A LECTURE DEMONSTRATION OF THE DONNAN EQUILIBRIUM
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HARRY P. GREGOR Polytechnic Institute of Brooklyn, Brooklyn, New York
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first mention of the effects which must arise in a system of two solutions of electrolytes, where a t least a single ionic species is restricted to one of the two solution phases, was made by Ostwaldl in 1890. Some twenty years later Domanz. a observed these effects experimentally while measuring the osmotic pressure developed across a membrane which was impermeable to a large dye ion. While these effectshave been widely observed and are known to be of considerable importance in many chemical and biological phenomena a ready classroom demonstration of them has not been available. This is primarily because the permeabilities of membranes which can show strong effectsare too low to permit the establishment of equilibrium within a short period of time. The development of the "Permselective" membranes described by Gregor and Sollnerd! b o w permits the easy demonstration of Donnan effects. A striking lecture demonstration is carried out by immersing a protamine-collodion membrane (which is permeable only to anions) filled with 1 N Mg(NO& solution in a beaker containing a large volume of 0.001 N Mg ClZ. After approximately 45 minutes of contact, and with properly prepared membranes, the external chloride concentration has dropped while the chloride concentration inside the membrane has risen until i t is several times the external concentration. Thus, the chloride ion has been concentrated manyfold against its own gradient. At the end of the experiment this effect is demonstrated by the addition of silver nitrate to diluted inside and outside solutions, observing the resultant turbiditv. PREPARATION OF MEMBRANES
The procedure for the preparation of the membranes required for this demonstration need not be as exacting as that described by Gregor and Sollner.'. Positive membranes are used because they are quite sturdy compared with the fragile negative membranes. They also retain their properties for prolonged periods of time. The author has made use of protamine-collodion membranes which after a period of four years have lost only -
' O S T W AWI., ~ , Z. Physik. Chem., 6, 71 (1890). 9
DONNAN, F. G.,
AND
A. B. HARRIS,J . Chem. Soc., 99, 1554
(1911). a
DONNA F., G., Z. Eledmehem., 17, 572 (1911). P., AND K. SOLLNER, J. Phys Chem., 50,
' GREQOR,H. (1946).
Ibid., 88 (1946).
10 per cent of their original electrochemical activity. The use of the large, polyvalent magnesium ion instead of the alkali metal ions permits a high degree of ion selectivity with membranes of high porosity. The membranes can be prepared as follows: Select a test tube which has a uniform diameter of one inch or, preferably, one which tapen toward the closed end. Blow a small (0.5- to 1-mm.) hole in the end and then close it by allowing a droplet of concentrated sugar solution to dry there. Make up a 5 per cent solution of collodion in 50-50 alcohol-ether mixture (a commercial preparation is satisfactory). Fix the tube in a horizontal position on a rotating mandrel. Hand rotation can be used but is much less satisfactow. The speed of rotation should be 18 to 20 r. p. m. The room temperature should be about 18 to 20°C. If the room temperature is higher increase the percentage of alcohol in the collodion solution accordmgly. Then pour the solution over the rotating tube, catching the drippings in a beaker. It is better to use an excess rather than a deficiency of solution. Repeat this procedure after a three-minute interval, then again in five minute? Finally, allow six minutes for drying. Remove the tube from the rotating mandrel rod and immerse i t in cold water. After several changes of water the membrane which is still on the glass tube is impregnated with a 4 per cent solution of protamine sulfate6 dissolved in a 0.05 M buffer of sodium borate-hydroxide (19 g. of borax plus 4 g. of sodium hydroxide per liter) a t pH 11. This is done in a refrigerator for two to four days. The membrane then is washed and allowed to drv for one dav in open air, if the relative humidity is in "the range 36 to 70 per cent; otherwise, a humidistat must be used. After more soaking in water, the membrane can he pulled off the casting tube and secu~edto the rim of a glass cylinder one inch in diameter. The selectivity of the membrane can be measured by fillmg it with 0.1 N NR&l and immersing it in an equal volume of 0.1 N KNO, solution. If the membrane is selective, it will permit the interchange of anion, but not of cation. Therefore, considering the outside solution, the ratio of the concentration of chloride ion to ammonium ion constitutes a measure of the selectivity of the.memIn carrying Out this experiment, both and external solutions should be stirred continuously. After 30 minutes, the external chloride concentration
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This is obtainable as salmin sulfate from Eli Lillv and Company. 260
.
MAY, 1949 should be a t least 0.02 N and the ammonium concentration not more than 0.004 N, or an ion selectivity ratio of 5 should be reached. If the ratio is less than 5, and the chloride ion concentration is greater than 0.02 N, the membrane is too porous. In this case, the membrane may be made less porous by allowing it to shrink in air without internal support. DEMONSTRATION
The lecture demonstration is set up by immersing the membrane bag, filled with 1 N Mg(NO& solution, in a beaker containing 100 times the membrane volume of 0.001 N MgC12. Both solutions must be stirred continuously, either by streams of air bubbles or by electric stirrers. If a test tube 4 inches long is used for casting, the membrane will contain 25 ml. of solution, requiring a 2500-ml. external volume. Thus, a t equilibrium the = concentration of (NOa-)in = 0.91, (N03-),.t 0.0009, (C1-)i. = 0.091, (Cl-).,, = 0.00009, calculated from the Donnan relationship (NOa-)i,/(NOa-)= (C1-)i./(C1-),,. It is not necessary to add a nondiffusible nonelectrolyte to the system to equilibrate the osmotic pressures, for the mass flow of solvent is negligible through these membranes. In practice equilibrium is not reached for several hours. However, in a period of 45 minutes one can usually achieve 50 per cent of the equilibrium state, or C1-)
