edited by GEORGEL. GILBERT oenison university Granville. Ohio 43023
A Simple Demonstration of Ion Migration SUSM~TTED BY John G. Llttle St. Mary's Hlgh Sehool Stocklon, CA 95207 CHECKED BY
Ronald Strothkamp Holatra unluerslty Hempatsad, NY 11550
I n t h e C H E M S t u d y film, "Electric Interactions In Chemistry"1, migration of copper(I1) a n d chromate ions is illustrated using time-lapse photography. T h e present demonstration accomplishes t h e same goal, easily within one class period, using simpler apparatus. Water glass (sodium silicate solution) a n d household vinegar are used t o oreoare t h e gel, which is'the same one c o m m h y used t o demonstrate cwstal erowth. Some advance oreoaration is reauired since thk gel rakes several hours to set up. T h e gel is piaced in a U tube: electrolvtenolutions are added t o each side once the gel is firm. Descrlption of Apparatus The macrosetup (Fig. 1) uses a commercialsalt bridge (e.g.,Flinn Scientific #AP1117) and graphite (or other) electrodes; we used Sargent-Welch #S29125-32. The electrodes are connected to the power supply. We have used a LabVolt adjustable-voltage unit, but a 9-V battery, fitted with a Radio Shack battery clip (#270-325) and small alligator clips (Radio Shack #270-3731, works just as well far short-term use. If the apparatus is to be allowed to run for some time, expect significant loss of water due to electrolysis,especially at higher voltages. For the microversion, a 10-15-cm piece of 6-mm-diameter flint glass tubing is bent into a U shape; a Radio Shack battery clip completes the apparatus, with the Leads from the clip sewing as the electrodes. If the microscale version is used, students may prepare their own tubes the day before the experiment: the same tubes may he used later to observe the electrolysis of potassium iodide.
Figure 1. left Macro version. right. Micro version.
Sponge packing covered with plosfic film B = Gel mixture
A=
A=Electrolytesolutions B=Gelisetup1
Preparatlon of t h e Gel Dilute 150 mL of saturated sodium metasilicate (water glass) (we use Flinn Scientific #0102) to 1 L; this is alot, but it keeps welland can be used for other experiments. The day before the demanstration is to be done, combine equal volumes of the diluted silicate solution and ordinary household (white) vinegar. Fill the tub&) with the gel mixture to just above the curve, and allow to set; overnight is best. A Beral or Pasteur pipet is helpful for filling the microversion. For demonstrations, you might consider using three tubes rather than just one. In that way you will be able to demonstrate the CuZt and CrOaz-seoaratelv. as well as to~ether.The tubes may be prepared sever2 dais in advance; after ;bat time, cracks begin to form in the gel. Procedure To begin the ion migration, fill one side of the tube with copper(I1) sulfate and the other side with sodium or potassium chromate; 1.0 M solutions work well, although there is little evidence of CuCrOl precipitate if the process is continued until the ions meet. If three tubes are being used so that the ions can he observed independently, a 1.0 M KC1 solution will serve as the colorless electrolyte.
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Electric lnteractlons In Chsmlstry; Campbell, J. A,, and Hollenberg. J. L., collaborators; Chemical Education Materials Study, University of California: Berkeley, CA. 1964. Llorens-Molina, J. A. J. Chem. Educ. 1988: 64, 1090.
Figure 2. Petridish method Insert the electrodes (or wire leads) and connect to the power (see Fig. 1). The anode (positive electrode) should be placed in the copper solution, with the cathode (negative) in the chromate. For the microversion, movementofthe solutions is evident within 15-20 min. The larger version takes somewhat longer, as much as 2 h (more, ifyau want the copper(I1)and chromate ions to meet), so you may wish to have two sets: one to prepare in front of the class, and another that has been running awhile. Volume 67
Number 12
December 1990
1063
Alternative Method for Presenlatlon on Overhead Projector
The silica gel may he used in the Petri-dish method described by Llorens-Molina2. We have found that any fairly rigid packing material, wrapped with Parafilm or other laboratory film, will do for the semicircular sections, which are taped in place (Fig. 2). The gel is poured between the sections, almost to the full depth of the dish; setup time is about 6 h. When you are ready to begin the demonstration, remove the foam pieces then fill one side with 1 M CuS04or Cu(N0h and the other with 1 M NanCrOa. As Llorens-Molina suggests, paper clips may be used as electrodes, but we found that alligator clips soldered to the ends of the battery clip leads served very well as electrodes when clamped on the sides of the Petri dish. Migration becomes evident within about 20 min, although the chromate ion appears to migrate faster than copper(I1).so students may at first think that only one of the two is actually moving. Because two or three dishes can be displayed on the overhead together, you might want to prepare several at a time; in that way each class can see the nrocess as it heeins and as it anoears . . after an hour or two. Prolonged operation (we once let one run for an entire school day) eventually results in hrrakdwn of the gel and seriws deterioration of the alligator clips. Acknowledgment The author is indebted to James Domheck for his skill and patience in preparing and testing the procedure and for his helpful suggestions leading t o improved performance of the demonstrations.
The demonshation polarirneter in use.
A Classroom Demonstration Polarimeter S U B M ~ E BY D
Samuel G. Levine
North Carolina State U d r ~ l t y
Ralelgh, NC 27695 CHECKED BY
Jason Miller Denlson Unlverslly Granville, OH 43023 Most modern polarimeters are more versatile, more accurate, and faster t o use than their classical predecessors. As instructional tools. however. these black boxes with hidden components are nearly valueless. A partial remedy may be found in any of several descriptions of demonstration polarimeters which have appeared in this Journal over the past 20 years.14 All employ an overhead projector and are therefore suitable for use in large classrooms. The demonstration polarimeter described here mas be assembled in minutes from common laboratory materiais and is particularly convenient for use in small classrooms or in the laboratory. As shown in the figure, the light source is a large flashlight (four 2B batteries), mounted vertically on a ringstand and pointing upward. A flattened piece of red celluloid from the label tab of a hanging file is Scotch-taped to the face of the flashlieht and serves as an adeouate lieht filter. A nair of polarzd clip-on sunglasses is support& so that one lens rests on the light filter and serves as a "polarizer". A 250-mL tall-form empty beaker (Coming 1140) is clamped above the polarizer and capped by a disk of aluminum foil with a 15mm hole in the center. A white card is mounted above this hole a t a 45' angle. Turning.on the flashlight casts a spot of red light in the Eard. The demonstrator then employs a second pair of polarized sunglasses as a hand-held "analyzer". Insertion of one lens in
1064
Journal of Chemical Education
the path of the beam leaves the red s ~ o with t its orieinal intensity, provided that the analyzer pair is oriented paFallel with the polarizer air. At riaht anales. the red mot entirelv disappears. ~ i l l i n the g beaker with water leads t o identic2 results. On the other hand, filling the beaker t o a depth of 10 cm (approx. 250 mL) with an aqueous solution of sucrose a t a concentration of 0.83 g/mL5 leads to a +50° to +60Dshift in the blackout point in accordance with the formula: where u is the observed rotation, la1 is the specific rotation (+66' for sucrose), c is concentration in grams per milliliter, and 1 is the length of light -path through the medium in decimeters. For a small classroom, this demonstration polarimeter has several advantaees over those based on the use of an overhead projector.-~he device is self-contained, lightweight, highly portable, and does not require a projection screen or darkened room. Also, all components are easily visible to the observer and correspond to the essential part8 of true polarimeter. ~
' Dorn, H. C.: Bell, H.; Birkett, T. J. Chem. Educ. 1984, 61, 1106.
Fernandez, J. E.: Gray, T. J. Chem. Educ. 1976, 53, 508. Hill, J. W . J. Chern. Educ. 1973, 53,574. 'Frank, F. J.; Kidwell, S.M. J. Chem. Educ. 1969, 46, 58. A 300-mL supply of this solution was prepared from 250 g of Sucrose and approximately 134 mL of water. Magnetic stirring and warming on a stirrer-hot plate will speed up the process. The cloudy solution (air bubbles) clears on standing.
