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 eomhus- tion 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 instruc- tor 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).

To oroduce such rines. two chemicals that form a sparing- - . ly solihle product are required; the aqueous solutionof one, added to the second in agel, can give rise to periodic precipi- tation. According to Flicker and Ross', this phenomenon was first reported in 1855 by Runge. However, the first systemat- ic studies were reported hy Liesegang in 1898, and the phe- nomenon bears his name.

Procedure The basic procedure to produce Liesegang rings is as fol-

lows. 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~il thete-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 so- lutions. students should not use these chemicals without instructor supervision.

Sample Results We have tried a variety of systems to produce Liesegang

rings. A summary of our successful and unsuccessful at- tempts is given in the table. Selected results are described below.

Systems Used To Try To Produce Liesegang Bandsa

Gal containino Solution on too

systems that have producedLissegeng rings within two wseks: cobalt(ll) chloride concentrated ("0.880") ammonia magnesium chloride sodium hydroxide (19 M) magnesium chloride concentrated c'0.880") ammonia copper(1l) sulfate silver nitrate solution (0.1 M) manganese(1l) chloride concentrated ("0.880") ammonia uranyl nitrate concentrated c'0.880") ammonia uranyl nitrate sodium hydroxide (19 M) uranyl nitrate silver nitrate solution (0.1 MI capper(1i) chloride sodium hydroxide (19 M) potassium chromate silver nitrate solution (0.1 MI

systems that did not produce Liesegang rings within two weeks: chromium(l1l) sulfate sodium hydroxide (19 M) cobalt(l1) chloride sodium hydroxide (19 M) chromium(ll1) chloride concentrated c'0.880") ammonia chromium(ill) chloride sodium hydroxide (19 M) mangaoese(l1) sulfate sodium hydroxide (19 M) nickel(1i) sulfate sodium hydroxide (19 M) nickei(1i) sulfate concentrated c'0.880") ammonia nickel(l1) chloride concentrated ("0.880") ammonia capper(i1) sulfate sodium hydroxide (19 M) copper(l1) sulfate concentrated ("0.880") ammonia

.Using me method outlined in he ~rosedure sectlon ot ths text.

concentrated ammonia

('0.880') solution

Liesegang rings --a gel with

cobalt (11) chloride

I Flicker. M.; Ross. J. J. Chem. Phys. 1974, W9). 3458. &sic equipment for producing Liesegang rings.

Volume 65 Number 4 April 1988 365

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 wid- ened the lower down the tube the rings travelled. The ammo- nia 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 sec- tions that were joined when the rings first formed but sepa- rated 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~itate in ~atches. which de- tracted from the appearance oithe result.

Uranyl Nitrate Rings formed after the addition of sodium hydroxide (19

M) or concentrated ("0.880") ammonia solution. With sodi- um 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 be- came 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 bot- tom, had a few more rings than the previois i n e so the bands of color between the white rings increased in thickness.

C o ~ ~ e d l l ) Sulfate . . . .

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 Wis- consin: 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. ~scihtions and 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.

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 normal- sized 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 sug- gested2 as an alternative to gelatine.

Dlmusslon There is an important reason for suggesting the produc-

tion 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 nro- posed to explain the formation of Liesegang rings: a dis'cus- sion 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 ex- plain 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 observa- tions, and stands up to further experimentation, it is accept- ed as "correct" in the sense of "reasonable and the best explanation to date".

~ u r i n g the project, there are many possible specific inves- tigations for students to pursue. At each stage. thev should he encouraged to formulaic hypotheses to explain the obser- vations 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 flasks rather 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 Liese- gang 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 os- cillators, of which ~ : e s e ~ a n ~ rings are but one example, are also available4.s.

366 Journal of Chemical Education