Luis M. Borrero F ~ C U I ~de O ~Medicina,
Universidad del Valle Cali, Colombia, South America
II
An Experiment O S ~ O &F ~ O W
This paper describes a procedure for the semiquantitative study of osmotic flow. A semipermeable membrane of cupric ferrocyanide is formed in the shape of a drop by the addition of a small quantity of copper sulfate solution, which also contains alhumen and sucrose or glucose in varying concentrations, to a solution of potassium ferrocyanide. Differences in 0smotic pressure, or more precisely differences in tonicity, between the two solutions, are obtained by means of the various concentrations of the sugars. The diffusion of water into the drop is accompanied by the formation of potassium sulfate. When the concentration of this salt becomes sufficiently large turbidity is produced in the drop due to the coagulation of the albumen. The time necessary for the initial appearance of this turbidity is an indication of the rate of penetration of the water into the drop which in turn is an indication of the 0smotic pressures of the solutions of sucrose and glucose. The procedure is a modification of the methods suggested by Chodat' for the qualitative demonstration of osmosis. Egg white, or a saturated solution of albumen, is added to a saturated solution of copper sulfate. A THOMAS, P., "Manual de Biochemie," Masson, Paris, 1936, p. 100.
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Journal of Chemical Education
1 :3 ratio of the albumen to the copper sulfate solution is recommended. From this solution are prepared solutions containing 1.25, 2.5, 5.0, and 10% (by weight) of sucrose and glucose. An 8% solution of potassium ferrocyanide is found to give excellent results. The potassium ferrocyanide solution is placed in a shell vial to a depth of about 5 cm. The diameter of the vial should be about 2 cm. The membranes formed should have the shape shown in Figure 1. This shape is obtained by using a dropping tube with an opening about 0.6 cm in diameter. The drop of copper sulfate solution should be released from a distance slightly above the surface of the potassium ferrocyanide solution. A little practice mill enable the experimenter to
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Conssntr.tion
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Figure 2.
produce uniformly satisfactory drops properly suspended from the surface of the solution in the vial. The drop is then observed for the coagulation of the albumen. The time necessary for the first faint appearance of turbidity is noted with a stop watch. This first makes its appearance near the neck of the drop. At least three satisfactory determinations should be mode with each concentration. The results of a series of determinations with the two sugars are plotted in Figure 2. The drop should be observed against a black background. It should be illuminated with a bright beam from above and the drop should be observed a t right wugles to the beam. Changes in the size and shape of the drop should also be recorded. I n general the enlargement of the drop due to the diiusion of the water manifests itself in a general lengthening of the drop. After the appearance of the turbidity the drop can be detached from the surface by means of a stirring rod. After it has dropped to the bottom of the vial, and any adhering portions cleared from the surface, a new drop can be formed in the same solution. I n this manner a number of determinations can be made with the same portion of potassium ferrocyanide. Egg white, or a saturated solution of albumen, give equally satisfactory results. However the egg white must he fresh or must have been stored in a cool place. The rate of flow into the drop is affected very appreciably by changes in temperature. Hence a series of determinations must be carried out at essentially constant temperature. The effect of temperature is shown in Figure 3. The solution in this case contained 1.25% sucrose. Because of the temperature effect on the rate of flow it is not possible to use this experiment
to study the direct effect of temperature change on the absolute magnitude of the osmotic pressure. As stated in the opening paragraph the time necessary for the first appearance of turbidity, at constant temperature, is a measure of the rate of flow of the water into the drop and hence a measure of the relative tonicities of the solutions inside and outside the membrane. As seen in Figure 2, the time necessary for turbidity for a given weight per cent of the two sugars is roughly 2: 1, essentially the ratio of the molecular weights, and hence the ratio of the osmotic effects produced by each. The formation of the turbidity is apparently due to the increasing concentration of the potassium sulfate in the drop. This is formed as the potassium ferrocyanide reacts with the copper sulfate. The membrane of cupric ferrocyanide is apparently permeable to the various ions present. The over-all processes taking place and the changes in concentration are probably quite complicated. The procedure described can be modified in a number of respects to produce a project-type experiment. The rate of flow with varying concentrations of various salts within and without the drop can be studied. Other solutes impermeable to the membrane could be added to the copper sulfate solution. Consideration can be given to the effect of changes in the concentrations of various added substances on the coagulation of the albumen.
Figure 3.
Acknowledgment
The author is most grateful for the critical appraisal of this experiment by Mr. Thomas B. Douglas, a senior chemistry major working under the direction of Professor H. D. Crockford a t the University of North Carolina, Chapel Hill. Volume 36, c
umber 5, Moy
1959
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