Apparatus for the Study of Liquid-Vapor Equilibrium Compositions R . M. WILEY
AND
E. H. HARDER, The D o w Chemical Company, Midland, Rlich.
T
using porcelain chips or a capillary air leak to prevent bumping and to keep the liquid in the flask thoroughly stirred. The vapor passes through the top opening of the sample cup t o the condenser, and is returned to the cup. The cup is allowed to fill nearly to the top with the joint closed, and the approximate time required is noted. The liquid in the cup is maintained at a constant level by controlling the rate of boiling and leakage through the glass joint. When thermometers and liquid level have remained constant for at least txice as long as is required t o fill the sample cup, the samples are taken, and the composition is changed by adding more of one component through the condenser. Variations in design may be made. A ball-and-seat type joint may be used in place of the regular ground-glass joint and will not break so easily, but is harder to adjust. A small pinhole may be substituted for the adjustable joint, in which case the rate of boiling alone must be controlled to maintain the proper liquid level. If the pinhole-type leak is employed, ground-glass joints may be substituted for the corks, and the apparatus operated under vacuum.
HIS apparatus, originated and used by the authors for
the purpose of obtaining samples of liquid and vapor in an equilibrium, is believed to have considerable advantage in speed, simplicity, and convenience over devices used by Othmer (3) and Brown (1). It checks the published curves for the system alcohol-water, and has been used to good advantage for the design of plant-size fractionating towers.
r l
The chief source of error in the apparatus as designed is in the vaporization of the liquid as it leaves the cup and before it becomes completely mixed with the liquid in the flask, but this is small in relation to the total amount of vaporization taking place in the flask. Furthermore, the vapors, thus made relatively too rich in the low-boiling component, tend to be absorbed by the liquid in the flask, thus restoring true equilibrium. S o error is incurred from fractionation in the reflux condenser, for regardless of the extent of separation during condensation, the liquid returned to the cup must be identical in composition with the vapor entering the cup. As the cup fills, the composition of the liquid in the flask and of the vapor being boiled off changes somewhat through removal of the lorn-boiling component, but when steady conditions have been established, the vapor and liquid approach equilibrium, except over the very small area where the reflux from the cup drops back into the boiling liquid. Care must be exercised in removing the sample from the
+-
FIGURE1. APPARATUS ASSEMBLY
Sources of error in the common distilling-flask method, which are eliminated by other methods only a t the price of greatly increased complexity of design or slowness of operation, are reduced to a minimum by this method, with little increase in complexity and even greater ease and speed of operation.
J'i
I
The apparatus is shown in Figure 1. The container is a standard 1-liter, three-neck flask, immersed as far as possible in a beaker (not shown) of a transparent heat-transfer fluid, such as glycerol. The cup, B, is of glass, about 20 ml. in capacity, with a ground-glass joint, D, welded to the bottom. The cup is welded directly to a glass reflux condenser, after fitting the cork as shown. The core of joint D is sealed at the bottom and welded to sampling tube A . A hole, H , is blown in the sample tube immediately above the joint, allowing the sample to pass up through the sampling tube into the cooled receiver, R, when the pressure is changed, either by blowing on tube T or applying slight suction to the receiver. Samples of the liquid in the flask are taken in the same way through tube E. When analyzing samples by the method of specific gravity, the jacketed funnel, R, is a convenient means of cooling, so that the samples can be drained directly into a pycnometer, immersed in a constant-temperature bath, and weighed when the temperature reaches the desired point. To operate, the liquid is boiled gently by heating the bath,
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OBTAINER BY AUWORS
t miNm O ~ T A I N ar ~ CMHV
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10
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LEWIS
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EQUILIBRIUM CURVEFOR ETHYL FIGURE 2. LIQUID-VAPOR ALCOHOL-~TATER
349
INDUSTRIAL AND ENGINEERING CHEMISTRY
350
cup to avoid contamination of the sampIe by the hold-up in the reflux. For very careful work, the sample should be removed by applying vacuum to the receiver, rather than pressure to the flask. TABLEI. LIQUID-VAPOR EQUILIBRIUM DAT.4 ALCOHOL-WATER
11 .~
12 13 15 17 18 19 20 21 22 25 26 27 28
0.8083 0.8329 0.8629 0,8839 0,8983 0.9166 0.9169 0.8348 0.8879 0.9301 0.9811 0.9777 0.9636 0.9890
ETHYL
Mole Per cent of Ethyl Alcohol Liquid Vapor
Density 259/25a Liquid Vapor
Run
FOR
81.8 66.5 49.3 39.3 33.4 27.0 27.0 65.3 37.7 22.7 4.8 6.0 11.0 2.5
0.8077 0.8231 0.8365 0.8442 0.8480 0,8516 0.8517 0.8230 0.8437 0.8547 0.8867 0,8820 0,8693 0.9230
82.5 72.8 64.0 59.0 57.0 55.0 55.0 73.5 59.5 53.5 35.7 40.0 46.0 25.0
VOL. 7 , NO. 5
AS a check on the apparatus, the curve shown in Figure 2 , was obtained, using ethyl alcohol and water. Samples were analyzed by the method of specific gravity, using a 10-ml. pycnometer in a constant-temperature bath a t 25" C., with the tables of the U. S. Bureau of Standards. Samples were drawn by suction on the receiver. Pressure was atniospheric (29.690 inches of mercury). The data of Carey and Lewis (2) are plotted with the authors' in Figure 2 and show good agreement, the authors points being slightly lower than theirs in the middle range, and slightly higher in the lower range.
Literature Cited (1) Brown, E.H., J. Chern. Education, 9, 1114 (1932). (2) Carey, J. S., and Lewis, W. K., IND. ENG.CHEM.,24, 882-3 (1932). (3) Othmer, D.F., Zbid., 20, 743 (1928). RECEIVED September 5, 1934.
The Germanium-Germanium Dioxide Electrode M. L. NICHOLS
AND
S. R. COOPER, Department of Chemistry, Cornell University, Ithaca, N. Y.
SI"
TCE the introduction of the hydrogen electrode by Bijttger (1) in 1897 and its popularization by Hildebrand (6) in 1913, potentiometric measurements and titrations have attained great importance. Because of the difficulties of obtaining the highest degree of accuracy with this electrode and its limitations, several electrodes, such as the quinhydrone and many metal-metal oxide electrodes, have been investigated as substitutes (2, 4, 6). In order for a metal to function as a metal-metal oxide electrode it should be slightly soluble, cathodic to hydrogen, and have a slightly soluble oxide. Germanium is a slightly soluble metal, whose dioxide is polymorphic in that it exists in a soluble and insoluble form. Laubengayer and Morton (8) state that the insoluble form of the dioxide is not appreciably soluble in 25 N hydrofluoric acid, 12 N hydrochloric acid, or 36 N sulfuric acid, but is 16.8 to 17.8 per cent soluble in 5 N sodium hydroxide solution. Since it is also cathodic to hydrogen, this electrode was studied to determine its application to pH determinations and electrometric titrations. For the electrode Ge/GeOz, H f , H20 the anodic reaction can be represented as Ge(s)2H20 --+
Ge02(s)
+ 4H+ + 4F
and therefore
Then for the cell Ge/GeOz, HA/salt bridge/KCl, HgzClz/Hg E = E,, - ECaI.and EZ5' = E;;L,25" - E?;, - 0.0591 log [H+] or E260 = E&Z50 - E26" cat, 0.0591 PH
+
Therefore the germanium-germanium dioxide should function as a metal-metal oxide electrode with the observed electromotive force for the cell changing 0.0591 volt a t 25" C. for each unit change in pH of the solution.
Experiment a1 The insoluble germanium dioxide was prepared according to the method of Laubengayer and Morton (8), and showed practically no solubility in a 48 per cent solution of hydrofluoric acid. The metallic germanium used had been prepared according to the method of Tressler and Dennis (IO). The germanium prepared as above was cast into a rod approximately 5 X 5 x 50 mm. A clay boat was filled with an alundum paste and a piece of wood the size of the ingot desired was pressed into the moist paste and the whole dried at 110" C. When the paste had dried, the wood was removed and the depression was filled with powdered germanium. The boat and its contents were heated in an electric combustion furnace at 950" to 1000" C.in an atmosphere of nitrogen, until the metal had melted. The resulting bar of germanium was smoothed with sandpaper. The electrode was mounted in a glass tube with Kronig's cement. The excess of cement was removed with benzene and the electrode was then washed with alcohol and distilled water. The glass tube was filled with mercury to make electrical contact with the metal. An electrode prepared in this manner could be immersed completely in any solution in which it was used. Pure calomel was prepared from mercury obtained by the electrolysis of mercurous perchlorate (9). A saturated calomel electrode was prepared from this pure mercury and calomel. The potassium chloride used was Kahlbaum's "for analysis" which was recrystallized twice from distilled water. The measurement of the voltage was made with a Leeds and Northrup Type K potentiometer and Type R No. E. galvanometer, shielded with a grounded copper case. The potentiometer was mounted on four rubber stoppers a n d all wires which were connected to it were either heavily insulated or covered with rubber tubing. A three-cell two-volt storage battery and a Weston standard cell supplied the voltage necessary. The Weston cell was calibrated by the Physics Department of Cornell University and found to give a reading of 1.0184 volts, the value of the last decimal place being uncertain. The solution which was measured and the saturated calomel electrode along with the intermediate vessel were immersed in a thermostat which was regulated a t 25' * 0.05" C. The electrode was first studied to ascertain if it gave a definite reproducible electromotive force and, if so, the time required for the electrode to come to equilibrium.
