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APPARATUS FOR ISOPIESTIC MEASUREMENTS C. H. BRUBAKER, JR., C. E. JOHNSON, C. P . KNOP, and FRANK BETTS Michigan State University, East Lansing, Michigan
IN
STUDYING the physical chemistry of solutions of electrolytes it is frequently desirable to determine the vapor pressure of the solvent, over a range of concentrations, in order to determine the activity of the solvent, the osmotic coefficient, and the activity of the solute,' all of which may be deduced from measurements of the vapor pressure of the solvent. The determination of the activity of the solute in moderately concentrated solutions (i.e., 0.1M to &OM) is of particular interest, since, in general, no method exists for calculating activities a t such concentrations. The isopiestic method is beautifully simple in principle and should make it possible to determine solvent vapor pressures quite easily. I n this method, a solution of the salt (or norivolatile solute of any kind) is examined in comparison with that of some reference salt. This reference solution will be one for which the vapor pressure of solvent is known as a function of solute concentration. Then the two solutions, each in its own crucible, are placed in a closed container and allowed to reach equilibrium. When equilibrium is attained, the molal concentrations of the two solutions are determined by suitable methods. The vapor pressure of the unknown solution is thus evaluated from that known for the reference solution. This equipment was developed from that described by Scatchard, Hamer, and Wood,= but embodies
HARNED, H. S., A N D B. B. OWEN,"The Physical Chemistry of Electrolytic Solutions," 2nd ed. Reinhold Publishing Corp., New York, 1950, Chap. 9. S C A T C ~ G., R DW. J. HAMER AND S. E. WOOD, J. Am. Chem. Soe., 60, 3061 (1939).
several novel features which facilitate construction, in the average college machine shop by persons with only modest knowledge of machine tools, and which permit the use of more economical materials. The apparatus consists of small gold-plated copper crucibles to hold the salt solutions, a brass vacuum bomb in which the samples attain equilibrium, a device for gentle rocking of the bomb, and a good constanttemperature water bath. Close control of temperature is important to the method2 and a good constant-temperature bath, capable of holding the desired temperature ~t0.005"C.,must be available. The rocking device consists of a wooden tray, suspended in the bath, which is slowly rocked through angles of 15'. VACUUM BOMB AND CRUCIBLES
A cut-away drawing of the vacuum bomb is presented as Figure 1. The cylindrical portion of the bomb can he made from 4'18in. brass rod or from brass tubing of suitable thickness. The top and bottom circular plates were turned from '/rin. brass plate and the hottom was silver-soldered into the cylinder. The circular brass block inside the bomb has six holes 3/4-iu. deep into which the crucibles fit snugly. These holes were cut with an end mill on the milling machine. The solid brass and the snug fit are quite important in order to insure rapid thermal equilibrium between the several crucible^.^ The neoprene "0" ring set in a shallow recess in the top of the bomb makes an airtight seal when the cover is screwed down. Since chemically inert, straight-sided containers are not readily available, we have had to draw crucibles of the proper shape and dimensions out of sheet copper and then plate first with silver and then with gold.s To accomplish this, a shallow cup must he drawn and successively redrawn into deeper and narrower forms until the desired cylinder is obtained. A typical die is shown in Figure 2. The dimensions of the set of dies is given in Table 1. Since we were working with 0.008-in. TABLE 1 Steel Die Sets for Drawinm C o ~ n e rSheet Female die Small diameter (inches)
Large diameter (inches)
Male die diameter (inches)
BLUM, W., AND G. B. HOQABOOM, "Principles of Electroplating and Elcctroforming," MeGraw-Hill Book Co., Inc., New
York, 1949, Chap. XII.
JOURNAL OF CHEMICAL EDUCATION
sheet copper, the male portion of the die is of such a dimension that the clearance between it and the sides 01 the female is only 0.0075 in., otherwise the desired drawing action will not take place. In metal drawing the reduction in diameter a t any stage should be about 20% or less. At t,he outset and after each drawing the copper had to be annealed, by heating to 600°C. and quenching in water, since c o p per work-harden8 during the drawing. A 2-in. disc of annealed eapper is plaeed in the top portion of the largest female die and bath dies, the retainer and the copper, are lubricated with lard oil. The retainer and male die are put in place and the assembly is placed in an arborbor press. The retainer is held in place by a stout coil spring which is compressed between the retainer and the top of the small arbor press. (This spring arrangement is important. If t,he retainer is not held rigidly in place, the copper buckles and tears during drawing.) The male is then slowly and steadily purhed through, until the whole of the capper has passed through the female die. The cup, thus formed, is removed from the male die by . arrrrlving .. . - a slight - air rrressure to the relief hole in the die. The cup is cleaned of lard oil and annealed. Then it is lubricated and plaeed in the second die. The operations are repeated until the copper has been d r a m through the four die sets and the cup is of the desired proportions. The top of the cup is samewhat jagged a t this point, unless a shoulder is made on the final die. The h a 1 die should be turned to the dimensions indicated only to the desired depth of the cup; above that it should widen t o a dirtmeter of 0.999 in. (which will just pass through the female.) This shoulder, then, serves to cut off the cup to the desired depth (1'/8 in.) and remove the uneven upper edge. Tighbfitting caps for the cups (to prevent evaporation of the solvent, while they me being weighed) can also be drawn from copper sheet but can be made in one step. The dies we used have the following dimensions: female, 1'1. in. s t the wide portion, 1.016 in. a t the small portion; male, 1.001 to a depth of in. and then above thin shoulder 1.015 in. Subsequent plating of the crueib!es and caps makes the fit even better.
pressure of pure water a t room temperature, it is evacuated through the bulb ten more times to insure complete displacement of air. The valve is closed, it is removed from the vacuum line, the cap is screwed down tight, the small cap which covers the evacuating port is put in place, and the bomb is put into the rocker in the constant temperature bath. Twenty-four hours is a sufficient time toattain equilibrium in the cases where the reference potassium chloride solutions have a concentration greater than 0.75 molal. The time required increases for more dilute solutions. Three or four days are needed when the potassium chloride concentration is about 0.1 molal. After equilibrium has been attained, the bomb is: removed from the bath, wiped dry and opened. The crucible caps are quickly put in place, and the crucibles are weighed and molal concentrations calculated. Duplicate samples should agree within one to three parts per thousand at equilibrium. I n Table 2 arelisted some typical equilibrium molalities for refereme potassium chloride and potassium octacyanomolybdate(1V) solutions; along with the length of time allowed for eauilibrium t o be attained. TABLE 2 Equilibrium Concentrations of KC1 a n d K4Mo(CNls
Time (daw) 1
Conc. of KC1 (molal) 1988 1.992
'
A". Cone. of osmotic K,MO(CN)~ eoefieient (molal) K,Mo(CNh 1.427 1.431
0.4906
MEASURING VAPOR PRESSURES
Since ample data on the activities, osmotic coefficients, and solvent vapor pressures of solutions of potassium chloride in water can be found in the literature,& this was used for the reference solutions. Sodium chloride might also he used.% Since the bomb has provision for six crucibles, duplicate samples of the reference salt and two "unknown" salts may he placed in it, and thus two salts can he examined simultaneously. The crucibles can be used conveniently with up to 5 ml. or 6 ml. of solution; it is desirable t o use a t least 1 ml. The range of reference solution concentrations employed has been 0.1 molal t o 2.0 molal. Sufficient potassium chloride (0.1 g. or more) is weighed into each of two crucibles, and water is added to make a solution of about the desired concentration. Similarly, two samples of an unknown salt solution are prepared. The apparatus has been used t o investigate solutions of such polyvalent electrolytes as potassium octacyanomolybdate. The uncovered crucibles are placed in the bomb and five drops of water are placed in the bottom of the bomb. It is closed and evacuated through a glass bulb of volume equal to that of the bomb. The evacua-. tion is thus controlled, and rapid boiling and spattering of the solutions are prevented. When the pressure in the bomb has been reduced to that equal to the vapor
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HIUINED, H. S., AND B. B. OWEN,"The Physiod Chemistry of Electrolytic Solutions," 2nd ed. Reinhold Publishing Corp., New York, 1950, Chap. 12.
VOLUME 34, NO. 1, JANUARY, 1957
It is unnecessary to weigh out new samples each time. After each run, solutions may be diluted by adding a little water and then placing the uncovered crucibles back in the bomb and proceeding as before. This practice may be continued until 5 ml. or 6 ml. total volume result or until reproducible results can no longer be obtained. Repeated handling of the cmcibles eventually (after four t o six runs) seems to reduce the precision, and the crucibles must be cleaned ~ n d dried and new samples must he prepared. When diluting the samples between runs, it is desirable t o add more water to one crucible of each pair than to the other so that one may he sure when equilibrium has been attained. When a sufficient number of samples have been examined, activities and osmotic coefficients may be calcu1ated.l ACKNOWLEDGMENT
The authors wish t o thank the Atomic Energy Commission, which has supported this work a t Michigan State University, 0.G. McMurray, and H. L. Womochel for their advice on the subject of metal drawing, and J. L. Dye for his help in connection with electroplating.
