An Interesting Student Chemistry Project: Investigating Liesegang Rings Renato A. Schibeci School of Education, Murdoch University, Murdoch, Western Australia 6150, Australia Connie Carlsen School of Mathematical and Physical Sciences. Murdoch University, Murdoch, Western Australia 6150, Australia One of the important aims of science education is to give students an appreciation of the way in which scientists build up knowledge. To some extent, this can he done through second-hand sources: for examole. students can be given an -~~~~~~~ historical account of the way ih which theories of eomhustion develooed. so as to eive them some insight into scientific proceduresaand scientiGc theory-building. ' A supplementary, powerful mechanism for appreciating scientific methods for investigating problems is, obviously, first-hand investigation. Chemistry laboratory experiments, it is hoped, will do this. However, many experiments require students to follow a predetermined sequence of steps to a final answer that the; believe (or knowj the instructor has. These experiments can be valuable as a way for students to acquire specific lahoratory skills, but they may not provide any additional insight into scientific procedure. A laboratorv oroiect. on the other hand, has the potential to provide this fnsi'ght;provided i t is properly designed and executed. The oroiect can be used to encourage intellectual independence Ln &dents (that is, not to rely i n the instructor for answers), and to teach them some of the inquiry procedures used in scientific research. This paper describes such a project, based on the production of Liesegang rings. Rings of this kind are observed in nature in minerals such as malachite and handed agate. Liesegang rings are one example of oscillating chemical reactions; these reactions can oscillate in space (as in Liesegang rings) or in time (as in many classic reactions, such as the Belousov-Zhabotinsky reaction). T o oroduce such rines. - . two chemicals that form a sparingly solihle product are required; the aqueous solutionof one, added to the second in agel, can give rise to periodic precipitation. According to Flicker and Ross', this phenomenon was first reported in 1855 by Runge. However, the first systematic studies were reported hy Liesegang in 1898, and the phenomenon bears his name. ~~
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Procedure The basic procedure to produce Liesegang rings is as follows. Add gelatine (1.5 g) and one of the chemicals (2.5 g) to 50 mL of distilled water. Heat with stirring until the solution is clear. Pour into a standard test tube (25 X 150 mm) until it is ahaut two-thirds full. Stoooer the tube. and leave to set: this usuallv takes about 12 h. Add thbscrond solution nhove thegel nearly tof~ilthete-t tube. Do nut stopper, but rover rrith watch glass, nnd leave overnight. The hasic 3rtup isshown m the fiwre. Safety
All chemicals must be treated with caution, especially concentrated ("0.880") ammonia and sodium hydroxide solutions. students should not use these chemicals without instructor supervision.
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Flicker. M.; Ross. J. J. Chem. Phys. 1974, W9).3458.
Sample Results We have tried a variety of systems to produce Liesegang rings. A summary of our successful and unsuccessful attempts is given in the table. Selected results are described below.
Systems Used To Try To Produce Liesegang Bandsa Solution on too
Gal containino
systems that have producedLissegengrings within two wseks:
cobalt(ll)chloride magnesium chloride magnesium chloride copper(1l)sulfate manganese(1l)chloride uranyl nitrate uranyl nitrate uranyl nitrate capper(1i)chloride potassium chromate
concentrated ("0.880")ammonia sodium hydroxide (19 M) concentrated c'0.880")ammonia silver nitrate solution (0.1M) concentrated ("0.880")ammonia concentrated c'0.880")ammonia sodium hydroxide (19 M) silver nitrate solution (0.1MI sodium hydroxide (19 M) silver nitrate solution (0.1 MI
systems that did not produce Liesegang rings within two weeks: sodium hydroxide (19 M) chromium(l1l)sulfate sodium hydroxide (19 M) cobalt(l1)chloride chromium(ll1)chloride concentrated c'0.880")ammonia sodium hydroxide (19 M) chromium(ill)chloride sodium hydroxide (19 M) mangaoese(l1)sulfate sodium hydroxide (19 M) nickel(1i)sulfate nickei(1i)sulfate concentrated c'0.880")ammonia nickel(l1)chloride concentrated ("0.880")ammonia sodium hydroxide (19 M) capper(i1)sulfate copper(l1)sulfate concentrated ("0.880")ammonia .Using me method outlined in h e ~roseduresectlon ot ths text.
concentrated ammonia
('0.880') solution
Liesegang rings
--a
gel with cobalt (11)chloride
&sic equipment for producing Liesegang rings. Volume 65 Number 4
April 1988
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Cobali(1fi Chloride After a day, a blue band with black stripes formed and the concentrated ammonia in the top of the tube changed color. Eventually, over the course of approximately a week, the black rings separated and became recognizable blue rings with transparent spaces between each one; the space widened the lower down the tube the rings travelled. The ammonia in the top of the tube itself developed a "banded" look. This is a verv effective experiment that can be carried out in larger quantities, but these take longer to produce bands. The thinner the tube, however, the more interesting the result. Coppettlfi Chloride A dark blue hand formed where the gel and the sodium hydroxide met; this slowly separated (from the bottom up) into thinner bands. The bands that formed were not as clearly defined or as neat as those produced by the cobalt(I1) chloride experiment described above but were interesting nonetheless. Manganese(1fi Chloride This was a very rapid reaction; the white bands usually resulted within 24 h when concentrated ammonia was added to thegel.Theformation ofthe bands wasvery unusualsince the hands were not solid across the width of the test tube (as in the previuus reactions). Rather, they formed in two sections that were joined when the rings first formed but separated as the rings mo\.edown the tube. Theonly drawback to the experiment was that the ammonia in the top of the tube developed a dark brown preci~itatein ~atches.which detracted from the appearance oithe result. Uranyl Nitrate Rings formed after the addition of sodium hydroxide (19 M) or concentrated ("0.880") ammonia solution. With sodium hydroxide, the colors were slightly darker and the rings more clearly defined. The rings were a mauvelgray color with a green background; the bottom part of the tube became cream and the top . Dart . brieht vellow. Lieht seemed to play quite an important part in this reaction. Originally, we notwed that bands formed onlv in the half of the tube that faced the light; the remainder had no bands. When the test tube was rotated 180°, bands formed to match those alreadv in evidence. When silver nitrate (0.1 M) was added to the gel (instead of sodium hydroxide or ammonia solution), many very thin multicolored rings formed in "sets". For example, several colored rings formed, followed by a white ring. Next, more colored bands were observed, and then another white ring formed (and so on). Each "set". countine un from the bottom, had a few more rings than the previois i n e so the bands of color between the white rings increased in thickness.
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C o. ~. ~ e. d. lSulfate l) Rings formed when silver nitrate (0.1 M) was added to the gel. They were dark gray and did not form a reaular oattern. For instance, one ring would form, then a mu& thinner one, Shakhashlri, B. 2. Chemical Demonstrations: University of Wisconsin: 1985;Voi 2, pp 305-307. Stern, K. H. Bibliography of Liesegang Rings, 2nd ed.: US Nati. Bureau of Standards: US Govt. Printina Office: 1967. Field. R. J.; Bur&, M., Eds. ~scihtionsand Travelling Waves in Chem. Syst.; Wiiey New York; pp 565-604, 615-60. [Chem. Abs. 102(19)165923rl Epstein, I. R.: Kustin, K.; De Kepper. P.: Orban, M. Scl. Am. 1983, 284(3), 96.
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then perhaps three rings together followed by two rings and a few single bands. We found that a greater number of rings formed in smaller test tubes (10 X 75 mm) than the normalsized test tubes (25 X 150 mm). In the laraer test tubes. the rings were so pale that they could only he seen when thetest tube was held up to the light. Other Results Other successful results are summarized in the table, along with some unsuccessful attempts. We did not have any success with sodium silicate solution, which has been suggested2 as an alternative to gelatine. Dlmusslon There is an important reason for suggesting the production of Liesegang rings as a project for chemistry students. Unlike many standard chemistry experiments, there are no "correct" hvpotheses. A number of theories have been nroposed to explain the formation of Liesegang rings: a dis'cussion of some of these is aiven in Flicker and Ross'. There is. however, no universall; accepted theory to explain the& formation. Students are able to propose hypotheses to explain their observations without feeling that they need to come up with the "correct" hypothesis, which the instructor has in his or her head. Provided that the hypothesis is a chemically reasonable one, is consistent with the observations, and stands up to further experimentation, it is accepted as "correct" in the sense of "reasonable and the best explanation to date". ~ u r i n the g project, there are many possible specificinvestigations for students to pursue. At each stage. thev should he encouraged to formulaic hypotheses to explain the observations they make. Some possible investiaationsincludethe. following. (1) How does the concentration of reagents affect the formation of
Liesegang bands? (2) Does the diameter of the test tube affect the formation of
bands?
(3) Can bands he produced in glassware of different shapes (for example, conical flasksrather than test tubes)? (4) What are suitable media for producing Liesegang hands? (5) What is the distance between bands? (Is it uniform?) (6) Are the bands discrete or joined in a spiral? (7) What happens to the hands when they reach the bottom of the
tube? Students are encouraged to pursue one or more of these investigations. It is crucial that they~. begin to hypothesize reasons for their observations as soon as possibie and to develop further experiments to test their hypotheses. The interplay between hypothesis and observation will begin to take on a direct meaning for them, and they will gain a realistic understanding of how chemical knowledge is huilt up. I t is also hoped that they will gain an appreciation of both the drudgery and exhilaration of chemical practice. Because the formation of Liesegang rings is a slow process (taking hours to days), these experiments are probably more suited to the high school situation, rather than the standard college laboratory format (meeting once per week). lnhtructors wishing to follow up the ;hemistry of Liesegang rings may like to consult the bibliography by Stern", whirh contains 786 references. He describes how Lieseeane. -~~~~ in 1896, found that, when a silver nitrate crystal was added to a glass slide that was coated with a dilute solution of potas&m dichromate in gelatin, silver chromate formed in bands around the crvstal. General accounts of chemical oscillators, of which ~ : e s e ~ arings n ~ are but one example, are also available4.s. ~~
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