Article by Brian Mansfield
Raphael Eduard Liesegang (1869-1947) well fits the description of an amateur scientist. Although he never completed a university education, he did independent research and published numerous papers on a wide variety of scientific subjects1. Today he is best known for his investigations of the curious phenomena bearing his name, Liesegang rings.
Liesegang rings are the periodic precipitates that form when a chemical reaction leading to an insoluble product occurs in a gel medium. What is a gel? We are all familiar with examples such as gelatin gels (think Jell-OTM). Agar gels containing nutrients are used for growing microorganisms. Here the gel structure immobilizes the growing colonies, allowing them to be counted. A gel is formed when a lyophilic (solvent-loving) macromolecular compound separates from solution (a sol) to form an extended network that entraps solvent (a gel). The sol-gel transformation can be thought of as a special case of precipitation in which all the solvent is retained within the precipitate.
Like sols2, gels are metastable systems, for over time the gel exudes solvent. This effect is called syneresis. Freezing also disrupts the gel structure, causing the solvent to separate. This process is used in the manufacture of agar. An agar gel is frozen, and on thawing most of the water separates out. This leaves the agar in concentrated form that can then be dried by conventional means.
Particularly pretty examples of Liesegang rings are formed when a solution of silver nitrate is placed on top of a gelatin gel containing potassium dichromate. The silver cation diffuses into the gel and reacts with the dichromate anion to produce a dark red precipitate of silver chromate,
Prepare a gel by dispersing 8 grams (g) gelatin in 200 milliliters (mL) of distilled or deionized water containing 0.2 g potassium dichromate. Heat to dissolve the gelatin and pour the hot solution into several Petri dishes or flat-bottomed evaporating dishes, varying the depths from a thin layer to a couple of centimeters (cm). Also, fill several test tubes of various diameters to within about 3 cm of the top. Similarly fill several long, narrow tubes made from about 5 mm inside diameter (i.d.) glass tubing (I used neon sign tubing) cut to about 20 cm in length and sealed at one end. Place in a refrigerator to cool. On cooling, the solutions will set to firm gels that are stable at room temperature. However, for best results a cool room is preferable.
Figure 1. Concentric rings form outward from the central puddle of silver solution we placed on the surface of the gel. Click image to enlarge.
Caution: Chromates and dichromates contain hexavalent chromium, which has a bad reputation as an environmental contaminant. Prolonged contact with the skin can produce "chrome sores," which are difficult to heal. An LDca (approximate lethal dose) of 12 mg/Kg has been reported for oral administration in rabbits.
It's always a good idea to wear safety glasses in a chemical laboratory. Wear gloves, too, if you want to be ultra cautious. Otherwise, handling and disposing of the small amounts used in these demonstrations pose no significant hazard (IMAO).
Precipitation of the dark red silver chromate within the gels will be seen within an hour or two after addition of the silver solution and will continue to develop over a period of several days. Simplistically, one would expect a continuously expanding zone of precipitate to extend outward from the Ag+ source. What we see, however, are discrete bands separated by spaces without any precipitate -- Liesegang rings. Over the century since Liesegang's original paper3 much study has been devoted to this phenomenon but a comprehensive explanation has yet to be found.4
Figure 2. Slicing the gel into vertical sections will show the three-dimensional structure of the rings. Click image to enlarge.
Figure 3. Instead of discrete disks, a helical precipitate may occasionally form. Click image to enlarge.
Other gel media, such as agar gel or silica gel,7 and other precipitation reactions can also be used. Substituting lead nitrate for silver nitrate produces yellow bands of lead chromate. Here are a couple of examples of reactions in silica gel. I haven't tried these, but quote the recipes:
Mercuric Iodide Bands
Let a solution, 0.1 N in potassium iodide, containing equal volumes of water glass (1.06 specific gravity) and of 1 N acetic acid, gel in a test tube. When gelled, cover with a 0.5 N mercuric chloride solution. In a week, bands of red mercuric iodide will be formed. The bands are particularly well defined from 2 cm below the gel surface to the bottom.8
Copper Chromate Bands
This is perhaps the most beautiful example of Liesegang rings. In a test tube make a gel from equal volumes of 1.06 specific gravity water glass and 0.5 N acetic acid. Enough potassium chromate should be added to make the solution 0.1 N in chromate. When gelled, cover with a 0.5 N copper sulphate solution. In a week, many well-defined bands of copper chromate will be formed.8
1. Henisch, H., Crystals in Gels and Liesegang Rings, Cambridge University Press. 1998 (ISBN 0 521 34503 0), p.116.
Copyright: 2003 Isis Publication