The FORMATION of MICROSCOPIC IJIESEGANG RINGS E. C. BOTTI, 0. W. LINK,
AND
B. P. CALDWELL
The Polytechnic Institute of Brooklyn.
S
INCE the pioneer investigations of Liesegang'
the phenomenon of rhythmic precipitation has attracted the attention of many workers.% This work has been done largely in test-tubes and Petri dishes. Lloyd and Maravek3 showed that it was much easier to obtain rhythmic precipitation in capillary tubes than in larger tubes. The present paper deals with the formation of Liesegang rings upon the microscope slide. The advantages claimed for the procedure are: (1) that the time required for the formation of the rings is quite shortfew minutes; (2) the process of ring formation can he followed by means of the microscope; (3) the amounts of material needed are very small; (4) if a permanent record of results is desired a photomicrograph can be easily obtained.
I
New
York
tant is then deposited on the gelatin drop from a fine capillary, care being taken to avoid rupturing the gelatin surface. If desired, four or more droplets may be placed upon each drop of cooled gelatin mixture. When proper concentrations of reactants have been employed the concentric rings of fine-grained precipitates form quickly. The accompanying illustrations show some of the results obtained.
For the precipitation of silver arsenate the solutions were as follows: 5 ml. of the gelatin mixed with 1 ml. of sodium arsenate solution (G g. per 100 ml. water); silver nitrate (10 g. per 100 ml. water). With single droplets upon the gelatin the rings formed very close to one another, the outer rings only FIoURes 1 & 2 -SILVERARSEWTE being clearly distinguishable. Better results were ohtained when two or three droplets of silver nitrate were EXPERIMENTAL METHOD placed on the gelatin-arsenate in close proximity to one The experimental method is simple enough. A another as in Figures 1 and 2. For the precipitation of silver chromate optimum gelatin solution (10 g. gelatin to 100 ml. water) is results were obtained when potassium dichromate (1 prepared in hot water and filtered through glass wool. g. per 100 ml. water) was mixed with the gelatin and To approximately 5 ml. of this solution is added 1 ml. of aqueous solution of one of the reactants, and uniform silver nitrate droplets were added as before. That the drops of the mixture are placed on a clean microscope hands follow the contour of the added droplet faithslide. This is then allowed to cool in a dust-free fully is shown in Figure 3. The regularity of the bands is well shown in Figure 4. chamber until the gelatin has set. The results of reaction between twinned droplets of A droplet or two of the solution of the second reacsilver nitrate and the potassium chromate (10 g. per 100 ml. water) mixture with gelatin are shown in Figure 5. Silver iodide precipitation was not so satisfactory. Small segments of rings appeared, and then precipitation occurred between the rings, thus obliterating them
in large degree. In addition, single crystals and clusters of silver iodide formed in the center of the droplet. The potassium iodide and silver nitrate concentrations were varied with little change in result. The photographic record presented in Figure 6 is that of the precipitation obtained with a solution of potassium iodide containing 5 g. per 100 ml. water. The results above reported, and others with other reactions, justify us in believing that the microscopic method of illustrating the phenomenon of rhythmic precipitation deserves consideration by instructors who wish to bring the subject of Liesegang rings to the attention of their classes either by lecture demonstration or by individual practice.
