DISTRIBUTIOX O F ACETOXE THROUGH A RUBBER MEMBRANE* BY D. S. MORTON
Introductory The law governing the distribution of a common solute between twc liquid phases was worked out by Sernst’ in 1891, and may be stated as follows: “If the molecular weight of the solute is the same in both solvents, the ratio in which it distributes itself between them is constant for a given temperature.” That is, Cl/CZ =
K
where CI is the concentration of the solute in the first phase, c p is the concentration in the second phase. K is called the distribution coefficient and depends upon the temperature and the nature of solute and solvent. The concentrations c1 and c2 correspond to solutions having equal partial pressures of the solute in the vapor phase, since both solutions are in equilibrium with the same vapor. The distribution of iodine between water and carbon disulphide was investigated by Berthelot and Jungfleisch.2 They obtained a constant distribution ratio when concentrations were expressed in grams solute per I O cc. of solvent,, that is, a constant ratio of the volume concentrations of the two phases. A thorough investigation of the distribution of succinic acid between water and ether was carried out by Forbes and C ~ l i d g e . This ~ case was complicated by the mutual solubility of the ether and water. The distribution ratio was not a constan4 when volume concentrations were used, but did come out nearly constant when compositions were expressed as mol fractions. It has been shown by Bell and Field4 that solutions of ammonia in water and chloroform deviate considerably from the distribution law at high concentrations. It is a t once evident that in cases of unlimited solubility the ratio of the volume concentrations cannot remain constant as concentration increases, unless that ratio is unity. For, as we add solute to the system, the composition of each liquid phase approaches pure solute, and the limiting value of the distribution coefficient is equal to the ratio of the density of the solute to the density of the solute, or unity. But in general the distribution coefficient is * A thesis presented to the Faculty of the Graduate School of Cornell University in rartial fulfillment of t h e requirements for the degree of Master of Arts. Thls paper 1s preiminary t o the programme now being carried out a t Cornell University under a grant t o Professor Bancroft from the Heckscher Foundation for the Advancement of Research, established by August Heckscher a t Cornell University. ‘ Z . physik. Chem., 8 , I I O (1891). *Ann. Chim. Phys., (4) 26, 396 (1872). J. Am. Chem. Soc., 41, 15 (1919). J. Am. Chem. Soc., 33, 940 (1911).
DISTRIBUTIOS O F ACETONE THROUGH A RCBBER XEMBRASE
385
not equal to unity, and hence the ratio of the volume concentrations is not a constant. A similar object,ion applies to expressing concentrations as mol fractions. Professor Bancroft suggests that the distribution equilibrium is primarily a function of mass concenbration (grams solute per gram solvent) rather than of volume concentration. X study of dilute solutions affords little evidence on this point because of the approximate proportionality between mass and volume concentration. It is the purpose of this thesis to investigate distribution equilibria between concentrated solutions and to find out if it is possible to derive a simple relation for the distribution in terms of mass concentrations.
X rubber membrane is readily permeable to acetone, much less permeable to methyl alcohol, and practically impermeable to water. Under ordinary conditions, methyl alcohol and water are miscible in all proportions; but they form a two-phase system when separated by an impermeable membrane. Khen acetone is added t o the system, it should pass through the membrane and distribute itself between the two phases in a manner analogous to the distribution of a solute between immiscible solvents. The system will attain equilibrium when the concentrations of the two phases become adjusted so as to give the same partial pressure of the acetone in the vapor over each phase. We have here a unique opportunity to study distribution equilibria over the entire composition range from zero to I O O percent, and to determine whether or not the effects are primarily dependent on mass concentration. One distinction between distribution through a semi-permeable membrane and the ordinary distribution between immiscible liquids should be noted. I n the ordinary case both solvents, as well as the solute, are in equilibrium with the same vapor phase, whereas in the case of the membrane the solute only is in equilibrium in the vapor phase. This circumstance in no way affects the general validity of conclusions drawn from distribution through membranes; in both cases the equilibrium is actually determined by the partial pressure of the solute in the vapor.
Experimental The first series of experiments is concerned with determining directly the distribution of acetone between methyl alcohol and water by dialysis with rubber membranes. The object of the second series of experiments is to check up on the results of the distribution experiments by means of measurements of the partial pressures of acetone over acetone-water and acetonemethyl alcohol solutions. Materzals. J. T. Raker's absolute methyl alcohol and Kahlbaum's acetone from bisulphite compound mere employed. Specific gravity determinations indicated less than 0.17~ water in the methyl alcohol. The acetone was redistilled before using; the specific gravity of the distillate indicated the presence of about 0 . j 5 - G water.
386
D. S. XORTOS
A p p a r a t u s and method. The simple form of apparatus shown in Fig. I gave satisfactory results. An inverted thistle tube' .I was supported by a cork stopper in the I O O cc wide-mouth bottle B. A rubber membrane cut from a toy balloon was fastened tightly across the bottom of the thistle tube. All membranes were tested for leaks before using. The thistle tube was corked at the top after introducing the solutions and the corks sealed with paraffine to preclude loss of vapor. About j cc of the acetone-xater solution were placed inside the thistle tube, and about 50 cc of the acetone-methyl alcohol solution outside, in contact with the membrane. The purpose of using such unequal quantities of the t.wo solutions was to allow for considerable gain or loss of acetone by the water solution without materially affecting the composition of the methyl alcohol phase. Due to slow permeation of the membrane by methyl aleohol, it was foilnd impracticable to obtain an equilibrium simply by adding acetone to one of the phases and letting the concentrations adjust' themselves. To locate the equilibrium point for a given acetone-mater solution an adaptation of the method of Bancroft and Kugent2 was adopted. An acetone-water solution of the desired composition was made up by weighing out and mixing the components. Portions of this solution were then placed in the distribution ---apparatus, in contact with a series of acetone-methyl solu-tions of regularly graded concentrations. Vnder these conditions the water solution would gain or lose acetone acFIG.I cording as the methyl alcohol solution mas more or less concentrated than the equilibrium value. The amount of acetone gained or lost by the water solution was determined by analyses. A good approximation of the concentration of the methyl alcohol phase that would exist in equilibrium with the water phase could then be obtained by interpolation between the highest observed concentration that gained acetone from the water and the lowest observed concentration that lost acetone to the water. For instance, to determine the distribution for a I O percent acetone-water solution, portions of the I O percent solution were run against acetone-methyl alcohol solutions centaining I percent, I O percent, 2 0 percent and 50 percent acetone respectively. The water gained acetone from the 2 0 percent solution but lost acetone to the I O percent. Hence the equilibrium point lay somewhere between IO and 2 0 percent. The experiment was therefore repeated using 12, 14,16, 18 and 2 0 percent acetone-methyl alcohol solutions. .Icetone was gained from the 18 percent solution and lost t o the 16 percent solution. By interpolation the equilibrium point mas estimated to be about 16.j percent acet,one in the methyl alcohol solution. The acetone in the water solution was cieterniined by the LIe~singer m e t , h ~ d .This ~ method, which requires :1 sample containing about 0.01g of .___
The draughtsman did not mnke a good thistle tube. Colloid Symposium LIonograph, 5 , I j q (1927,. "Goodwin: J. .Im. Chem. Soc., 4 2 , j y ( I ~ Z O ~ .
1
2
D I S T R I D U T I O S O F ACETOXI: THROCGFI I RUBBER XLMI3RASE
387
acetone, gave results xvhich w r e consistent but invariably a little low. For this reason the original Lvater solution was analyzed, although its composition was known accurately, in order to get a figure with which to compare results obtained after contact with the methyl alcohol solutions. .Icetone-mcthyl alcohol solutions were made up to the concentrations desired by measuring the liquids cartfiilly out, of calibrated burettes. Esperimcr&d conditiom. -411 ir01.kwas done at room temperature, about, ?jo(' Solutions were allonwi to remain in contact 2 4 hours, and were shaken thoroughly three or four times during the standing period. *IO.
Errors. The most annoying source of error was the tendency of the methyl alcohol to p a s through the nienibrane into the water solution. The,voluine of the water solution phase would usually show a iiiarked increase inside of n. week and nould continue to increase 2s long as the solutions remained in contact. In some instances an osmotic prersure suficient to rupt,ure the inernbrane or blow out the corks mas developed. L-nfortunately there is no good method for determining small amounts of methyl alcohol in the presence of acetone. I t is possible, howver, to infer indirectly the effect of the methyl alcohol hy anaiyzing solutions for acetone at frequent intcrvals. 1le:wlts of such a series of analyses indicated that the equilibrium was approachc>din three stages : Srtfici.ntion of the rubbcr m p m b r a n c irith ncctone. AIllwater solutions, (1) whether more or less concentrated than the equilibriurn composition, showed a small initial loss of acetone. This lo ppertred to be due to absorption of the acetone by the rubber membrane, equilibrium being attained within five hours or less. Osniosis o j the ac,ptone t h r o u y h the rubber membrane from the water (2) side to the niethyl alcoho! side, or vice versa, depending on the ratio of the acetone concentrations in the two phases. This flow of acetone proceeded fairly rapidly and gave an easily determinable concentration change within 2 4 hours. The direction of the osmosis, as has been pointed out, is an index of the position of the acetone-niethyl alcohol solu'tion with respect to the equilibrium point for the given acetone-water solution. s the m e t h y l alcohol accompanied by acetone. ,111 acetone( 3j S20ij. o ~ m o s i ' of water .elutions, regardless of composition, began to gain acetone a t the end of t w o days and continued to do so as long as observations were taken. This effect can be explained b y assunling that methyl alcohol, especially in the presence of acetone, permeates the rubber membrane slowly and introduces methyl alcohol eventually into the water solution. Since the methyl alcohol solution alxvays contains more acetone than the mater solution in equilibrium with it, it seems reasonable to expect that, as the concentration of the methyl alcohol increases in the water solution, the concentration of the acetone will increase correspondingly. The water solution is changing over into a methyl alcohol solution, so t o speak, and is approaching the composition of the methyl alcohol solution on t'he other side of the membrane. This hypothesis
388
D. S. MORTON
seems to be the only one that accounts for the observed continuous increase in the concentration of acetone in the water layer. These three effects undoubtedly proceed more or less simultaneously. So far as could be determined from the results of the analyses, the second effect was the predominant one after a standing period of 2 4 hours; estimates of the equilibrium point were therefore based on analyses taken 24 hours after placing the solutions in contact. Values for the equilibrium concentration were thus obtained agreeing within I percent or less on the lower concentrations and I and 2 percent on the higher concentrations. Check results obtained at different times indicated that the error introduced by variations in room temperature was slight compared with the erro: due to diffusion of the methyl alcohol. The analytical method used gave results concordant to I part in I O O parts of acetone-a degree of accuracy entirely adequate for the purpose of these experiment's. An occasional extraordinary result was rejected as being due to a defective rubber membrane. Results. The equilibria obtained for the various acetone-water solutions are shown in Table I. There is no evidence of constancy in the distribution ratio regardless of the mode of expressing concentration. Curves. In order to determine whether the distribution equilibrium could be represented by any simple type of equation in terms of mass concentration,
TABLE I Distribution of Acetone between Kater and Methyl Alcohol a t 2 '3
h
E F
= = = = = =
G
=
B C
D
I€ = I = A 0
Gram percentage acetone in water solution Gram percentage acetone in alcohol solution Ratio of gram percentages = B/'A Mass concentration acetone in water solution = h/roo - A Mass concentration acetone in a,lcohol solution = B/IOO- B Ratio of mass concentrations = E/D Mol fraction acetone in water solution Mol fraction acetone in alcohol solution Ratio of mol fractions = H/'G B 0
E
D
C
-
0
0
F
G
H
I
-
0.000
0.000
-
IO
17
1.70
0.111
0.205
1.8j
0.033
0.102
3.05
20
32
1.60
0.2jo
0.471
0.072
0.206
30
46 59 69.5
1.53 1.47 1.39
0,429
0.119
0.319
2.86 2.68
0.666
0.8j2 1 44
1.88 1 ' 99 2.16
0.171
0,443
I .0 0
2.28
2.28
0,239
O.jj7
2 59 2.40
3.14 5.66
2.30
0,318
0.6;;
2.06
2.42
0.420
o.ij7
1.80
CL:
1.000
1.000
I . 00
40 50
60 70 IO0
7j.j
1.29
1.50
8j
1.21
2.34 'x
IO0
1.00
'
CL)
DISTRIBUTION O F ACETONE THROUGH A RUBBER MEMBRANE
389
concentrations of the acetone-methyl alcohol solutions were plotted against corresponding concentrations of acetone-water solutions on logarithmic paper. The result is a very satisfactory straight line. This means that the data can be represented by log ( G A , / G ~ l , ) - a log (G'AJGw) = log K, as shown in Table 11.
TABLE I1 Distribution of Acetone between Water and Methyl Alcohol at 23' GAC
log-
G'A~
- 1.09log - = log K =
70acetone in water solution
% acetone in alcohol solution
0.3502
Gw
GAla
GAO
GAlo
found
GdGw
calc.
IO
I7
0.205
0.I 1 1
0.I 1 1
20
32
0.471
0.250
30
0.852
0.429 0.666 I .oo
0.239 0.412 0.667
60
46 59 69.5 77.5
70
85
40 50
I .44
2.28 3.44 5.66
log K calc.
1,017
I . 50
I . 482
2.34
2.341
The agreement is satisfactory over the whole range covered by the experiments-up to seventy-six mol percent of acetone in the methyl alcohol solution. If the equation is to be tested further, some more exact method of measuring the distribution ratio at high concentrations must be devised. Vapor Pressure Measurements I n any case of equilibrium of a solute between two liquid phases, the partial pressure of the solute in each phase must be the same. Therefore a distribution curve for the system acetone-water-methyl alcohol can be deduced from vapor pressure data, by plotting acetone-methyl alcohol solutions against acetone-water solutions having equal partial pressures of acetone in the vapor phase. I n view of the possibility of error due to osmosis of the methyl alcohol in the membrane experiments, it seemed desirable to check up on the results by means of partial pressures. Since up to the present writing there appear to be in the literature no accurate vapor pressure data on the system acetone-water, and none a t all on acetone-methyl alcohol, it was necessary t o obtain these data experimentally. The partial pressure of a constituent of a vapor is equal to the product of the total vapor pressure and the mol fraction of the constituent, assuming validity of the gas laws and normal molecular weights. To compute the partial pressure we must determine in some manner both the composition and total pressure of the vapor. The usual method of determining vapor compositions is to condense the vapor completely and analyze the condensate. Unless considerable care is taken, error will be introduced by refluxing during the vaporization and by
3 90
D. S. MORTOX
incomplete condensation of the vapor. The composition of the original liquid may be measurably changed by the distillation, and if this occurs there is always some uncertainty as to what composition of residue corresponds with the observed composition of the distillate. Various ingenious methods have been devised to eliminate or reduce these causes of error.’ By using special precautions and rather elaborate forms of apparatus it seems to be possible to secure results of a fair degree of accuracy by the isothermal distillation method. A second method, that has received less attention, consists in measuring some physical property of the vapor which is a known function of its composition, such as refractive index. This method is theoretically the ideal one since it permits a direct analysis of the vapor phase under equilibrium conditions. Cunaeus2 attempted to work out the system acet’one-ether by analyzing the vapor phase with an interferometer, but got into difficulties which he attributed to condensation of vapor in the end of his tubes. More recently Hoover and Glassey3 have used the interferometer to determine vapor compositions in systems of two volatile liquids, and report good agreement with Wrewsky’s results for the systems ethyl alcohol-water and methyl alcohol-water. A method similar to that of Cunaeus was adopted for the present investigation. The vapor in equilibrium with a given solution was admitt,ed into the interferometer and its refractivity measured against air as a standard. Assuming refractivity to be a linear function of vapor composition, the vapor composition and the partial pressure of each component were obtained by a simple calculation. The interferometer used was found incapable of compensating for the difference between the refractivity of air a t atmospheric pressure and that of the vapors under their own vapor pressures. Hence in every case the pressure of the air was reduced in the interferometer chamber until its refractivity exactly equaled that of the vapor. In this way calibration of the instrument was avoided, temperature corrections applicable to difference in refractivity were eliminated, and calculations were greatly simplified. Formulas. Following is a derivation of equations expressing refractivity and composition in terms of measured quantities. Let RA = refractivity of gas h a t pressure p a , absolute temperature TA RB = ” ” ” B ” pressure pB, absolute temperature TB Rc = ” ” C ” pressure pC, absolute temperature Tc RM = refractivity of mixture of A and B a t pressure pPI.1,temperature TAI a = mol fraction of A in mixture b = )I JJ ” B ” ” = respective refractivities a t 760 mm., 273’A. R A ,RB, Rc, J J
Rrewsky: 2. physik. Chem., 81, I (1912);Sameshima: J. Am. Chem. SOC., 40, 1482 (1918). * Cunaeus: Z. physik. Chem., 36, 232 (1901). 3 Hoover and Glassey: Trans. Roy. SOC. Canada, I l l ( 3 ) , 19, 35 (1925)
DISTRIBCTIOS O F ACETONE THROUGH .4 RUBBER MEJIBRASE
391
By definition, R = n - I By law of Gladstone and Dale'
(I) (2)
rl
=
where d = density of gas, a constant called the specific refraction.
Assuming validity of the gas l a m ,
(3)
da piT or R 273
For R.A = R A ,k
I1.i.
Subs
(j)
=
k(p,'T)
Ka
Pt 760 'I'a 273
-
= --- HA
(6) So far as is known the refractivity of a gas mixture is very nearly additive. Therefore
R11 = a R i
+ bRB = aRy + (I-%)
RB*
Solving for a ,
(7)
(8) Under experimental condition? 1131
(10)
==
R,.,Tit
=
In these experiments C' i s dry nir free from carbon dioxide, and
-
Hc =
.OOOZ9Ij2
i11)
SLlbS, 1x11 =
(.000?91;)
p31 1
Tc
Phil. Trans. ( 1 8 ~ ~ 8 ; .
* R.,; ItB, R ,for same temp. and pressure. lleggers and Peters: I3ur. Standards B~ull.,14; 698-740 :191Sl
392
D. S. MORTON
I n the following experiments the zero refractivities of acetone, methyl alcohol, and water were first determined, and the values substituted in equation (7) for EAand RB. The zero refractivities of various mixed vapors were then determined by observing the vapor pressure of the solution p31 and the pressure pc of air having equal refractivity, and applying equation (11). The mol fraction of acetone in the vapor could then be calculated by substituting for RAf in equation (7). Apparatus. (Fig. 2.) The solution under investigation is contained in the glass-stoppered flask F. This flask is provided with a side tube communicating with the righthand gas chamber Is of the interferometer, and with the manometer 31.12. There are also tubes connecting with a vacuum pump with stopcocks c, d , and e inserted as shown. The far end of the chamber Ipis connected through a stopcock with tubes containing calcium chloride and soda-lime (not shown in the figure). A tube is run from the pump line to the remote end of the gas chamber 11, with stopcock a t b. The near end of chamber I, communicates with manometer XI1 and with the atmosphere through stopcockaand drying tower D, which is filled with calcium chloride and soda-lime. Ordinary 7 mm. glass tubing was used for the manometers and connecting tubes. Stopcocks were of z mm. bore. Rubber connections were avoided as far as possible in those parts of the apparatus coming in contact with acetone Apparatus for determination of partial vapor;it was necessary, however, to use vapor pressures two short lengths of rubber tubing to connect the glass tubing with the brass nipples on the extremities of the interferometer chamber. The interferometer was of the Zeiss laboratory type, with gas chambers roo cm. long. I t was easily capable of measuring refractivities to I X IO&. Experimental procedure. The liquid under investigation was introduced in flask F and the stopper inserted. The flask and its contents were then thoroughly ccoled with carbon dioxide-alcohol mixture. With stopcocks b, c, and d closed and e and f open, the space over the solution was evacuated for R n hour or more to remove the air. Little vapor was lost in the evacuation because of the low vapor pressure of the solution a t the temperature of solid carbon dioxide. After the air had been pumped off, stopcock e was closed and the cooling bath removed. A water bath regulated a t zo°C was put in contact with the flask F. While waiting for the solution to warm up t o 20' the interferometer was adjusted by drawing air a t atmospheric pressure into both gas chambers and
DISTRIBUTION O F ACETOKE THROUGH A RUBBER MEMBRANE
393
turning the compensator screw so as to bring the interference bands to the zero position. This was repeated two or three times, or until the setting was constant. Stopcock d was then closed, c was opened, and the interferometer chamber I,, manometer M,, and connecting tubes evacuated to .I mm. or less, when c was closed again. Stopcock d was opened to admit the vapor into the interferometer, flask F being shaken to hasten the equilibrium. At the same time the pressure of the air in chamber I1 was slowly reduced, by opening b slightly and pumping, until the interference bands as viewed through the eyepiece of the interferometer were brought back to the zero position, when b was closed. The heights of the mercury columns in the two manometers were then observed. The difference between atmospheric pressure and the reading on M2 was equal to p ~ the , vapor pressure of the solution, while the difference between atmospheric pressure and the reading on AII gave the pressure p. of air having the same refractivity as the vapor. The refractivity as calculated from the first few readings generally came out too low, probably because of some residual air remaining in the solution. Therefore some of the vapor was pumped off through e and the observations repeated, until p~ and pa became constant. Some difficulty was experienced in establishing a state of equilibrium between the liquid and the vapor in remote parts of the apparatus. By jarring and shaking flask F persistently, however, one could usually succeed in bringing the intederence bands to a fairly stable position. It would have been bett.er to provide F with a magnetic stirrer. A very gradual but steady displacement of the bands took place even after equilibrium appeared to be established; this may have resulted from slow vaporization of heavy impurities in the acetone. As a rule this effect was too slow to interfere much with the readings. Duplicate runs gave values of ps checking to I mm. or less in most cases.
TABLE I11 T'apor Pressures of Acetone-Water Solutions a t 2ooC A Percent acetone in Solution
B Mass conc. = acetone 100 -A
PU Total vapor pressure
E M
Zero refract. of vapor
mm
10,3
,116
17.1 49.2
996
20.0
,250
81.1
IOjI
'9,3 39.0 58.3 79.6
,414
103.3 119.8 141.5 156.3 179.2
I094
0.0
100.0
,000
,639 1.39 3.90
255
a P .A hlol Partial fraction pressure of acetone of acetone -in vapor - R M - - a z pA = apM 877 mm
(10)-6
1100 I112
,000
'845 ,913 ,956 ,964 ,975
1113
,976
1132
I .oo
0.0
41.2 T3.2
98.5
115.8 138.1 1jz.5 179.2
PW Partial pressure of water pw=pX-p., mm 17.1
8.0 7.9 4.8 4.0
3.4
3.8 0.0
3 94
D. S. MORTOS
All solutions mere made up by weighing out the components. About jo 1111. of solution were used for a run. I n the case of acetone-water solutions the specific gravity of each solution was determined after the experiment and the composition obtained from Young’s’ table. The results are given in Tables 111 and IV. TABLE IV Vapor Pressures of Llcetone-31ethyl AllcoholSolutions a t Z O O C -
A Percent acetone in solution
E P \I Mass Total cone. vapor acetone pressure = 100--;l
IL,
a
Zero Refract. of vapor
&IO1
fraction of acetone in vapor a - R,-boo 534
P %. Partial pressure of acetone p k = ap,
Ps
mm 0.0
,000
10.0
,111
24.2 30.0 40.0 49.8 60.0
,319 ,423 ,666 ,992
600 IO)-^ , 0 0 0 i38 . 2 j8 83 7 ’ 444 888 ,540 928 ,614 959 ,672
96.0 110.8 128.4
28.6 57.0
82.2
72.4
67.4 61.8
jO.1
2.3j
80.1 90.1
4.02
9.06
1i9.0
1082
,902
179.2
1132
1.000
179.2
100.0
,729
1014
.,,>
1044
,831
r - *
pr-p,
mm 96.0
103.8 118.8 131,6 144.2 161.4
989
=
mm o o
134.2 145.1 154.4 162.9 169.8 173.5
1.50
1’s
Partial pressure of methl-l alcohol
89.2
j6.z 50.6 44.1 38.2 ‘9.3 1j.6 0.0
C i m c s a n d discitssion. In order to check up on the results of the mcmbrane experiments partial pressures of acetone were plotted against percentage of acetone. Curve ( I ) , Fig. 3 , shows the relation for the system acetone-water, and curve i 2) that for the system acetone-methyl alcohol. The compositions required for quilibriuni between the two solution phases are obtained by taking abscissae corresponding to the same partial pressure ordinate. Thuy, the ordinate 41 iii~n.intersects the water solution curve at I O percent and the methyl Zilcohol solution curve a t 16 percent. lherefore a I O percent acetone-water solution will exist in equilibrium with a 16 percent acetone-methyl alcohol solution. Table 1- consists of corresponding abscissae read off the, two curves: 7
water
i; aceton? 0 0
10.0
hretonemethj-I alrohol c i acetone 0 .0 I6 I j
2 0 . 0
?O.
,jo.0
-46.6
’ I-oung: “Distillation I’rinriples
I A IILJ: T7
.Icetonewater 1 acrtone 40.0 i0.S
.\cetanemet h>-l
‘
alcohr)l acptone j9 4 6g 0
60 o
76
3
;o 0
$1
;
a n d Processes.‘ 261
(1~22
DISTRIBTTIOS (OF 9CETOSE THROUGH h RUBBER MEMBRAXE
39 j
The curve corresponding to these values is shown in Fig. 1, ( 2 ) . Curve is the one obtained from the rubber-membrane experiments. The agreement between the two curves is fairly satisfactory when one considers t'hat they were obtained by entirely independent' methods, each involving more or less error. The primary object of this investigation has been t o determine the form of the vapor pressure and distribution relationships in concentrated solutions, rather than to carry o u t measurements to a high degree of precision. The fact, that the dktribution equilibria can he represented by a simple linear equation in terms of mass concentration suggests that a similar equation can be found for the partial vapor pressures. Professor Bancroft proposes the formula' (I)
p z ' = K \Cis CT.i)%, P'
1)
where p = the vapor pressure of pure
FIG.
3
Partial pressure of atetone in vapor over acetone water and aretonemethyl alcohol solutions.
Curve KO. 2 . Equilibria between acetone-methyl alcohol and acetone-water solutions I I ) By separation Tvith rubber membranes ( 2 ) From vapor pressure data.
acetone = 1 i 9 . 2 at zo°C, p' = the partial pressure of acetone over a solution containing Ga grams of acetone and Gs grams of solute, and K and a are constants depending on the solvent and the temperature. This equation is equivalent t o Raoult's law for a = I and K = b I ~ , ' L l s . Taking logarithms on bothsides, we get
1
Bancroft and Davis: J. Phys. Chem., 33, 361 (1929)
396
D. S. MORTON
which is the equation for a straight line. The calculated data are given in Tables VI and 1'11. The agreement is quite satisfactory for acetone and methyl alcohol; but there is considerable discrepency for the acetone-water solutions a t the highest and the lowest concentrations. It may be that the method of vapor analysis used is unsuitable for such readily condensable mixtures. It is hoped that this point may be cleared next year.
TABLE T'I Methyl Alcohol and Acetone a t Galc
0.83 log -- logp""
~
G.h - log P A l c - P'Alc p'llc
GAk
7cacetone
log K1 calc.
in liquid
=
log K, = 0.14
=
log Kz = 0.03
~ ' A c
GAC
0.62 log
-
20'
log R2 calc.
p' acetone found calc. mm mm
0.0 10.0
0.0710
24.2
0.1235 0.1361 0.1419 0.1416 0 . I475 0 . I337
30.0 40.0 49 8 60 o 70.I
80. I 90. I
O . I I I ~
0.1625
100.0
28.6 57 o 72.4
89.2 103.8 118.8 131.6 144.2 161.4 179.2
32.7 62.j 73.8
0.1834 0.0642 0.0188 0.0407 0.0449 0.0384
88.8 103.6 118.1 132.1 146.0 160.6
o.ojj8
0.0162 I . 9698
TABLE
in liquid 0
10.3 20.0
0.3210 0.3811
41.2 73.2 95' 5 115.8 128.1
0.402
0,4274
1
0.3954 0.2245 __
0.
--
20'
found mm
-
65.8 62.4 55.6 49.9 43.6 36.8 29'9 20.6
T'II
log l i l calc.
0.0
152.5
179.2
PlAC
73.7
96.0 82.2 67.5 61.8 56.2 50.6 44.1 38.2 29.3 17.6
-
Water and Acetone a t
76 acetone
p' alcohol found calc. mm mm
calc. mm
-
46.5 74.1 95 4 113.8 137.7 160. I
-
DISTRIBUTIOS O F ACETOSE THROUGH A RUBBER MEMBRANE
397
Summary ( I ) The distribution of acetone through a rubber membrane between methyl alcohol and water has been measured by two independent methods over a composition range of o t o 70 percent, and has been shown to agree with the equation
G AG~A I=~
2.24
(GA,//G,)~@'
The partial pressure of acetone over acetone-methyl alcohol solu(2) tions a t zo°C is represented fairly well by the expression
( 3 ) The partial pressure equation for acetone-water solutions as derived from the tu-o preceeding equations is
2.655
1j9.2
- p' -
P'
-
(ao@
This equation agrees with experimental values for intermediate concentrations, but not with those for high and low concentrations. It is possible that the discrepancies are due to experimental error. (4) The results indicate that the distribution of a solute between two phases is primarily a function of mass concentrations. Cornel1 Cnaiersity
