The Temperature-Composition Diagram of the Partially Miscible System Water-Isobutyl Alcohol JOSEPH J. JASPER, CYRILL J. CAMPBELL, a n d DONALD E. MARSHALL Wayne University, Detroit, Michigan THEORETICAL DISCUSSION partially miscible, and (3) components completely HEN two pure, chemically similar, volatile immiscible. Of these, the second offers the greatest liquids are mixed, no appreciable energy or number of difficulties in obtaining the necessaty data volume effects are observed, regardless of the for the construction of the temperaturecomposition proportions of the components employed. Since the diagram. This is due to the fact that the distillate concentration of each component is decreased by samples break down into double liquid layers a t the addition of the other, the vapor pressure of each is relatively low temperature (of the room) a t which the correspondingly decreased. The partial vapor pressure compositions are determined. The purpose of this of either component in the vapor phase, however, is paper, therefore, is to present a relatively simple exactly proportional to its mol fraction in the liquid method for obtaining the necessary data for the temphase, and the requirements of Raoult's law are ful- perature-composition diagram of partially miscible filled. Such solutions exhibit a constant environ- liquid systems. The authors believe that the method mental effect throughout the entire composition range described will apply to all such systems with possible slight variations as the case requires. The system and are accordingly classified as ideal. If the two components are chemically dissimilar, water-isobutyl alcohol was used in this study. deviation from ideality occurs, which becomes increasTHE RERERENCE CURVE ingly pronounced as the dissimilarity increases and the temperature decreases. Such deviations become maniThe initial step in the procedure was to construct a fest in vapor pressurecomposition diagrams by de- reference curve by plotting the refractive indexes of parture from the straight-line relation of Raoult's prepared solutions against the corresponding known law. With increasing dissimilarity, it is obvious that compositions. In order to obtain a smooth, cona condition should eventually be reached a t which a tinuous curve for the entire composition range, it was vapor pressure-composition diagram would exhibit necessary to employ a third solvent in which both both a maximum and a minimum for some definite water and isobutyl alcohol are soluble. The necessity temperature. These are not actually realized, for for the third solvent is obvious, since water and isoa t the given temperature two saturated phases appear, butyl alcohol are but partially miscible in each other which are in equilibrium and whose compositions and, therefore, form two saturated-solution layers over depend upon the condition that the partial pressure a rather large total composition range. It is important of each component must be the same from each phase. to obtain homogeneous systems for the whole range These two-component systems are, therefore, univari- in order to determine the refractive indexes. The third ant, and for a given temperature the composition of solvent used was 95 per cent ethyl alcohol. each phase is constant and independent of the proA series of 24 mixtures of isobutyl alcohol and water portions of the components. The addition of either was used in the preparation of the reference curve. component results only in changing the relative vol- These were prepared by adding increasing quantities umes of the two liquid phases. The vapor pressure of isobutyl alcohol in increments of 0.5 ml. to successive of such a system remains constant during distillation, 10-ml. volumetric flasks. Water was then added to and, consequently, a distillate of constant composition each flask until the liquid level was a t the 10-ml. results as long as the two liquid phases are present. mark. The last five solutions of the series were made These systems, therefore, behave like constant-boiling up by the addition of 0.2-ml. increments of the alcohol. mixtures. During distillation, the liquid phases con- A 10-ml. graduated pipet was used and care was taken tinually decrease in volume and eventually one of them to drain the pipet slowly. A double liquid layer was disappears; the composition of the remaining liquid observed in most of the flasks. Exactly 2.5 ml. of phase depends upon the total composition of the ethyl alcohol were then added to each flask; the original two-phase system. contents were carefully mixed. A clear, homogeneous It is clear, therefore, that the variation of inter- mixture resulted in each case. The flasks were tightly molecular attractive forces, acting between different stoppered and numbered; the solutions were reserved components, gives rise to varying degrees of deviation for refractive index measurements. from ideality. This leads to the simple and convenient An Abbe refractometer was used and the temperamethod of classifying solutions according to the type ture was regulated to 20°C. The mol fraction of isoand magnitude of the deviation shown. Thus we have, butyl alcohol in each mixture was calculated on the (1) components completely miscible, (2) components basis of a total volume of 10 ml., the effect of the alcoh& L i40
w
present being disregarded, since this factor 'is automatically canceled in the reading of the unknown mixtures which were obtained during the distillation. The isobutyl alcohol used boiled between 106.8'C. and 108.0°C. a t 748.2 mm. pressure. The data are shown in Table 1. The refractive indexes n were plotted as
Flask
Cornposirion Isobu~yl ~lrohol. warn, MI. MI.
Mol Prodial ~sobutyl Alcohol
*lo-
bent sharply downward (about 45') to insure rapid passage of the distillate. The neck of the flask was wrapped with several layers of asbestos to prevent uneven heating. A thermometer with a lens front having O.1° scale divisions was used.
D.
ordinates against the mol fractions of isobutyl alcohol as abscissas. The curve thus obtained (see Figure 1) was used to determine the composition of the samples obtained from the distillation of the water-isobutyl alcohol system. Since i t was necessary to obtain samples of distillate and residue simultaneously, arrangements had to be made to remove small quantities of the liquid from the flask without interrupting the distillation process. To accomplish this, a residue delivery tube D was made by extending a length of capillary through the stopper nearly to the bottom of the flask. The upper end was bent downward as shown in the figure. The residue receiving tube E consisted of a small length of 7-mm. tubing closed a t one end and equipped with a side arm. This receiving tube was carefully graduated to one ml. The distillate receiving tube F was also graduated to one ml. A small rubber washer G, wbich fitted into 1.3395 1 I the neck of tube E, was drawn over the end of tube D, as shown in the figure. To obtain a sample of the 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 liquid within the flask, the delivery end of tube D was Mol Fraction of Isobutyl Alcohol inserted into the residue receiving tube with the rubber washer effectively closing the neck. Liquid from the flask was then drawn into the receiving tube by suction THE DISTILLATION PROCESS through its side arm. To insure that the residue Two distillations were made, one with an excess of samples withdrawn from the distillation flask were isobutyl alcohol and the other with an excess of water. representative of the mixture present, ,the withdrawal The procedure was essentially the same in both cases. tube was each time blown clear of trapped liquid and The apparatus assembly is shown in Figure 2. A the samples quickly withdrawn as described. Instead of heating the flask directly, i t was immersed 500-ml. distillation flask A was used, wbich had a deliverv tube 45 cm. lone and of 5-mm. bore. The in a strong solution of calcium chloride contained in a delivery tube functioned as an air condenser and was 4-liter beaiker. About 250 ml. of isobutyl alcohol were
-
placed in the distillation flask and enough water added until, after shaking for several minutes, a definite double layer was formed. The temperature of the bath was raised slowly nntil the liquid in the flask just started to boil. The boiling was regulated to such a rate that the vapor front was, held withm 15 cm. of the outlet of the air condenser. After discarding the first few milliliters of distillate, but while there was still a double layer present i n the jlask, exactly one ml. of the distillate was collected. As nearly simultaneously
I 0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.91.0 Mol Fraction of Isobutyl Alcohol
PICURE 3.-TEMPERATURE-COMPOSITION DIAGRAM O F THE WATER-ISOBUTYL ALCOHOLSYSTEM as oossible with the collection of the distillate samole. a one-ml. sample of the residue was collected from the alcohol layer. The temperature and barometric pressure were noted immediately. A double liquid layer appeared in the distillate sample after cooling. Exactly 0.25 ml. of ethyl alcohol was then added to both the distillate and the residue samples by means of a one-ml. graduated pipet. The samples were thoroughly shaken and then placed in 10-ml. test tubes. These were tightly stoppered, labeled, and reserved for the refractive index measurements. For convenience, the fractions were labeled Id and l r , 2d and 2r, etc., with the corresponding temperature a t which they were collected. The temperature remained constant until the double layer disappeared. After this, the temperature gradually increased. A number of samples of both the distillate and the residue were collected a t temperatures varying between the boiling point of the two-phase mixture and the boiling point of the isobutyl alcohol. Great care was taken to raise the temperature very slowly after the double layer disappeared. When the boiling point of the isobutyl alcohol was approached closely, the distillation was stopped. The distillation system was then carefully cleaned and 250 ml. of water were added to the flask. Isobutyl alcohol was now added until, after several minA
utes of shaking, a double layer was formed. The distillation process was repeated exactly as described above, distillation being stopped when the boiling point of pure water was closely approached. In every case, one-ml. samples of the distillate and the residue were obtained; 0.25 ml. of ethyl alcohol was added to each sample. After the second distillation was completed, the refractive indexes of the various fractions of the distillates and the residues from both distillations were determined a t the reference temperature of 20°C. By applying the refractive indexes to the reference cnrve, the mol fraction of isobutyl alcohol in each sample was determined. The data are assembled in Table 2. The rnol fractions were plotted, as abscissas, against the temperatures (corrected to 760 mm. of mercury) a t which each sample was collected, as ordinates. The boiling points and refractive indexes of the pure components were also plotted. A temperature-omposition diagram of the partially miscible system waterisobutyl alcohol was thus obtained. This is shown in Figure 3. It is to be noted that the effect of the ethyl alcohol on the refractive indexes of the mixtures is canceled in the described procedure; it functions only as a mutual
.
Tm9nnlrrc Obsnmd. Conrclrd Obreracd 'C. la 760 Mm.. Prrrsurc.
T.
Mm.
Rcfrorriuc Index
Mot Froclion of
01 20°C.
I s o b ~ l y Alcohol* l
Dislillolc
Rddw
DirliUelc R~sidue
Distillation 6th ercenn water
Distillation with excess iaobutyl nleohol
* Interpolated fmm the reference refractive index-omposition
curve
solvent to produce homogeneous solutions of the components. Without this procedure i t is impossible to determine the refractive indexes of the distillates (and, therefore, their total compositions) except for those samples whose compositions approach very closely those of the pure components. Only one or two points would be obtained for the liquid and the vapor curves a t opposite ends of the temperature-composition diagram, a number certainly insufficient to construct a complete diagram.
