A Simple Educational Method for the Measurement of Liquid Binary Diffusivities

Abstract

A simple low-cost experiment has been developed for the measurement of the binary diffusion coefficients of liquid substances. The experiment is suitable for demonstrating molecular diffusion to small or large undergraduate classes in chemistry or chemical engineering. Students use a cell phone camera in conjunction with open-source image processing software to measure concentrations of a colored species in a clear liquid, as a function of position and time. Three fundamental principles in mass transfer and spectrophotometry are demonstrated: Fick's first and second laws of diffusion and the Beer-Lambert law for absorption of light. The measured value for the binary diffusion coefficient for potassium permanganate in water using this method was found to be within 10% of literature values at the 95% confidence interval. M olecular diffusion is a core component of the syllabus for the teaching of undergraduate chemistry, physics, and chemical engineering. It has been reported that students find this particular concept difficult to grasp, 1−3 and therefore, practical demonstrations of the phenomenon are valuable teaching aids. This article describes a simple, low-cost experimental technique which enables students to measure binary diffusion coefficients (D AB ) of a colored species dissolved in a clear liquid.Many studies have shown that students find molecular diffusion difficult to conceptualise, even at third-and fourth-year undergraduate level. Westbrook and Marek 1 showed that, despite advanced instruction, undergraduate-level science students had no better understanding of diffusion than seventh-grade life science students. In an investigation into the effect of simulated experiments, Meir et al. 2 identified the following specific misconceptions and lack of knowledge:• the concept of concentration • the relationship between an amount of substance and its concentration • kinetic energy and random molecular motion of a species • species supposedly having a directional motion toward regions of lower concentration of that species • equilibrium and the effect on diffusion rates as equilibrium is approached Marek et al. 3 showed convincingly that students who underwent a laboratory-based interactive learning cycle showed a 94% improvement in understanding. Students who attended expository teaching activities (such as lectures and discussions) showed only a 58% improvement in understanding. Similarly, Meir et al. 2 demonstrated improved understanding after students performed simulated experiments on a computer. These studies highlight the effectiveness of practical activities in the learning of diffusion concepts.Many experimental methods for the study of diffusion have been reported, all of which use costly apparatus, and/or require knowledge of various physical properties of the species which are not readily available. Kraus and Tye 4 used a backlit capillary tube and cathetometer telescope, having first measured the refractive characteristics of the apparatus and the solvent. Nishijima and Oster 5 developed an op...

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