Chemical Education Today
Letters Demonstrating Corrosion Using Galvanic Cells
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E / mV
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1% NaCl 3% NaCl 200 0.0
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5% NaCl 7% NaCl 1.5
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10% NaCl 15% NaCl 2.5
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Time / min Figure 1. Influence of concentration of NaCl solution on EMF of a Fe–Cu galvanic cell.
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Increasing attention in chemical education focuses on practical technical problems, including the particular problem of corrosion. A paper (1) published in this Journal is a good example: the authors provide a description of various demonstrations of corrosion projected for the entire class to see. We have several comments on the paper. The concentration of phenolphthalein indicated in the paper is too high (2 g phenolphthalein dissolved in 50 mL of ethanol). After addition of 1 mL of phenolphthalein of such concentration to 10% NaCl solution, a white precipitate of phenolphthalein appears. Experiments with students at the Kaunas University of Technology have established that the optimum concentration of phenolphthalein is 0.1 g/100 mL ethanol. These graduate students also raised the question: why is the concentration of NaCl 10%? Accelerated investigations of corrosion are carried out in 5% NaCl solution or in salt spray (2). Students measured the EMF of a Fe–Cu corrosion galvanic cell. The surface area of the electrodes was 58 cm2, and the pH of the NaCl solution was 7. Before the electrodes were immersed into the NaCl solution they were cleaned with emery paper and activated for 15 s in 5% H2SO4 solution. Figure 1 shows that the most significant changes of EMF occur during the first 1–2 minutes. At longer times a slight decrease of EMF is observed; after 30 hours it was about 20 mV lower than after 3 minutes. This can be explained by the fact that the surface of the Fe electrode is covered with Fe(OH)3 precipitate, which also appears in the NaCl solution. The increase of EMF at the initial stage of the experiment can be explained by activation of the electrode surface, which is apparently not accomplished during 15 seconds in 5% H2SO4 solution. The highest EMF is observed in 1% NaCl solution. This observation can be explained by the decrease of the solubility of O2 with the increase of the concentration of NaCl (3). The lowest EMF was observed in 7–10% NaCl solutions. Since the standard (2) required 5% NaCl solution, the influence of temperature was studied in 5% NaCl solution. No great influence of temperature on EMF was observed; Figure 2 shows that the biggest differences are observed at the beginning (0.5–1 minute). We hope that the data presented here supplement the paper (1), which is interesting and helps students to understand the problems of corrosion.
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50 C 40 C 30 C 20 C
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200 0.0
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1.5
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Time / min Figure 2. Influence of temperature on EMF of a Fe–Cu galvanic cell in 5% solution of NaCl.
Literature Cited 1. Tejada, S.; Guevara, E.; Olivares, E. Slide Projector Corrosion Cell. J. Chem. Educ. 1998, 75, 747–748. 2. International Standard ISO 9227:1990. Corrosion tests in artificial atmospheres—Salt spray tests. http://www.iso.ch/iso/en/ CatalogueDetailPage.CatalogueDetail?CSNUMBER=16859 (accessed Jul 2003). 3. Uhlig, H. H.; Revie, R. W. Corrosion and Corrosion Control. An Introduction to Corrosion Science and Engineering, 3rd edition; J. Wiley and Sons: New York, 1985; p 456. Algirdas Sulcius Department of General Chemistry Kaunas University of Technology LT-3028 Kaunas, Lithuania
[email protected] Journal of Chemical Education • Vol. 80 No. 10 October 2003 • JChemEd.chem.wisc.edu
