Clyde R. Dillard and Pony Hall Kammeyer Brooklyn College Brooklyn, New York
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I
An Experiment with Galvanic Cells For the general chemistry laboratory
The inclusion of an experiment on galvanic cells in the general chemistry course is somewhat inconvenient, particularly a t institutions with very large enrollments in the beginning course. Perusal of recent editions of general chemistry laboratory manuals reveals that four out of fifteen do not include any experiment on galvanic cells; the others use a "conventional" set-up involving a voltmeter and a U-tube salt bridge or a porous barrier. Only one manual1 suggests a means other than a meter for detecting the direction of electron flow through the external circuit. For very large enrollments, even with students working in pairs, the cost of voltmeters and of the relatively large amounts of chemicals required is considerable. Moreover, among beginning students, the casualty rate of U-tube salt bridges is high. Porous cups or H-tube cells with sintered glass harriers2 are more suitable for small classes. Some electrochemical experiments for freshmen have been described by G ~ r m a n . These ~ are done hy students working in groups of two or three. The galvanic cell design descrihed below has heen used by first semester general chemistry classes of approximately 1000 studeuts workinp individually. I t requires no special apparatus, and because the quantities of materials used per student are small, the total cost is relatively low. Each student constructs a workable cell using metal-metal iou couples desiguated by the instructor, determines the direction of electron flow in the exterual circuit, and compares the relative electromotive potentials of several redox couples. To introduce the experiment on galvanic cells, the concepts of oxidation-reduction couples, half-cells, and electrode reactions in both galvanic and electrolytic cells are r e v i e ~ e d . ' . ~I t is stated that a complete galvanic cell circuit cousists of two half-cells, an external conductor, and an electrolytic contact between the half-cells. The electrolytic contact is usually a salt solution which separates the two half-cells while providing ions to couduct a current between them. Consequently it is termed a salt hedge. The form and constit,utiou of the salt. bridge is a matter of practical convc:iietrce. I LAUBENGAYER, A. W., "Experiments and Problems in General Chemistry," Rinehart and Co., New York, 1959, Expt. 34. ANDREWS, D. H., AND KOKES,R. J., ''Laboratory Manual for Ft~ndamental Chemistry," John Wiley and Sons, Yew York, 1962, rhap 12. ( ~ R M AJ.. N .J. CHEM.EDUC.. 34. 409 (1957). . . ' LATIMER; W. M., "Oxidation ~otentials,"2nd ed., PrenticeHall, New York, 1952. "IANIELS, F., A N D ALBERTY,R. A,, Thysicit1 Chemistry," 2nd. ed., Jdm \Wey and Sons, New Ynrk, 1961, Chap. 14.
The electrolysis of water is discussed. It is pointed out that a t the electrode of the electrolytic cell which supplies electrons from the external circuit there is an increase in the concentration of hydroxide ions; while a t the electrode from which electrons are drawn into the external circuit, there is an increase in the concelltration of hydrogen ions. Thus by using the galvanic cell t o electrolyze water containing a suitable indicator, the direction of electron flow from the galvanic cell may be deduced. The Experiment The galvanic cell consists of three parts: a coppercopper ion reference half-cell, a salt bridge, aud a metal-metal ion half-cell. Construction of each part is described below. Copper Reference Half-Cell. About 4 in. of a 10-in. piece of fine copper wire (B & S gauge #24) is made into a coil hy winding onto a small nail or a straightened paper clip. The tube containing the salt bridge is filled with 1.0 A[ CuS04 by means of a capillary pipet, and the copper coil is inserted. See Figure l a . The cell is then assembled as is shown in Figure lb. Salt Bridge. A &in. length of 6-mm od soft glass tubing is fire-polished on one end, and the other end is heated and rotated in the fl.ame until it contracts t o about one-half its original diameter. When cool the tube is inserted into a #1 one-hole rubber stopper with the constricted end down, and a very small wad of cotton (or a few strands of fine glass wool) is inserted
Metal Electrode
(b Figure 1.
Cu, Cui+,
half-cell, lo1 and orrernbled golvonic cell.
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down the tuhe to the constriction. The constricted end of the tube is dipped into a hot NaNOa-agar solution (prepared by the i ~ s t r u c t o r )so ~ that the liquid rises just above the cotton. The function of the cotton is merely t o give the agar gel mechanical strength. (See Figure la.) The tube is removed and set aside ahout 10 min. Metal-Metal Ion Half-Cell. Each student is assigned a specific metal in the fonn of a strip or a rod. After the surface of the metal is cleaned with sandpaper or treated according to the detailed instmctions for specific metals (given below), a 6-in. piece of copper wire is attached. Twenty ml of 1.0 M solution of the appropriate metal ion is put into a 6-in. (18 X 150 mm) test tube, the metal electrode is inserted, and the copper lead wire is bent over the edge of the test tube so that only the electrode is in contact with the solution. Certain metals require special treatment before use. The following directions are given: Aluminum. Dip into a test tuhe of 6 M HCI until bubbles of hydrogen are observed all over the surface. Do not rinse. Use immedistdy, before the oxide coating reforms. Magnesium, Zim. If the metal appears oxide coated, dip quickly into dilute HCI then rinse with water. Silver. Enough silver is produced by reduction on the surface of a graphite rod to function as a ailver electrode. After cleaning the surface of a. graphite rod with fine sandpaper, attach a 6in. copper wire to the rod. The silver deposit is formed when the cell is assembled.
Indicator Strip. To indicate the direction of current, a strip of Universal pH indicator paper7moistened with saturated NaN03 may be placed across the lead wires as shown in Figure lb. A blue color develops about the wire carrying electrons away from the galvanic cell, while a red color develops a t the other wire. A strip of filter paper moistened with NaN03 containing a few drops of phenolphthalein provides a less sensitive indicator; also, a color change is observed at. only one wire. Comparison of Cells. Cells are compared by connecting their copper reference electrodes together and placing the indicator strip across the leads from the respective metal electrodes. (See Figure 2.) The number of comparisons made will depend on the metals available and the amount of t i e allotted to laboratory work. I n a three hour period, students were able to construct cells and make comparisons with a t least two others. Metals and comparisons were assigned in patterns of the type shown in Table 1. Such patterns are designed to provide for an appreciable difference in voltage between cells being compared, to insure that each individual would have different results to report, and to avoid congestion and confusion. One or two voltmeters per class are sufficient to permit students to obtain numerical values for the voltages of their cells. These can be compared with Eo calcu-
To prepare the salt bridge solution, add 30 ml of water to 0.6 grams of powdered agar in a 150 ml heaker. Heat on a water hath until the mixture appeam homogeneous. Then add 8 drops of saturated NaNO, to the hot solution and leave on the water hath at low heat until all of the students have constructed their copper half-cells. The above quantity is sufficient for 2 5 8 0 students. ' "Universal pH Indicator Paper," from Will Scientific, Inc., New York, New York, or "pHydrionn Paper, from MicrrrEssenti$ Laboratory, Brooklyn, New York.
364/ Journal o f Chemical Education
lated from tables. Though agreement with the calculated E" is seldom good, the experimental values are consistent and precise. The students may be encouraged to investigate the reasons for the discrepancy. Reports. The student is required to make schematic diagrams of the cells which he has constructed, labeling the parts of the cell and indicating the directions of electron flow and ion migration. He must write electrode reactions and balanced cell reactions and then compute the EO's for the cells. Finally, he is t o arrange the metals which he compared into an abbreviated electromotive series. All of this is done in the laboratory as part of the experiment, and all work is handed in before the student leaves the class. Results
Nearly all students were able to determine the direction of electron flow from their cells by observing color development on indicator paper. Measurements using inexpensive voltmeters8 gave results which were always lower than the W's calculated from tables. However, students working independeutly on the same assigned cells obtained voltages that were similar and reproducible. Voltages from combination cells were also lower than theoretical but were consistently in the same order as the electromotive series. This is shown in Table 2. In the table the values under "Cu" are those obtained from single cells. Rough indications of the experimental electromotive series were obtained also from comparisons of the Weston Instruments Model 201, 0-3 v dc, 1000 ohms/v.
Moist Indicator Paper
Figure 2.
Arrangement for comporiron of cells
Table I.
Assignment Pattern for Construction and Comparison of Galvanic Cells
Student no.
Metal electrode
1
AP.
To be compared with Zn Mg Pb Mg A1 Mg Ag Pb
and and and and and and and and
Cd Sn Ag Sn Fe
Zn
A1 Ag
relative times required for the developmeut of perceptible color changes on the indicator paper. Some precautions should be noted. Obtaining a color change in a reasonably short time requires that the maximum current be delivered by the cell. Aside from faulty connections of the lead wires, the chief factors which reduce the current are a p?orly constructed salt bridge and inadequately cleanedelectrodes. Table 2.
Tvpicol Voltages from Combination Cells
(A1 and Zn are particularly bad offenders.) For a good salt bridge, the constriction a t the end of the 6 mm inner tube should have an opening of a t least 2 mm. A minimum of cotton should be loosely inserted a t the constriction. To avoid rapid oxide formation on the
metals, the solutions of A13+ and Zn2+ should be slightly acidified. If the electrodes are used promptly after being cleaned with acid, they will work properly. The cells are compact, sturdy, and portable. Copper reference half-cells left standing in water were still usable after 10 days. Thus, if necessary, it is possible to extend the experiment over more than a single laboratory period. Although they have not been used in the general laboratory classes, half-cells containing mixed ions such as Fez+, Fe"+, and a graphite electrode have been shown to be feasible in the cell design described herein. For small classes or for selected students, many variations are possible. As was suggested by Gormaq3 "unknowns" may be assigned to be located in their appropriate places in the electromotive series, and concentration cells or nonmetal-anion cells may be designed. Student response to this experiment has been gratifying. They have derived considerable pleasure in constmcting and comparing the cells, and their interest has been reflected in a het,ter understanding of t,his part of the general chemistry course.
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