January 15, 1930
ISDL-STRIAL d S D ESGIL%ERISG CHEMISTRY
top of the rod and the top of the buret. At the bottom the rod is connected by means of rubber tubing to a tee. From one branch of the tee tubing runs to a valve on the vacuum line, E. The handle for operating the valve is placed on the front of the table. The other branch of the tee is connected through the trap bottles, F and G, to the open glass tube, H, which just projects through a rubber stopper set in the top of the table. The trap bottles are filled with soda lime t o filter the air which enters at H. T o fill the buret, the vacuum is turned on and the stopcock
109
of the buret is set to connect with the stock bottle. By placing a finger on the open end of the tube, H, the flow of the solution into the buret can be controlled. S n y number of these units can be combined to provide for the standard solutions which are in regular use. The only parts of the apparatus visible above the table are the burets, supporting rods, and short lengths of glass tubing attached to the bottoms of the burets. A nickel-plated finish for the rods and clamps is satisfactory in the laboratory. The burets can be removed easily for cleaning or calibrating.
Physical Methods of Separating Constant-Boiling Mixtures' Arthur A. Sunier and Charles Rosenblum U K I V E R S I T Y OF
ROCHESTER, ROCHESTER, h-. Y .
The physical methods of separating constant-boiling separating constant-boiling mixtures are reviewed. Examples of industrial applimany solutions of liquids mixtures has been of great cations are given wherever possible, although emphasis cannot be separated into commercial importance. The is placed upon general principles involved in the propure components merely by superiority of absolute alcocedures. Engineering detail is omitted, the theoretical distillation. I n a binary syshol over the azeotrope conbasis being stressed. Certain possibilities are sugtem, for example, only one taining 4.43 per cent water as gested which have as yet had no trial. of the constituents can be a solvent for certain nitrocelThe methods are divided into two classes, according obtained in a pure condition. luloses has interested industo the existence of mass or vapor pressure differences I n addition there is formed a trial chemists. The use of between components. Among the first group are iiiixture of constant boiling absolute alcohol mixed with atmolysis, non-equilibrium evaporation, thermal difpoint and of definite composihydrocarbons as a low-temfusion, and centrifuging. The second division depends tion depending only on the p e r a t u r e m o t o r fuel h a s upon reduction of pressure, formation of three-compressure. T h i s property further directed attention to ponent systems by addition of a liquid or solid, or the b y v i r t u e of w h i c h c e r the problem and has resulted production of solid phases. The use of silica gels is t a i n liquid mixtures distil in numerous patents involvmentioned. a t a constant temperature ing both physical and chemiunder a constant Dressure cal methods of setmation. without change in composition is called "azeotropisni." The use of some chemical reaction to remove a n undesiraThere are t n o classes of constant-boiling mixtures, de- ble component from a liquid system has been a common pending on whether the mixture boils above or below the practice for centuries. For example, quicklime was known boiling point of either pure constituent. Their properties as a dehydrant of ethyl alcohol as early as t'he tenth century can readily be seen with the aid of a composition-boiling (26). Howel-er, it is not the immediate purpose to review point diagram. Consider a t constant pressure mixtures the problem from the chemical standpoint. 'The works of of components -4 and B having boiling points t d and le, Patart, (%), Pique (26, E), hlariller (17, 18), and Cooley (5), respectively. Figure 1 shows the variation with temperature on the prepa1,ation of absolute alcohol, from the constantof the composition of the liquid and vapor phases. It is boiling mixture of greatest industrial import'arice have disevident that during distillation both phases approach a cussed quite fully the chemical methods of separation. I n state of constant composition. Obviously, regardless of this paper only methods based on well-defined physical nhich component is in excess, the difference between the principles are to be considered. Certain lines of attack composition of liquid and vapor in equilibrium decreases which have not yet' been imestigatetl but ~ ~ h i seem c h t'o be until a t c the composition of the liquid phase is identical valid will be suggested. For simplicity only binary systems with that of the vapor. Therefore continued fractionation will be discussed, since the theory of ternary systems is not produces no further change in composition. The mixture yet very well developed. Any process which pernianently of composition c and boiling point t , is the maximum azeo- changes the equilibrium concentration of a.n azeotrope, trope. Figure 2 shows similar curves involving a minimum thereby allowing separation by fractionation, will be conconstant-boiling mixture. sidered a solution to the problem in hand. Approximately two thousand cases of azeotropism are Classification of Methods known. Examples of the first kind of azeotropism are the systems: nitric acid-water, halogen acid-water, formic acidThe physical methods of separating constant-boiling mixwater, and chloroform-ethyl acetate. Typical minimum tures fall into two general classes. Certain of these depend azeotropes are: ethyl alcohol-water, ethyl alcohol-carbon on differences in mass of the components. Abmolysis, nontetrachloride, methanol-chloroform, acetonecarbon disulequilibrium eyaporation, and thermal diffusion are examples fide, and ethyl alcohol-water-benzene. More detailed in- of this division. Centrifuging has also been suggested ( $ 2 ) . formation concerning this phenomenon is given by Young Other met'hods involving vapor-pressure relations have (41) and Lecat (14, 15). found greater commercial application. Among these are Aside from its purely scientific interest, the problem of variation of pressure and addition of a third component, ' Received M a y 10, 1929 either a liquid t o form other azeotropes or a sohd which will
T I S well known that
Ai$-L4LYTICALEDITIOS
110
change the vapor pressures of the components to a different dcgree depending on its solubility in the respective members of the system. The “salting-out” effect ma)- be applied. Freezing niay also quite conceivably be used as a method of separation. 1-ery recently silica gels ha\-e been found t o break u p azeotropic mixtures. METHODS DEPENDING ON MASS OF COMPONENTS
Atmolysis
I n his studies of the molecular motion of gases Graham (6.7 ) effected the separation of oxygen from air and from a mixture with hydrogen by allowing the gases to diffuse through a porous wall. He stated that the diffusibility of a gas was inversely proportional to the square root of its density. From the ideal gas equation PV = n mv2 = RT it follows that a t constant temperature the velocities of molecules of different gases are inversely proportional to the square root of their molecular xeights
If we consider two gases in a closed container. the lighter gas will strike the wall more often than the heavier, depending on the respective molecular weight ; or If there is a small opening in the wall, of the order of magnitude of a molecular diameter, more molecules of the lighter component will leave the container. Looking upon a porous wall as made up of countless minute openings, ive should expect, by diffusion of the gas mixture through it, an enrichment of the denser component in the residual gas. This phenomenon is called “atmolysis.” The theory of atmolysis for gaseous systems has since been established by Rayleigh (29) for two components, considering enrichment of the heavier fraction. Nore recently Sameshima ( S I ) has treated both residual and diffused portions for any number of components. E. and R. Urbain (34) have extended these considerations to the case of vapors from an azeotropic mixture, which differ from simple ga,qeous diffusion in that partial condensation of enriched vapors produces a residue of changing composition. Obviously If a constant-boiling mixture were distilled and its vapors allowed to diffuse into a wicuiiiii through a porous wall, a concentration of the heavier component in the reridual vapors should result.
T-ol. 2. KO, 1
upright tube through which vapors poor in the heavier constituent will diffuse and be removed. The porous tube is connected through a descending spiral condenser to a threeway arrangement by means of which the distillate can be re-admitted for further distillation and atmolysis or lead to a receiying Aask. By this procedure 99.8 per cent etliyi alcohol and 99.6 per cent nitric acid were obtained. Figure 3 gives diagrammatically the apparatus used by Urbain and differs a little from the figure given by Pique (26). Khether or not the enriched distillate is to be re-admitted to the distilling flask depends, of course, upon the properties of the particular system concerned. Consider the distlllation of a water-rich mixture of ethyl alcohol and water xhich form a minimum azeotrope as shown in Figure 2 , where R is alcohol and c is 95.5 per cent. At first the distillate consists of the constant-boiling mixture s h i l e the residual liquid becomes richer in mater. There is. of course, no advantage in uniting the enriched distillate with the impoverished residue. However, when the azeotrope itself is distilled, both distillate and residue are enriched-the former by vapor diffusion and the latter by the partial condensation of atmolyzed vapors-and recombination of the portions is desirable.
.i
S O
Concentratton
E
’
6
Point Diagram Figure 2-Composition-Boiling f o r M i n i m u m Constant-Boiling Mixture
The reieree is true for the system nitric acid and water forming a mixture of maximum boiling point. Distilling a water-rich nitric acid solution increases the acid concentration in both phases and therefore reunion is beneficial. But when the constant-boiling mixture is obtained it is futile to recombine the acid-rich distillate with the residue, for only nitric acid would distil over until the azeotropic point is again reached. Evidently the success of this method dependb on various factors. The difference between the molecular weights of the components, the rate of distillation, length and porosity of the atmolyzer wall, and efficiency of maintaining a vacuum determine the success of the separation. Non-equilibrium Evaporation
A
SO
Concentration E
I O O h
B
Figure 1-Composition-Boiling P o i n t Diagram for M a x i m u m Constant-Boiling Mixture Continuous line, liquid phase; dotted line, vapor phase
Such a separation was successfully attempted by E. and R. Urbain (35, SG), who applied the equivalent of Graham’s (?) atmolyzer tube to the removal of water from ethyl alcohol and from nitric acid. Their apparatus consisted of a distillation flask surmounted by a porous tube tightly jacketed in glass for the purpose of producing a vacuum outside the
Of the methods suggested to take advantage of the mass of the components, only atmolysis has been applied. Several of the others have been used in the separation of isotopes and it is quite conceivable that they should be applicable to the separation of constant-boiling mixtures. The method of non-equilibrium evaporation or “free evaporation” has been applied to the partial separation of the isotopes of mercury and chlorine (2, 3, 10, 23). From kinetic considerations of the equilibrium between liquid and vapor phases, it can be shown that the weight of a substance evaporating per second per square centimeter of liquid surface is w = p d m , where JI is the molecular weight of the liquid, p its partial pressure in dynes a t absolute temperature, T ,
January 15, 1930
I S D C S T R I A L S S D EA-GISEERISG CHEJIISTRY
and R the gas constant. If a good vacuum is maintained above the surface of the liquid, equilibrium is destroyed, and u: represents the weight of the substance which will be removed per second per square centimeter of exposed surface. Evaporation of a mixture of liquids a t a constant temperature under such non-equilibrium conditions must proceed according to the relation where p now is the partial pressure of a component. Sote-Apparently t h i s relation was first suggested b y Sunier (32). Since t h e present article was written Washburn (37) has studied t h e applicability of t h e non-equilibrium method t o t h e separation of complex mixtures found in t h e petroleum industry. Washburn has also discussed t h e applicability of t h e method t o t h e present problem a n d has given t h e same equation a s t h a t presented b y Sunier.
I n applying these considerations to the separation of isotopes, the partial vapor pressures of the components are supposed equal due to atomic similarity, and therefore differences in mass control the efficiency of separation. For azeotropic mixtures the vapor pressures of components are usually not equal; and in favorable cases vhere the ratio of partial pressures is of the same order as that of the respective molecular weights, the ratio ~ L , / I C ~would vary considerably from unity. Katurally a decided separation should be expected. Since few data are available concerning partial pressures in constant-boiling mixtures a t reduced pressure, prediction concerning the possible efficiency of the method becomes uncertain. For one thing a very high vacuum cannot be maintained over a system of relatively high vapor pressure. However, it seems reasonable to expect a continuous change in composition of both phases until there is left the particular azeotrope of the components which boils at the temperature under which the vacuum is being maintained. This procedure is not, of course, strictly non-equilibrium evaporation, and therefore the effect of mass differences is subordinate to differences in partial pressures. Nonequilibrium conditions may be more closely approximated by introducing a rapidly flowing stream of some inert gas into the vapors or bubbling through the liquid to sweep away the vapors formed. Here, too, predictions would be futile, for such experiments have not been reported. However, from the data to be discussed later on the change in composition of azeotropes with pressure, rre should expect an effective separation of the alcohol-water system. Centrifuging
The methods of centrifuging and thermal diffusion have been suggested (32) as possible ways of separating constantboiling mixtures. Both have been used in attempting to separate isotopes and other mixtures, but no efforts have been made to apply them to the separation of azeotropic solutions. Considering the extremely great gravitational field produced by a centrifuge, Poole (28) approximates the separatory effect on a mixture of two isotopes to be given by where nl and n2 are the number of atoms of the isotopes per unit volume, Adl and M 2 , and their respective atomic weights, T is the absolute temperature, and g the acceleration a t the distance 5 from the center of the centrifuge tube. Mulliken (28) has given a mathematical analysis of the whole problem, including equations showing the effect to be expected on centrifuging any solution, It is true that both Joly and Poole (13) and Xulliken (21) have not been able to detect appreciable separations (after centrifuging) of the isotopes of lead or mercury, respectively, but it is almost certain that vibration of the apparatus aids diffusion to such an extent that negative results were obtained for these extremely difficult cases.
111
However, Joly and Poole ( I S ) have obtained definite separations, with alloys of lead-tin and lead-tin-bismuth, of 0.63 and 1.8 per cent, respectively, thus establishing the t'heoretical possibility of such separations as well as the practicability in some cases. It would seem, then, that constant-boiling mixtures might be partially separated by such a process. Thermal Diffusion
Thermal diffusion shows more promise. If a mixture of gases of different molecular weights is allowed t o diffuse freely through a tube, the ends of which are kept a t different temperatures, a concent,ration of the gas of higher molecular TTeight will be found at the colder end. This phenomenon has been used by Chapman and Dootson (4) to effect t'he partial separation of hydrogen from carbon dioxide and from sulfur dioxide. Starting with a mixture of 61.0 per cent hydrogen and 39.0 per cent carbon dioxide in one of his experiments, Ibbs (12) has effected a 3.5 per cent separation between the gases from the hot and cold ends. The composition of the gaseous mixture in the hot end, kept a t 570' C., was 62.76 per cent hydrogen and 37.24 per cent carbon dioxide, while that kept a t 16.2" C. was 59.24 per cent hydrogen and 40.76 per cent carbon dioxide. If vapors from a boiling azeotrope are superheated instead of condensed, there should result a concentration of the heavier component.. Of course, the methods so far suggested, except for the case of atmolysis, have not yet had experimental application to our problem. However, it was thought useful t o mention them as possibilities since their physical bases seem to be valid. METHODS
INVOLVING VAPOR P R E S S U R E
The next class of methods involves the changing of the partial vapor pressure relations of the two components by some process; such methods have been more widely used than those which are dependent on mass relations. .Vole- Readers who are familiar with t h e concepts of fugacity and activity developed b y G. N. Lewis (16) will appreciate thmt i t would appear more accurate t o state this proposition a s follows: Any process b y which t h e relative fugacity or activity of t h e components is changed should effect a t least a partial separation of t h e constant-boiling mixture. Such a statement covers all of t h e cases yet t o be discussed.
Reduction of Pressure
A very simple way of separating certain constant-boiling mixt.ures is distillation under reduced pressure. The works of Roscoe and Ditmars (SO) and of Hulett and Bonner (11) show that the composition of the constant-boiling hydrochloric acid varies with pressure. The change is not considerable, increasing from 18 per cent hydrogen chloride a t 250 cm. to 23.2 per cent at 5 cm. pressure. However, the ethyl alcohol-water system allows of a much better separation into components, as is evident from Tahle I, which is extracted from Merriman's (19) experiments with the iystem in questmion. These data are plotted in Figure 4. T a b l e I-Composition
PRESSURE .Wm .. . 1451,3 1075.4 760.0 404.6
of Alcohol-Water A z e o t r o p e s a t Different
Pressures WATERI N AZEOTROPE PRESSURE Per cenl .bf m. 4.75 198.4 4.65 129.7 4.4 94.9 3.75 70.0
WATERI N AZEOTROPE Per cent 2.7 1.3 0.6 0.0
Here a complete separation takes place under a pressure of 70 mm. a t 27.96' C., the boiling point of absolute alcohol under such conditions, Thus, fractionation of the minimum constant-boiling ethyl alcohol-water mixture a t a moderately reduced pressure produces absolute alcohol.
112
A.t-A L I’TICA L ED1 T1O.V
This fact has actually heen embodied in the Barbet patent, which is now in operation in France (25, 26). Apparently this work was anticipated by the patents of Winter (38) &s has been pointed out by Cooley ( 5 ) .
s-01. 2 , s o . 1 Addition of Third Component
Another procedure which has found profitable industrial use involves the addition of a third subst>ance, usually a hydrocarbon, to form a ternary constant-boiling mixture having a boiling point below those of the binary system and of the pure liquids. The experiments of Young (,p, 45) and his eo-workers on the systems involving alcohol, Tvater. and beiizene yield the significant re,sults incorporated in Table 11, n-hich is the basis of a number of patents granted to the United States Industrial Alcohol Company. The word “basis” is used advisedly! since Young’s TTork provided a firm foundation for the subsequent years of research coidiicted 11sthis company, which finally led to the development of a continuous practical process by which several millions of gallons of anhydrous alcohol are now produced annually. Table 11-Composition a n d Boiling P o i n t s of All Constant-Boiling P h a s e s Formed from Alcohol, Water, a n d B e n z e n e BOILING -----COMPOSITION--~IIXTURE POIST Alcohol Water Benzene c. Per cent Per cent Per cent Vv’ater- alcohol-benzene 64. S5 18.5 7.4 74.1 Alcohol-benzene 68.25 32.41 .... 67.59 Water-benzene $9.25 8.83 91.17 Alcohol-water 88.15 7 4.43 .... Alcohol 78.3 1 .... Benzene 80.2 100’ , Water 100.0 100’ ‘
.. ... ...
Figure 3-Atmolysis
Apparatus Used by Urbain
Similarly almost complete separations have been effected for the systems ethyl acetatewater, ethyl acetateethyl alcohol, and ethyl acetate-alcohol-mater (20). From a consideration of the azeotropic properties of a great many mixtures, Young (42) concluded that the concentration of t h a t component in a binary system which has the lower d p / d t increases as the pressure under which the mixture exists is lowered. T h a t this is only an approximation is shown by the very important ethyl alcohol-water case, which is an exception to the rule. It has been pointed out t h a t the basis for Young’s suggestion is not fundamental, and attempts have been made to account for all cases in terms of the latent heat of vaporization. Krewsky (39) and more recently Tanaka and Kuwata (33) have shown that for all azeotropis mixtures of the lower alcohols, the concentration of the component with the greater heat of vaporization increases when the boiling point or pressure of the boiling liquid is greater. Further experimental tests of these theories are being made in this laboratory. From the data given in Table I it is apparent that increase of pressure changes the composition of the axeotropic mixture. In one scheme proposed for dehydrating ethyI alcohol with benzene, the pressure is raised to a point where alcohol is no longer contained in the water-benzene mixture distilled.2 The method previously suggested of sweeping away vapors becomes more reasonable in view of these data. CertainIy the variation of partial pressures with temperature is different for components of a mixture. Therefore a solution of a coniposition which at atmospheric pressure corresponds to the azeotrope would at room temperature be in equilibrium with vapors of decidedly different composition from that of the solution. Thus a steady f l o ~(not necessarily rapid) of a sweeping gas should tend to resolve the mixture into its components. Information kindly supplied by A. A Backhaus in a private communication.
Evidently if a sufficient amount of benzene, readily calculable from the aboye data of Young, is added to ordinary 95.5 per cent alcohol, it is possible to remove the water simply by distilling off the ternary system, which boils about 14 degrees below absolute alcohol. The benzene can be recovered simply by dilution with water. The patent of the Ricard-dllanct Company (27) covers the use of trichloroethylene, which gives a ternary system boiling a t 67.10” C. Other substances, such as chloroform, carbon tetrachloride, carbon disulfide, ethyl acetate, and certain ketones and hydrocarbons, have been suggested.
Per Cent Woier in Areoirop,c Mixture
Figure 4-Water in Alcohol-Water Azeotropic Mixtures a t Different Pressures
X study of the fundamental lams of solution leads to still another possibility. If a solid solute is added to a mixture of liquids, it should distribute itself according to the distribution law, and its effect on each component bhould be approximately as is demanded by the law of Raoult. The difference in solubility of the solute in the components of a solution should change the relationship between their respective partial pressures. Some preliminary experiments by Mariller and Coutant (18) bear out this contention concerning the effect of a substance soluble in a mixture but of different solubility in the constituents. Thus compounds such as calcium chloride, sodium chloride, or glycerol, which are more soluble in water than in alcohol, enrich the vapors from an ethyl alcoholwater azeotrope; and conversely mercuric chloride, which has a greater solubility in alcohol, impoverishes the vapors. They conclude “that the added substance facilitates the re-
I S D C S T R I A L A X D E S G I A ~ E E R I X GCHEJIISTRY
January 15, 1930
moval of the component which tends to make it more insoluble in the mixture." A somewhat similar method is the so-called "salting out" effect, which consists in the production of a state of immiscibility by addition of a new substance. This has been found successful in separating water from alcohol. Addition of potassium succinate3 causes the separation of a salt solution in water, allowing the decantation of alcohol comparatively free from both salt and water. An analogous case involving the addition of a liquid is seen in the dilution of the azeotropic system, alcohol-water-benzene, with water, resulting in the formation of a benzene layer. One manufacturer of ethyl acetate gets a ternary constant-boiling mixture of ethyl acetate, ethyl alcohol, and water. This ternary mixture, which a t ordinary room temperature is a homogeneous liquid, is washed with water a t room temperature, thereby separating the original ternary mixture into two liquid phases; one phase containing nothing but water and ethyl acetate and the other containing all of the alcohol originally in the ternary mixture and ethyl acetate u p to the saturation point. The binary mixture of ethyl acetate and water can then be readily separated by distillation, since the constant-boiling binary mixture of ethyl acetate and water contains nearly twice a$ much water as the solubility of water in ethyl acetate at room temperature.* It is of interest to mention a mechanical method presented by Cuvillier (26) some thirty years ago in connection with the ethyl alcohol problem. An immiscible liquid such as olive oil of higher boiling point and lower density than the components in question would form a film over the surface. The liquid of lower boiling point, having a higher partial vapor pressure, would be hindered in its passage through the film less than would the component of lower vapor pressure. In this way it was expected that by heating such a system the vapors would be enriched with respect to the lower boiling component. This suggestion has proved only of historical importance. . Fractional Recrystallization Before leaving the methods depending on vapor pressure relations, it should be pointed out that fractional recrystallization is a quite possible method of separating components if their freezing points are reasonably easily reached. By freezing down a liquid system, we have departed from the region where vapor pressures of liquid phases govern separation. It is now a question of equilibrium between a solute and solution. Whichever component of the system crystallizes out first depends. of course, on the composition of the mixture and upon the phase relations in the neighborhood of the freezing point, a discussion of which is beyond the scope of this paper. However, i t should be very easy to separate a n azeotrope of benzene and acetic acid into its components simply by freezing. This system forms a t 760 mm. a constant-boiling mixture containing 2 per cent of acetic acid and boils at 80.05' C. (1, Z$). T h a t this azeotrope can be resolved is evident from the freezing point data given in Table 111. Table 111-Freezing
P o i n t Data for A z e o t r o p i c M i x t u r e of B e n z e n e and Acetic Acid FREEZING ACETIC FREEZING ACETIC POINT ACID POIXT ACID o c M o l per cent M o l p e r cent
c.
If the constant-boiling mixture is cooled to about -5' a Observation of C.
hf. U'hite in t h i s Laboratory.
C.
113
benzene will crystallize out leaving a liquid phase considerably enriched in acid. Distilling this enriched mixture will separate the acid from the azeotrope, which can be again subjected to the same treatment. Evidently this procedure can be employed to effect a considerable separation of benzene from acetic acid. This is a convenient system for illustrating the method because of the relatively high freezing points of both components. Adsorption by Silica Gels The last method left to be discussed cannot be included in either of the classes originally named. It depends on the adsorption by silica gels of one component in preference to others from a mixture of liquids. Very recently Grimm and his co-workers (5,9) have shown that silica gels are capable of greatly changing the composition of liquid mixtures. The gel may be in contact with either liquid or vapor phases, and can be regenerated by heating in vacuum. As an example of the efficiency of separation it is interesting to find that the concentration of carbon tetrachloride in the azeotrope carbon tetrachloride-ethyl alcohol is changed from 84.15 per cent to 60 per cent. It was noted that the size of the pores and grains of the gel affected the separation, finer grains and narrower pores being more efficient. Further, that component having the greater heat of wetting is the more strongly adsorbed. This application of silica gels to the separation of azeotropes into components is the last method to be discussed. It is of very recent conception and secms to be very promising. Literature Cited Beckmann, Liesche, a n d Gabel, Z . physik. Chern., 92, 421 (1917). Bronsted a n d Hevesy, Xalure, 106, 144 (1920); 107, 619 (1921). Bronsted and Hevesy, Phil. Man., [SI 43, 31 (1922). Chapman and Dootsofl, Ibid., 33, 248 (1917). Cooley, Chem. M e t . E n g . , 34, 725 (1927). G r a h a m , Phil. T r a n s . , 153, 385 (1863). G r a h a m , A n n . chim. phys., [11 4, 154 (1864). Grimm a n d Wolff, Z . annew. Chem., 41, 98 (1928). Grimm, Raudenbusch, a n d Wolff, I b i d . , 41, 104 (1928). Harkins and Mortimer, Phil. M a g . , [7] 6, 601 (1928). Hulett a n d Bonner, J . A m , Chem. So< , 31, 390 (1909). Ibbs. Proc. Roy. SOC.( L o n d o n ) , 99A, 385 (1921). Joly and Poole, Phil. .Mag., [6] 39, 372 (1920). Lecat, "La tension d e vapeur des melanges de liqtiides: L'azeotropisme," Lamertin, 1918. Lecat, A n n . sac. sei. Bvzrrelles, 46, 169, 284 (1926); 47, 21 (1927); Rec. trau. chim., 45, 620 (1926); 46, 240 (1927). Lewis a n d Randall, "Thermodynamics," Chap. 17 a n d 22, McGrawHill, 1923. hlariller, Bull. assocn. chim. sucr. dist., 42, 247 (1925). Mariller a n d C o u t a n t , Ibid., 42, 288 (1925). Merriman, J . Chem. Soc., 103, 628 (1913). hlerriman, I b i d . , 103, 1790 (1913). Mulliken, J. .Am. Chem. Soc., 44, 1033 (1922). hlulliken, Ibid., 44, 1729 (1922). Mulliken a n d Harkins, Zbid., 44, 37 (1921). Nernst, Z . physik. Chem., 8, 110 (1891). P a t a r t , Bull. S O L .encouv. i n d . nat., 136, 188, 201 (1924). Pique, Bull. assocn. chim. w c r . disl., 41, 337 (1924). Pique, Zbid., 41, 386 (1924). Poole, Phil. M a g . , [SI 41, 818 (1911). Rayleigh, Ibid., [ 5 ] 42, 493 (1896). Roscoe a n d Ditmars, J . Chem. Soc., 12, 136 (1860). Sameshima, J a p a n . J. Chem., 2, 3 3 (1925). Sunier, J . Chem. Education, 6, 879 (1928). T a n a k a a n d Kiiwata, Chem. S e ; ; s , 137, 1 3 (1927); J. Faculty Eng. T o k y o I m p . Cniv., 17, 117 (1927). Urbain, Compi. r e n d , 176, 304 (1923). Urbain, I b i d . , 176, 166 (1923). Urbain, J. Soc. Chem. Znd., 42, 201 (1923). LVashburn, Buy. Standards J . Research, 2, 476 (1929). Xvinter, U. S. Patents 1,427,885-8 (1922). Wrewsky, Z . physik. Chem., 81, 1 (1912); 63, 551 (1913). Young, J. Chem. Soc., 81, 707 (1902). Toung, "Distillation Principles a n d Processes," hlacmillan, 1922. Young, OD. c i t . , p. 61. Young a n d Fortey, J . Chem. Soc., 81, 717, 739 (1902).
