simple and inexpensive laboratory setup relies on the deflection of a laser beam across a sharp interface between the aqueous mixture and pure water. The refractive index gradient that results from a change in the concentration of the solute species causes refraction of the laser light. As diffusion occurs, the sharpness of this concentration gradient declines and is manifested in a decreased bending of the laser light. Experimental measurements of binary diffusivities for KCl in water and sucrose in water using this setup are combined with a simple data fitting algorithm to determine the relative fractions of KCl and sucrose in a ternary mixture with water. The experimental setup is easy to assemble and visually informative for instruction of diffusional mass transfer in engineering or physical chemistry. In addition, we show that it is relatively simple and inexpensive to incorporate concepts such as digital capture of the laser deflection using a web camera, digitization of the concentration profiles, and multi-variable, non-linear curve fitting. The laboratory experiment can also be extended to reinforce a variety of concepts ranging from simple statistical and error analysis to more advanced concepts of mass transfer such as the importance of cross-diffusional contributions in mixtures.