VAPOR-PRESSURE R E L h T I O N S IN M I X T U R E S O F TWO LIQUIDS, I
BY A. ERNEST TAYLOR
T h e object of this series of papers is to make an exhaustive study of the vapor-pressare relations which obtain in mixtures of two consolute liquids under varying conditions of temperature and pressure, the system thus far studied being that of acetone and water. A full series of measurements has been made of the boiling-points of solutions ranging in composition from pure water to pure acetone, over a wide range of pressures. T h e composition of the vapor given off from these solutions has also been determined, and the change of the composition with the pressure noted. Work of a similar nature has been carried on in this laboratory during the year by Dr. Carveth,I and in England by Lehfeldt.” This first paper will consist largely of a description of the methods of work employed, the difficulties to overcome, and the precautions necessary in order to obtain accurate boiling-points.3 Any one who has worked on ordinary boiling-point work knows how difficult it is to eliminate completely the phenomenon of superheating; and with mixtures of two volatile liquids the difficulty is greatly increased. Another important feature is the regulation of the pressure. T h e first measurements were made on the change of boiling-point with the change of pressure. A nionth or more was spent in perfecting an apparatus in which the phenomenon of superheating might, as far as possible, be elimi-
J
Jour. Phys. Chem. 3, 193 (1899). Phil. Mag. [5] 46, 46 (1898) ; 48, 215 (1899). T h e experimental data will be given in the second paper.
Va$or-$ressuye Relutions in Mixtures of Two L i p i d s'
291
nated, and in which the pressure could be kept constant or be varied by carefully regdated increments. T h e boiling-point apparatus (Fig. I ) consisted of a thick-
Fig. I
walled, two-necked flask containing scrap platinum and garnets. Into one neck was tightly fitted, by means of a rubber stopper, a small glass adapter, I O cm long and 2 cm in diameter, having a side arm ending in a IOO cc separatory funnel, by means of which the solution used was introduced. This separatory funnel
292
A. Ernest Taylov
was added in order that the apparatus might first be pumped out before the solution was admitted. If the solution should be added to the flask before the air was removed, evaporation would take place during the evacuation and the composition of the residual solution would be unknown. This difficulty is obviated by the use of the separatory funnel. T h e adapter was connected with two nickel condensers (such as used in the Beckmanii boiling point apparatus) joined together by means of a tight-fitting glass tube, the joints being made air-tight, by means of a sealingwax joint. T h e second condenser was connected by a similar joint to a glass T-tube, one arm of which led to a manometer, the other to a mercury pump. T h e separatory funnel could also connect with a Chapman p i m p , in which case the solution was of course not poured into the funnel until after the evacuation was complete and the connection with the water pump broken. T h e other neck of the flask contained a thermometer, the whole bulb of which was immersed in the liquid. This thermometer was divided into fifths of a degree and could easily be read to twentieths. It was carefully compared with a normal thermometer. T h e three rubber stoppers used in the adapter and in the two necks of the flasks were protected from the acetone vapors by cork stoppers about 5 mm in thickness. This simple device was found to be very effective, for as long as the rubber stoppers were kept well soaked out with water, the joints were found to hold the pressure very well. At first, and for a long time, cork stoppers smeared on the outside with a rubber cement, made from rubber and beeswax fused together, were used in place of rubber. These stoppers were fairly effective, but very troublesome. T h e boiling flask of 250 cc capacity, but so filled with garnets and platinum that it would hold up to the necks only 160 cc of liquid, was warmed by means of a water-bath, in which i t was immersed u p to the level of the acetone solution in the flask. This water-bath, in size 23 cm x 18 cm x I O cm, was kept at a temperature about 8' higher than that of the boiling liquid and was made of very uniform temperature by means of a funnel-
Yajor-pressureReZations in Mixtures of Two Liquids
293
shaped stirrer run by a small electric motor. T h e source of heat was a small Bunsen burner placed under the bath, and it was controlled by means of a mercury regulator in the bath. T o prevent as far as possible the radiation of heat to the apparatus above, the bath was covered by a thick sheet of asbestos. I n a series of measurements, the temperature of the bath, of course, was made to vary through quite a range of temperature, and this would, if no provision against it were made, affect the temperature of the condenser somewhat, and thus make the volume of vapor vary. That the various measurements may be comparable, it is necessary that the temperature of the condensers should not change much. T h e water flowing through the condensers was passed through at high speed so that the vapor from the solution might be condensed as rapidly as possible. T h e temperature of the water, all through the winter, was very nearly constant at 4" C. I t is desirable that as small an amount of vapor as possible should be given off from the liquid, and that it should be condensed quickly. for its composition is never the same as the liquid from which it is given off. For this reason the flask was filled as nearly as possible, the volume of the adapter was made small, and metal condensers of large surface were employed. T h e glass tube leading to the mercury pump contained two stop-cocks about 9 cni apart, the one nearer the boiling-point apparatus being a three-way cock. Thus, when working under diminished pressure, the small space between the two taps could be shut off from the rest of the apparatus and connected with the outside air. By again turning the three-way cock, thus connecting this space with the apparatus, a small amount of air would be let in and the pressure within raised slightly. Repeating the operation as often as necessary furnished an easy means of raising the pressure slowly. T h e mercury pump used was of the very simplest construction and was much the same as one described by Guglielmo.' I t consisted of a tube 155 cm in length and 3.5 mm in bore conI
Rend. Accad. Lincei, ( 5 ) 6, 11, 324 (1897).
294
A . Ernesi! Taylor
nected with a larger tube 14 cm long and I 5 mni broad. A small tube at the side of the larger tube led to the boiling-point apparatus, and another small tube at the top, furnished with a stopcock, led to a large reservoir of mercury. With this very simple pump it was found possible to lower the pressure in the whole apparatus to four or five millimeters of mercury in a few minutes (a much lower pressure than was ever necessary). T h e Chapman water pump used was also very effective, as the following measurements will show. One day, March 7th, when the barometer stood at 732.2 mm the apparatus was pumped out till the manometer registered a lowering of 725.5 mm, the water used being at 3.5". T h e vapor-pressure of water at 3.5" is 6 mm. When a series of measurements was to be made, the apparatus was first pumped out till the pressure was soinetliing less than I O mm and, if possible, allowed to stand over night to test for leaks. T h e liquid (120 cc) was then introduced through the separatory funnel, and in consequence the pressure would rise to about 150 mm or 2 0 0 mm, according to conditions. This would bring the boiling-point somewhere between 2 2 " and 30°, temperatures which were about as low as were practical for work. T h e bath, as has been stated, was kept 8" above the boilingpoint. It was found well at the beginning to have the bath at about the temperature needed for the first reading. I t took some little time for the apparatus to come to equilibrium, so that the reading could be depended upon, perhaps ten minutes. When equilibrium was established, a reading of temperature and pressure was made, and repeated in five minutes. During that period the pressure should not change more than one or two millimeters. Such a rate of change would introduce no inaccuracy. T h e thermometer readings were made to tenths of a degree, and were accurate to about that extent. With the ten percent and twenty percent acetone solutions, the limit may sometimes have been exceeded, especially at the very low pressures. These measurements under much diminished pressure are very difficult, for the boiling-point rises rapidly as the pressure on the solution is increased. I n fact, all the sources of error are introduced to
Va$oy+yesszwe ReGaLions iz &fixtures o f Two Liquids 295 the most marked extent in the solutions containing small percentages of acetone. T h e concentration of the vapor given off, in these cases, is fully sixty percent of acetone greater than that of the boiling liquid. An accuracy of one percent can be claimed for most of the work, and of course with much of the work a greater accuracy was obtained. When a constant pressure had been maintained for five to fifteen minutes, the pressure was gradually raised 50 mni by means of the two stop-cocks, near the mercury pump; 'and the temperature of the bath correspondingly raised. When equilibrium was finally reached, measurements were made as before, and so on for every fifty millimeters from 150 mni to 7 jo mm pressure. In cases where the rise of temperature with rise of pressure was rapid, measurements were made every twenty-five millimeters. For a final rise of fifty millimeters, it was necessary to change the, pressure about thirty-five or forty millimeters, the rise in temperature of the bath doing the rest. At first a great deal of trouble was caused by superheating. This is a most troublesome factor, for its workings are not always very evident. I t is perfectly possible to obtain an apparently constant boiling-point, which may be very far from the true boiling-point. Until the effects of superheating are absolutely eliminated, one's work has little value. This soiirce of error, however, I think that 1 succeeded in entirely removing. At least the inaccuracy thus introduced was not greater than my limit of error in readings. I do not feel at all sure that anyone else, who has worked along this line, has obtained a boiling-point free from the effects of superheating ; for I found that the ordinary methods as usually described, are wholly inadequate, in fact nearly useless. For example, I do not see how Lehfeldt could possibly obtain a correct boiling-point by simplyusing a piece of pumice weighted down by means of a coil of copper wire. However, he seems to be able to read his temperatures with very much more exactness than I claim for my readings. At first I used a large amount ( I O g) of scrap platinum cut into small sharp-pointed pieces. This seemed to work well
296
A. Enzest Taylov
enough for the first measurements, that is so long as there was air dissolved in the liquid. T h e different measurements made, however, did not agree well among themselves. T h e principal trouble came when a change of pressure was made and the liquid, in consequence, stopped boiling. On raising the temperature to bring about boiling, the liquid would become superheated; and very severe buinping would ensue, which it was impossible to prevent. Even after this bumping has been partially stopped, the liquid was sure to boil in a peculiar way characteristic of a superheated liquid, viz., in very large flat bubbles rising from the surface of the boiling liquid. T h e next step was to add to the platinum a large amount of garnets, so that the total volume of garnets and platinum mas nearly equal to that of the liquid used. T h e effe'ct was marvellous. T h e solution boiled in an ideal manner, small bubbles rising from all parts of the liquid. T h e boiling could be stopped any number of times and be again resumed even at the lowest pressures without the least trouble. Usually no effects of superheating were observed, but occasionally the liquid would boil a little irregularly for a minute or two at the beginning. T h e acetone used in this work was all obtained from Roessley and Hasslacher, of Perth Amboy, N. J., and was of very good quality. It was carefully dried over calcium chloride and redistilled. T h e acetone thus obtained had a boiling-point of 56.0" at 740 mm pressure. From this acetone the solutions used were made by adding distilled water in varying amounts, so that a series of solutions were obtained, differing from one another by ten percent, from o percent acetone to IOO percent acetone. In most of the calculations afterwards, these will be rediiced to molecular concentrations. After this set of measurements to determine the vapor-pressure of mixtures of acetone and water at varying temperatures was completed, another set of measurements was made to determine the concentrations of the vapor given off under similar conditions. This, of course, required an entirely different apparatus. T h e work was carried out with an apparatus not very
Vapoy.@essziye Relnkioizs
1.72
Mixkures of Two Liquids 297
different from that used by Lehfeldt when working on the composition of mixed vapors, and was constructed as follows. As distilling flask there was used a small round bottom flask of 300
Fig. 2
cc capacity, containing a goodly supply of garnets and scrap platinum. A three-hole rubber stopper, provided with a cork guard at the bottom, fitted in at the neck, one hole being used to admit a thermometer, the second a separatory funnel, and the third the condenser. This condenser consisted of a glass tube 9 mm in diameter directed vertically upwards from the flask for 8 cm, then bent at an angle of about sixty degrees, and after
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A. Erizest Taylov
about 15 cm bent directly downward, expanding into an elongated bulb of about 50 cc capacity, 2 0 cm long and 2 cm in diameter, terminating in a short tube 5 mm in diameter, fitted with a carefully ground glass cap. This expanded portion of the tube was surrounded by an inverted bell-jar provided at the bottom with a rubber stopper, through which the last mentioned small tube of the condenser projected, thereby making it easy, by removal of the glass cap, to empty the contents of the condenser. A small glass tube, 4 mm in diameter, bent sharply upward, was connected with the expanded portion of the condenser at a point a few centimeters above the mark which indicated that 20 cc of solution had been distilled over. This small tube ended in a T tube, one arm of which led to the condenser and the other to the manometer. T h e bell-jar was completely filled with a mixture of cracked ice and water, to render the condensation as complete as possible. T h e distilling flask, as before, was immersed in a water-bath, provided with a stirrer operated by a small electric motor. I t was necessary, of course, to prevent condensation of the vapor in that part of the condenser which would allow the condensed liquid to run back into the distilling Bask, otherwise the actual composition of the vapor given off would not be determined correctly, but a mixture too rich in acetone would be obtained. This was obviated by electrical heating. T h e distilling tube, from a point well past the first bend to the neck of the distilling flask, was wrapped in a very thin layer of asbestos (to distribute the heat) and was then wound with nianganin wire, the wire continuing down the neck of the flask to the level of the liquid in the flask. By passing a current of one ampere at a potential of three volts through the wire, it was found easy to keep that part of the apparatus at a temperature which effectually prevented condensation of the vapor till it had passed the point at which it was possible for i t to run back into the distilling flask. T h e same solutions were used as in the previous determinations. T h e change of the composition of the vapor with change of pressure was found not to be very great, so each solution was
Va$or-pressure ReZntions in Mixtuves
of'TWOLiquids
299
distilled under but two different pressures, viz., under barometric pressure and under a pressure of about zoo mm of mercury. Two hundred and twenty cubic centimeters of solution were always used in the distilling flask, and twenty cubic centimeters distilled over into the condenser, that point being indicated by a mark. It is necessary to use an exactly known amount of solution and distill over a known amount for the concentration of the solution varies during the distillation, especially in the solutions containing the smaller amounts of acetone. T h e concentration at the beginning and at the end of the operation must be known and the mean wed. An actual distillation was performed as follows : T h e distilling flask was filled and the bath heated gradually till it reached a temperature of about three degrees higher than that at which the solution should boil, this being about the temperature necessary to make twenty cubic centimeters distill in twenty minutes. T h e electrical heating of the neck of the flask and the first part of the condenser was started at the same time as the heating of the bath. When twenty cubic centimeters of the distillate had been collected, it was run out into a small apparatus to determine its boiling-point and hence its composition, T h e distillate was made to run out smoothly without loss from spattering, by diminishing slightly the pressure in the apparatus before removing the cap at the bottom of the condenser. In determining the boiling-point of the distillate] with care, little loss is necessary. So, in general, the distillate was put back again into the distilling flask with the solution from which it had been distilled ; and the whole redistilled under diminished pressure. For solutions containing less than fifty percent of acetone a pressure of about 2 0 0 mni of mercury was used and a pressure of 250 mm of mercury for higher concentrations. If, on distilling, the pressure increased] the mercury pump mas run from time to time for a few seconds to lower the pressure and thus keep it constant. As the concentration of the vapor given off varies but slightly with the pressure, it was not necessary to measure the pressure with great accuracy. When twenty cubic
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A. Ernest Taylor
centimeters had been distilled over, the boiling was stopped at once by opening the three-way cock of the mercury-pump, thus raising the pressure to that of the atmosphere. When distilling at atmospheric pressure, the boiling was stopped by cooling the bath rapidly with ice water. I n the second distillation it was often difficult to start the boiling quietly, the air in the solution having been removed. A slight superheating would be produced which was liable to make the liquid boil over. After this, the liquid would boil quietly enough, but usually not until after the distillation had been thus destroyed. In either of the other pieces of apparatus, a slight superheating for a few seconds would have produced no trouble ; still, the tendency toward superheating was much greater in this apparatus, undoubtedly due to the fact that the relative amount of garnets and platinum was much less than in the other cases. T h e trouble, however, was obviated by disconnecting the apparatus after the first distillation and raising the garnets and platinixm out of the solution repeatedly, thus introducing a certain amount of air. T h e next two distillations were made with a fresh lot of solution, but in the reverse order, viz., first under diminished pressure and then, on putting back the distillate, under atmospheric pressure. This furnished a check on the error which might be introduced from loss of distillate. T h e twenty cubic centimeters of distillate were each time carefully analyzed by determining the boiling-point in a little apparatus designed for that purpose. I t consisted of a broad tube, 13 cm in width and 2 . 5 cm in length, blown out slightly into a small bulb at the bottom. T h e bulb was filled u p with garnets and silver tetrahedra so that twenty cubic centimeters of solution would fill the flask very nearly up to a side arm which was attached to an ordinary glass condenser 35 cm in length. T h e flask was provided with a rubber stopper and cork guard through which passed, to the bottom of the solution, a so-called normal thermometer, graduated to tenths of a degree. This thermometer, Xo. 3060, made by Gerhardt, of Bonn, had been
Vajor-pressuve Relations i i z &Gztures of Two Liquids 301
carefully compared by Dr. Carveth with the Reichs-Anstalt in the Department of Physics. Corrections were made, in reading the thermometer, for the height of the mercury column above the
J; P
d
h
Fig. 3
flask and for the air temperature by using the tables given for such a correction in Landolt and Bornstein’s Tabellen. T h e solution was heated by means of a water-bath kept at constant
A. E m e s t TayZoy
302
temperature and very thoroughly stirred by a spiral stirrer, which was especially effective. A half liter beaker served as a water-bath. At first, to produce constant boiling one hundred and twenty-five silver tetrahedra were used, which would seem to be a sufficient supply for twenty cubic centimeters of solution, as they have been described as especially effective. It was found, however, that the boiling-point varied with the level of the bath, with the temperature of the bath, and with the rate of stirring. In fact, a change of half a degree could be easily obtained by simply varying the temperature of the bath. I t will be seen from Table I that by changing the bath seven degrees
TABLE I Bath at top of silver Boiling-point
I
Bath temperature
57.15O 57.05 56.95
70° 67 65
57.7 57.6
67 65
Boiling-poin t
Bath temperature
57.85O 57.75 57.45
63O 62.5 60
Vapor-jressuveRelatiom in Mixtures of Two Liquids 303
B. Garnets added 57.35O 57.55
60' 62.5
57.35O 57.35 57.35 57.35 57.35
62.5' 62.5 65 69 68
solution is taken and the level of the bath kept the same as that of the boiling liquid. In part A, the silver only is used, in B garnets are added, and in C more garnets are added till the total mass of the garnets and silver displaces ten cubic centimeters of solution, that is, their volume is just half that of the solution. I n A it can be seen that raising the temperature of the bath has a marked effect, in B much less, while in C raising the temperature of the bath ten degrees above that of the boiling liquid has no effect. I n Table 111, other conditions remaining the same,
TABLE I11 Bath at top of garnets Boiling-point
I
Bath temperature
68" 74 75 68 64 65 62
57.1s" 57.1 57.1 57.157.05 57.05 57.0
Bath half-way from garnets to top of liquid 57. I5O 57.20 57.20 57.25 57.20 57.20
I ,
~
60" 63 62.5 74 72 70
A. Ernest Taylor
304
Bath at top of boiling liquid 57&
I
65O
the effect of changing the level of the bath is shown. This still has an influence, but much less ; and as long as the level is the same, even a temperature twenty degrees above the boiling-point of the liquid has little effect. This shows that it is necessary to keep the bath at a constant level. Undoubtedly the level of the bath and of the boiling liquid should be the same, for if the bath reaches simply to the lower part of the interior liquid, the lower part only is boiling vigorously, while the upper part is somewhat cooled by the air and the boiling-point indicated is one or two-tenths of a degree too low. This shows very conclusively that boiling-points as ordinarily made would not be very accurate. This mixture of silver and garnets seems to be especially effective. T o determine the composition of these solutions, a series of determinations were made of solutions of known concentration from 50 percent to IOO percent acetone at various pressures, from 730 mm to 750 mm of mercury; and a curve plotted for 740 mrn, the variation of the boiling-point with change of pressure being determined, and all measurements corrected to 740 mm. On determining the boiling-point of any solution of unknown concentration, and noting the barometric pressure, it was easy to read off its concentration from the curve. I n all this work, the bath was kept at the level of the inside liquid. T h e conclusions which I would draw from this work are as follows : I. That without special precautions to prevent superheating, the boiling-points observed for mixed liquids are very inaccurate. 2. That often the effects of superheating may be overlooked, if conditions are not varied. 3. T h a t when foreign substances are added to prevent superheating, they must be added in large quantities, and their volume must be at least half that of the liquid used.
Va$or$ve.sszwe Relations iiz Mixtuves of Two Liquids 305
4. T h a t a mixture of platimm and garnets or silver tetrahedra and garnets is especially effective in overconiing the effects of superheating. In conclusion, I wish to express my thanks to Professor Bancroft, for it was at his suggestion and under his direction that this work has been carried out. Cornell University, July, 1x99
