30 April 2004

Liesegang Rings

My father's life-long dislike of machinery struck home; I did not want any apparatus.  A couple of glass plates, some test tubes and a few dishes were all I needed.  I am glad I never had to teach; it allowed me to be a student all my life.  R. E Liesegang

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-O^TM).  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,

               
         4Ag⁺ + Cr₂O₇^-2 + H₂O -----> 2Ag₂CrO₄ + 2H⁺

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.

For the precipitating reagent, dissolve 1 g silver nitrate in 10 mL water.  Place a few drops of this solution on the surface at the center of the gels in the shallow dishes and cover the dishes with glass plates to retard evaporation.  Place about 1 cm of solution on top of the gels in the tubes.  Let stand at room temperature to allow the silver ions to diffuse into the gels.

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

When precipitation takes place in a shallow dish, a series of rings develops outward from the central puddle of silver solution we placed on the surface of the gel (Fig. 1).  The rings become more widely spaced and more diffuse over distance.  Commonly the rings are discrete, though spiral structures sometimes occur⁵.  Irregularities in the rings reflect the irregular boundary of the silver solution atop the gel.  Since diffusion occurs downward as well as outward,  slicing the gel into vertical sections will show the three-dimensional structure of the rings (Fig. 2).  If one were to inject the silver nitrate below the surface of the gel, then ring development would be free to develop upward as well.  Ideally a series of concentric shells would be formed.

Downward diffusion occurs when the silver nitrate is layered on top of  the gel in a tube.  Again, precipitation is periodic.  Most commonly a series of discs of silver chromate forms.  These become thicker and more widely spaced farther down, eventually becoming bands of discrete particles.  Occasionally, however, a helical precipitate forms instead of discrete discs (Fig. 3).  This has been described as a most uncommon phenomenon.⁶  My experience has been that it is more likely to be produced if a narrow diameter tube is used.  The pitch of the helix increases as the precipitate develops downward and eventually breaks away.

1. Henisch, H., Crystals in Gels and Liesegang Rings, Cambridge University Press. 1998 (ISBN 0 521 34503 0), p.116.

2. Colorful Colloids, SAS E-Bulletin, 10-31-2003.

3. Liesegang, R. E. (1896), Naturwiss. Wochenschr. 11, 353 1.1, 5.1.

4. Henisch, Op. Cit., pp.131-175, discusses, in lengthy detail, attempts at modeling this phenomenon.

5. Henisch, Op. Cit., p. 120.

6. Thomas, A. W., Colloid Chemistry, McGraw-Hill 1934. p. 481.

7. Silica gel (polymeric silicic acid) can be prepared by mixing equal volumes of water glass (sodium silicate solution) and 1 N acetic acid.  The water glass should be added to the acid drop by drop with constant stirring, rather than all at once. This is to forestall localized gelling resulting in an inhomogeneous gel structure (Henisch, Op. Cit., p. 9). The reagent for producing Liesegang rings should be uniformly mixed into the water glass before it is added to the acid.  Silica gel has the peculiar property of ringing audibly when the tube containing it is struck.