In the Classroom edited by
Overhead Projector Demonstrations
Doris K. Kolb Bradley University Peoria, IL 61625
Slide Projector Corrosion Cell
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Silvia Tejada, Estela Guevara, and Esperanza Olivares Posgrado Edificio “B”, Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, México D.F. 04510 México
Corrosion of metals, especially iron, is a serious problem that costs billions of dollars every year. The process of corrosion can be demonstrated in a slide projector (since the cell is in the shape of a slide) or on the stage of an overhead projector by setting up a simple galvanic cell. Corrosion occurs as the result of a galvanic cell reaction, in which the corroding metal acts as the anode. Several simple demonstrations relating to corrosion are described here. We have found this kind of presentation to be very successful. Even in a large lecture room, the demonstration can be seen easily by all the students. To introduce the cell into a slide projector it is necessary to take off the carrousel and put the cell with the solution and electrodes into the projector orifice, as you would do with a conventional slide. The cells are manufactured by using an opaque acrylic plate 3 mm thick; the plate is cut with the dimensions stated in Figure 1. Afterwards it is covered with transparent thick polyethylene by using an instantaneous glue, which must seal the area to prevent water leaks. The thickness of the cell is approximately 4 mm including the acrylic and the polyethylene. Using a clear acrylic sheet to cover the cell faces is not recommended because the cell becomes very thick. As an instantaneous glue we use cyanoacrylate (Kola Loca is the commercial name). In the top part of the cells a Velcro ribbon can be placed to hold the metal plates functioning as electrodes. These cells can also be used for other kinds of chemical reactions involving change in color, gas evolution, etc.. To project the cell with an overhead projector we recommend putting the cell on the stage of the projector with an inclination of approximately 10°, to prevent the solution from overflowing. For this purpose it is possible to use a base of the same acrylic plate of approximately 1 cm high by 7 cm long. We also recommend flat acrylic cells, using a tilted acrylic support of the type described by Alyea (1). (Alyea also describes the flat acrylic cells for overhead projectors.) The electrolyte solution for the cell is 10% sodium chloride (NaCl) to which has been added a little phenolphthalein and potassium ferricyanide. To prepare the solution dissolve 10 g of NaCl in about 100 mL of water, and then add about 1 mL each of potassium ferricyanide solution (1 g in 100 mL of water) and phenolphthalein (2 g in 50 mL of ethanol). Adjust the pH of the solution to 7 by dropwise addition of 0.01 M HCl. Galvanic Cell This is a type 1 cell (Fig. 1A) containing an iron (Fe) strip and a copper (Cu) strip immersed in the electrolyte solution and connected by an external copper wire. For this demonstration copper and iron metals are especially recommended. By having a little phenolphthalein and potassium
Velcro
8 cm
Polyethylene
Acrylic
6.5 cm
5 cm
A
B
Figure 1. Two cell types. A: type 1; B: type 2.
ferricyanide in the sodium chloride solution, we can readily determine which electrode is the cathode and which is the anode, since the cathode turns red and the anode turns blue. Reduction takes place at the cathode, with the following reaction occurring at the copper strip: O2 + 4 e{ + 2 H2O → 4 OH {
(1)
The hydroxide ion causes the phenolphthalein to change from colorless to red. Meanwhile, oxidation takes place at the iron anode: Fe – 2 e{ → Fe2+
(2)
2+
The Fe ion reacts with ferricyanide to produce ferrous ferricyanide, Fe3[Fe(CN)6 ]2, which is blue. The more electropositive metal acts as the anode and the other metal as the cathode. The electromotive force (emf ) of the cell, which is the potential difference between the metals, can be measured with a voltmeter. Differential Aeration Cell This kind of cell can produce significant damage in metal areas without air. For example, the airless surfaces in buried pipes tend to become corroded. The areas with air have higher oxygen concentration, which favors reduction according to equation 1. The differences in oxygen concentration along a pipe can produce a cell potential and a flow of electric current. This demonstration cell is of type 2 (Fig. 1B), with iron strips in each of the two compartments. The metal strips are immersed in the electrolyte solution and are joined by an external copper wire. The compartments are also connected with a salt bridge, which might be a piece of yarn or filter paper saturated with the electrolyte solution. To carry out the demonstration, inject air into one of the compartments with a syringe. The electrode with more air turns red, since it is the cathode. The other electrode, the anode, turns blue. (The blue color appears very soon; the red color usually takes a little longer. If the solution has not been adjusted to pH 7, the red color may be quite slow in appearing.)
JChemEd.chem.wisc.edu • Vol. 75 No. 6 June 1998 • Journal of Chemical Education
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In the Classroom
Differential Temperature Cell The rate of corrosion, like most chemical reactions, increases with increasing temperature. Higher temperature areas in a metal favor the oxidation half-reaction. Examples of the damage from differential temperature cells can be found in boilers. To carry out this demonstration use a type 2 cell with iron strips in both compartments joined by an external copper wire. The solution in one of the compartments is heated before being added to the cell, and the two solutions are connected by a salt bridge. The half-cell with the higher temperature turns blue, indicating that it is the anode; the lower-temperature half-cell turns red. Stress-Corrosion Cracking This demonstration shows that oxidation and reduction can occur in the same piece of metal because of stress-corrosion cracking. Although this can be shown in a type 1 cell, it can be done more easily in a small Petri dish. Place a nail in the electrolyte solution and let it stand for a while. Areas that are under stress (anodic areas) turn blue; the others turn red. Instead of a nail try using a large paper clip. Take the outer half of the clip and make a few bends in the metal. Let the ends of the clip touch each other, but do not otherwise disturb
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the inner half of the paper clip. Immerse the twisted paper clip in some of the electrolyte solution in a small Petri dish, and note that the stressed areas turn blue, while the unstressed areas turn red. Cathodic Protection There are various methods that can be used to avoid corrosion. One of them is cathodic protection, which involves the use of an external electric current. A more active metal is used to provide electrons to the metal that is being protected from corrosion (the cathode). The more active metal acts as a “sacrificial anode”. This cell consists of an iron strip and a zinc strip connected by an external copper wire and immersed in the electrolyte solution in a type 1 cell. Zinc is the more active metal, so it acts as the anode and iron acts as the cathode. A red color is observed around the iron strip, since iron is the cathode. There is no blue color because the blue appears only when iron is the anode. Literature Cited 1. Alyea, H. N. J. Chem. Educ. 1989, 66, 765–768. Tested Demonstrations, 6th ed.; Alyea, H. N.; Dutton, F. B., Eds.; Division of Education of the American Chemical Society: Easton, PA, 1965; pp 46, 104, 171.
Journal of Chemical Education • Vol. 75 No. 6 June 1998 • JChemEd.chem.wisc.edu
