LECTURE-TABLE DEMONSTRATION OF OSMOTIC PRESSURE J. W. RAMSRY AND R. N. MAXSON, UNIVERSITY OB KENTUCKY,
LEXINGTON, KENTUCKY Several methods have been used to demonstrate osmotic pressure, many of which frequently fail to give convincing results in the period of the lecture. Chapin ("Exercises in Second Year Chemistry") describes an excellent experiment using a dialyzing thimble with a solution of sugar which shows a rise of one-half centimeter per minute in a tube with an internal diameter of one millimeter. The apparatus described in this paper is perfectly dependable, easy to prepare, and will, under IW+ Plafe I proper conditions, show a 95 ,fiem1wne i-' rise of one and one-half 90,,,nmamne.g ,',',+iembmnc)l' centimeters per minute. 85 1,) # /, a0 t, J Dnring the purification 75 (7 t' of a ferric oxide sol by /I / 70 dialysis, a high osmotic ,,1,1,1 65 ,/// pressure was noted which $ , ;// 1 1 agrees with an earlier ob6 rr.F servation by Zsigmondy. ,,,;',' x\ 37r ,,,y ("Chemistry of Colloids," ;/, t 6Trans. by Spear, 1917, p. .$ 40,'$ ,7,' 8, , 33.) This fact suggested .2: 2s,'," that such a sol might be + 30,',,' ,I, 25 used, with suitable appara,;y 20?;,, tus, to demonstrate os15motic pressure. As speed of A? 10, g dialysis and simplicity of 5-.p. apparatus were the prime O , * , . , , C ~ ~ C # ~ , , o 6 W B Q I I I ~ ~ factors ~ ~in this ~ case, a memT,me in minutes brane was desired which would give the most rapid dialysis per square centimeter of surface. Collodion, parchment, cellophane, and gold-beater's skm were used. According to Zott ( A m . Physik., 27, 229 (1889)), gold-beater's skin is the best membrane for dialysis and such proved to be the case in this experiment. The concentration of the sol, which was prepared by the hydrolysis of ferric chloride, was the next factor to be considered. As the osmotic pressure of a ferric oxide sol varies directly with the number of particles per unit volume (Svedberg, Report of Faraday Society and Physical Society of London, Oct., 1920, p. 5 ) , i t was desired to make the sol as concentrated as possible without running into danger of coagulation.
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VOL. 5, NO. 4
DEMONSTRATION OP OSMOTIC MSS#
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Preparation of Apparatus and Solutions Thirty cubic centimeters of a stock solution of femc chloride, made by dissolving 35 gms. of anhydrous salt in 100 cc. of water, were added to 100 cc. of boiling water. This gave by hydrolysis a femc oxide sol, the disperse phase, according to Pauli (Report of Faraday Society and Physical Society of London, Oct., 1920, p. 14), being a complex salt with the represents the anion of the formula, x F e ( ~ ~ ) ~ . ~ Fwhere e l ~ n"An" , salt from which the sol was prepared. The apparatus was made by cutting the stem from a thistle tube about two centimeters from the Plate ll bulb. T h e membrane was then placed over the mouth of the bulb and held in place with a rubber band. A watertight fit was insured by cementing the skin in place with collodion. The bulb was filled by means of a small funnel which can be made by drawing out a test tube into a fine stem. A piece of glass tubing about one meter long with an internal diameter of 2 mm. was next attached with a short rubber connection and the bulb immersed in distilled water. The liquid rose in the tube due to hydrostatic pressure, and the zero reading was taken a t this point. Experimental Work As would naturally be expected, diierent samples of gold-beater's skin gave slightly diierent results. In order to show how they might vary, three samples were used and curves plotted, showing the rise per minute. A reading was taken every five minutes. The curves on Plate I show the result using the sol mentioned above. The w e s on Plate I1 show the result obtained with a sol, one-third as concentrated. All experiments were carried on a t room temperature, the osmotic pressure of a ferric oxide sol having been found to diminish with rise in temperature. (Zsigmondy, "Chemistry of Colloids," Trans. by Spear, 1917, p. 44.)
