In the Laboratory
A Simple Undergraduate Concentration Step Experiment John Cassidy, Fiona Fitzpatrick, and Sean Culhane Chemistry Department, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland
The theory associated with diffusion and Gaussian distributions is widespread, ranging from the expansion of a depletion layer in electrochemistry (1, 2) to the evolution of peak shapes in chromatography (3). Experimental methods for diffusion coefficient (D) determination have included the uptake of solvent by polymers (4), the examination of oxygen permeation into 1% agar (5), and water vapor diffusion (6). The notion of random walk as a pedagogical model for this effect has been expounded along with the thought experiment of a concentration step in a homogeneous solution. The aim of this experiment is to emulate a concentration step using two immiscible layers in a cuvette: the lower 1,1,1trichloroethane layer containing dithizone (diphenylthiocarbazone) and the upper layer containing a nonabsorbing metal ion in aqueous solution. The metal forms a complex with the dithizone (7), which has a spectrum different from the free dithizone; and the diffusion of the 1:2 metal dithizone complex away from the interface along with the diffusion of dithizone to the interface may be monitored spectroscopically. When a process is diffusion controlled, the relationship between absorbance A and time is given by (8) A = 2 ε C (Dt / π)1/2 and from a plot of absorbance against t1/2 it is possible to characterize whether the process is diffusion controlled and to determine D once ε is known. Experimental Procedure A single-beam or double-beam spectrometer may be employed. Zinc solutions (10 mM) were prepared in phosphate buffer pH 9. Dithizone was 0.02% wt/vol in 1,1,1trichloroethane (TCE). (CAUTION: Exposure to halogenated hydrocarbons may cause damage to kidney, liver or nervous system and should be minimized. The halogenated hydrocarbon should be used in a fume hood and a Teflon lid should be used for the cuvette. Dichloromethane may be used instead of 1,1,1-trichloroethane). Two milliliters of the 1,1,1-TCE layer was placed in the glass cuvette and the cuvette was placed in the spectrometer with a lid. Following this, 1 mL of the zinc solution was added carefully as a second layer on top of the organic layer, the timer was started, and the absorbance was monitored at 534 nm and/or 618 nm. A schematic view of the experimental arrangement is shown in Figure 1. The absorbance of the cell was monitored for 15–30 min at 534 nm for the complex and 618 nm for the unreacted dithizone.
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Figure 1. A schematic view of the cuvette in the spectrometer where the beam passes through the upper aqueous and lower organic layer.
Treatment of Results Linear A vs. t1/2 plots were obtained with a positive slope for 534 nm and a negative slope at 618 nm, and from Beer’s law plots drawn previously the extinction coefficient of the complex and the unreacted dithizone could be measured. Diffusion coefficients could be determined; for example a value of 2.6 ± 0.3 × 10{5 cm2 s{1 was determined, using a value of ε for dithizone to be 2.34 dm3 mol{1 cm{1. The value of ε was measured under the same experimental conditions (spectrometer, slit width) with water as the upper layer. The assumption is that the metal transfers cleanly across the interface; and that the kinetics of the metal ligand reaction are fast is borne out by the linearity of the A vs. t1/2 plots. The concentration of zinc is at least ten times that of dithizone, ensuring fast reaction. Literature Cited 1. Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; p 128. 2. Palombari, R. J. Chem. Educ. 1992, 69, 473. 3. Perry, J. A. An Introduction to Gas Chromatography; Dekker: New York, 1981; p 78. 4. Shanthamurthy Aithal, U.; Aminabhavi, T. M. J. Chem. Educ. 1990, 67, 82. 5. Fate, G.; Lynn, D. G. J. Chem. Educ. 1990, 67, 536. 6. Nelson, R. N. J. Chem. Educ. 1995, 72, 567. 7. Irving, H. M. R. H. Dithizone; Wright and Sons: Bristol, 1977. 8. Christensen, P. A.; Hamnett, A. Techniques and Mechanisms in Electrochemistry; Blackie Academic: London, 1993; p 200.
Journal of Chemical Education • Vol. 74 No. 8 August 1997
