Soap Bubbles and Precipitate Membranes Two Historical Semipermeability Experiments Adapted for Teaching Purposes Nicoletta Nicolini Universita degli Studi "La Sapienza", P.zzle Aldo Moro 5, 00185 Rome, ltaly Antonio Pentella' lstituto Professionale Femminile, "A. Diaz", via Acireale 8, 00182 Rome, Italy Studies on semipermeability and osmosis during the 19th century led to a series of unsystematic experiments being carried out on a wide range of organic and inorganic material (1-5). The physiological measurements carried out by Pfeffer (6) on plants in one of these experiments led to J. H. Van't HofPs theory of solutions (7). The present paper sets out to show how "historic" experiments can be effectively adapted and used for teaching purposes. The experiments presented herein provide a useful qualitative approach to osmosis and semipermeable membranes and offer potential for further elaboration in areas such as measurements of osmotic pressure. Soap Bubbles This experiment, based on work by Roloff (a),is suitable for general school use due to its ease of execution and absence of any danger. I t is designed to demonstrate the permeability of soap bubbles to COz and the lower permeability to Nz and 0 2 . This is based on the fact that a t room temperature the coefficient of C o p absorption in water is much greater than that of 0 2 and Np. Consequently, the liquid membrane (while preventing the passage of Nz and 02) presents little obstacle to the diffusing of carbon dioxide. Equipment COzgenerator (an Erlenmeyer flask with a one-holestopper and a glass tube comected on the outvidr to another glass tube by means Presented at a meeting of Socleta Chlmica Italians, Modena (Italy), 10-13 December 1985, IV Convegno Nazionale Didanica Chimica. Author to whom correspondence should be addressed.
'
614
Jouinal of Chemical Education
of a section of rubber tubing 50 cm long equipped with a clamp, or a Kipp generator); air pump; liquid soap solution;materials to generate C02 gas, e.g., from NaHCO3 + vinegar (or any other dilute acid).
Method Two bubbles of CO? are blown-one into an atmosphere of air, and thesecond into theatmosphere of COz. TheCOz passes through the membrane in both eases so that it is ultimately distributed uniformly inside and outside the bubble. However, the resulting effects are different. are mixed Case I: Bubble with COI Inside. NaHCO? and vineear " inside the flask. The CO? (being more dense than air) very quickly replaces the air inside the flask. The flaskis thenstoppered with the one-hole stopper tube apparatus, and the gas is allowed to flow out through the tube for several secands.The tube (B) isdipped intothe soap solution and withdrawn allowing a fairly large bubble to be formed at its end (Fig. 1). The clamp on the tube is then closed so that the bubble is isolated and the subsequent events observed. First the bubble visibly decreases rapidly in volume, a clear indication thatthe Conis exiting through the soap film and that the latter is permeable to this gas. Case 2: Bubble with COz Outside. Here the experimental eonditions are reversed. The tube that will be on the inside of the flask (now called B) is dipped into the soap solution (Fig. 21, and COz is allowed to be given off inside the flask. The flask is sealed with the one-hole stopper, with the soapy tube inside the flask, and air is pumped in through the free end of tube A until a small bubble forms inside the flask. The clamp is closed so as to isolate the air bubble surrounded by COz. The bubble is observed to increase rapidly in size. This is further evidence that COI can pass through the liquid, thereby allowing the internal and external concentrations to balance and causing the bubble itself to swell perceptibly.
Figure 1.C02 bubbiesunoundedbyair; the bubble shrinksasthe C02leaves it. Figure 2. Air bubble surrounded by GO2;the soap bubble grows in sire as the Preclpltate Membrane In the following experiment, a cupric ferrocyanide membrane is formed, and its semipermeability properties are. tested. This substance can be ohtained in the form of a brown precipitate by mixing solutions of copper sulfate and potassium ferrocyanide. However, if the two solutions are stratified, a cup& ferrocyanide memhrane is distinctly seen t o form a t the interface. T h e semipermeable properties of this membrane were investigated by M. Traube (9) and G. Tammann. T h e latter used this membrane in order t o develop a new method for determining osmotic pressure (10).A solution of potassium ferrocyanide and one of copper sulfate were prepared in such a way t h a t the osmotic pressure of the former was much greater than t h a t of t h e latter, b u t the density only slightly greater. Having thus enhanced the contact between the two liquids and with the formation of the membrane a t t h e interface, the solvent could be observed t o migrate from the CuS04 solution t o t h e potassium ferrocyanide solution, with a consequent increase in t h e density of the CuS04.
COPenters It
Material and Equipment Balance (0.1 gl, two 250-mL measuring rylinder~,10M)--1100gml. densimeter, eyedropper, glass rod, watch glass, distilled water, spatula, K4Fe(CNji4H!0. CuSOv5Hd3, two 5~0-mI.volum~rrir flasks.
motion ultimately slows to a halt, and the cell begins to sink again. Once it has reached the bottom of the cylinder, it continues to swell irresistibly. Furthermore, in the course of the experiment, no cupric ferrocyanide (brown solid) will he seen to form either inside or outside the cell, showing that the ions in solution cannot pass through the memhrane. It is thus demonstrated that an osmotic pressure gradient is set up between the twosolutionsand separated by the membrane. In order to offset this gradient, the solvent (and only the solvent) migrates from the hlue solution (with its lower concentration of dissolved particles) to the yellow solution,
Method 500-mL solutions of potassium femoeyanide 0.28 M and CUSOI 0.43 M are prepared in the volumetric flasks. Portions of the two solutions are then transferred to the measuring cylinders, and the densitiesaredctcrmined using the denrimeter. The yellua fernryanide solution must h a w 8 slightly greater density (d' 'C = 1.006 g/ mL, = 1.005 r mLJ. - than the blue wooer .. sulfate solution Since the successful outcome of the experiment requires that a drop of CuSOI solution immersed in the other solution should rise slowly, the experimenter should add small quantities of solvent to either solution as required. At this stage, several milliliters of the copper sulfate solution are withdrawn using the eyedropper. The tip of the eyedropper is then carefully immersed in the yeilow solution contained in the other measuring cylinder. At a depth of several centimeters from the hottom of the latter, the bulb of the dropper is eentlv. soueezed so that the comer sulfate emerees. Durine this . nnemtion., n- "cell" of~, ihluel CUSO* is seen to form~senaratabfrom ~~~~. . . the outer (yellow, solution by a cupric ferroryanide membrane that has farmed in theinterface between the t w ~ , ~ d u t i m (Fig. s :XI. Aiter the eyedropper is rapidly removed, the hlue copper sulfate cell is seen to begin to ascend, protected by the brown cupric ferrocyanide membrane. Because of ionic dissociation, the potassium ferrocyanide solution will have a considerably higher oamotic pressure than the copper sulfate at the concentrations used. Consequently the cell (bubble) will shrink as it ascends since the solvent is mieratine .. .. toward the ~xternslsoluti~n. Further convincing evidence that the solvent migrates is the gradual increase in the density of the copper aulfntp cell. The upward
-
~
~~~~
~
~
Figure 3. Oeneration of the "cell", i.e, of a cupric ferrocyanidefilm
Conclusion Our studies on electrolyte solutions and osmosis have shown how (bv usine-s i m.~ l eand c h e . a ~e a.u i.~ m e n t )distinguished scie&ta have succeeded not only in studiing this phenomenon b u t also in devising simple instruments and effective methods of measurement. T h e very simplicity and rigorousness of the experiments developed by these scienti&convinred us that return to the his;(rricai origins is nor only culturally worthwhile in itself but also provides a good teaching point, with built-in scope for further didartir dtvclopment.
a
Literature Clted 1. Devilbe Saint Clair, H. E.: Troosf, L. Compi. Rend. 1863.56.977
2. Graham, T. Phil. M a g 1866.32.401. 3. Raouit,F.M. Compl.Rmd. 1890,121,187. 4. Nernst, W. H.Zaiischr.phys. Chem. 1890.6.37. 5. De Vries. HZeilschrphys. Chsm. 1888.2.423:1889.3,103. 6. Pfeffer, W. Osmofische Unterruchungen; Engelmsnn: kiprig, 1877; p 30. 7. Van't Haff, J. H. Arch. neerlond. 1885.20: Zeibchr. phys. Chrm. 1887,l.481 8. Rnloff. M. Zeilarhr. oneem. Chsm. 1902.22.44.
Volume 65
Number 7
July 1988
615
