M. De Paz
Institute of Physics University of Genoo Genoo, ltaly
A Quantitative Diffusion Experiment for Students
A
properly conducted experiment on diffusion can play an important role in the education of chemistry students. Most of the kinetic processes are in fact governed by diffusion, and this must be well known in order to solve many technological problems. Furthermore, since a diffusive process is strictly related to the molecular structure of the system in which it occurs, the comparison of experiments with theoretical models is particularly stimulating. This approach must be stressed because of its importance. Experiments which can be compared quantitatively with their corresponding theories should be devised. An experiment is presented to the students as a complete problem. Quantitative measurements are taken and compared, when possible, with previous results. After a thorough discussion of the errors, a comparison of data with the predictions of available theories is carried out. Works describing diffusion experiments for students were previously published in THIS JOURNAL. Kraus and Tye put two liquids in contact within a capillary tube and followed the rate of mixing by measuring the change of refractive index with concentration (1). Nishijima and Oster, to obtain short time measurements on liquids, developed a microscopic technique using an interference wedge pattern (@. Watts described a class experiment consisting of the quantitative analysis of the total amount of a liquid diffused into a capillary containing another liquid (5). Finally Gover reported a study of gaseous diffusion performed by students; the concentration was measured in a gas chromatograph (4). I n this paper a simple colorimetric method is described which has been satisfactorily used in the physics laboratory course for chemistry students held at Genoa during 1967. The svstem under studv is an ionic salt solution. To stimulate the interest of students a comparison of their results with current theory is discussed. When the correlation between the diffusion process and the electrical conductivity of ionic solutions is met, new experimental problems are naturally suggested.
PURE WATER
u Standard KMnO,+ solution
Figure 1.
Experimental set up and wall effect on diffusion.
solution is required during the first minutes of the experiment (see Treatment of Data). A wall effect is observed which is discussed later in the section on Comparison with Existing Data. Quantitative concentration measurements as a function of height in the tube are performed after 1 to 2 hr by means of the colorimeter illustrated in Figure 2. The simple transistor circuit of Figure 3, assembled by the students, amplifies the A3PL photovoltaic cell outputdjo that it can be measured with a common 100yA 2000-Q meter. The circuit is subject to zero drifts and must be frequently adjusted and calibrated; this, however, is very good training for students. The calibration is done with several samples of known concentration between the
Experimental Procedure
A thin-walled, soft glass tube (length 10 em, i.d. 2 mm, 0.d. 2.4 mm) sealed a t one end is completely filled with distilled water. Owing to the cohesive forces, the water remains in the tube upon inversion. At time t = 0 the open end of the tube is vertically immersed into a large beaker (200 cm3) containing a standard IWInOa solution (0.4% weight) slightly acidified with HzSOa (Fig. 1). The I
