T H E DETECTION OF CONSTANT-BOILING MIXTURES BY T. R. B R l G C S
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Professor Orndorff has called my attention to the apparent lack of any simple laboratory method of distinguishing1 an azeotropic* mixture from a chemical individual, since both boil at constant temperature, and thus may be distilled without change. The following experiments were therefore undertaken to supply such a method and were carried out and confirmed independently by several of my students during the course of their undergraduate research. Elaborate apparatus was avoided purposely, so that no great degree of exactness can be claimed for the results. The principle underlying the method is well known; but since the experiments are instructive and well fitted, by reason of their simplicity, to be included in a laboratory course in physical chemistry, they are deemed worth describing. It has long been recognized that the coinposition of a constant-boiling mixture varies with the pressure under which the process of ebullition is carried out. Thus it is known that a decrease in pressure displaces the composition of the azeotropic mixture toward the acid side in the case of hydrochloric acid and water, and toward the alcohol side in the case of ethyl alcohol and water3. It is evident, therefore, that a mixture which is azeotropic at one pressure will not be azeotropic at some other pressure, and will change in composition if distilled at this pressure. The extent to which this change in composition takes places will depend upon the effectiveness of the still-head, the slope of the temperature-composition curves in the neighborhood of the azeotropic points and the magnitude of the displacement which the pressure change produces. Such, in brief, is the principle on which these experiments are based. Two general cases arise ( I ) where the azeotropic boiling point is a maximum, ( 2 ) where the azeotropic boiling point is a minimum, and as examples of these I have chosen the systems, hydrochloric acid-water and methyl alcohol-benzene, respectively. The azeotropic mixture (A,) in each of these systems, prepared at atmospheric pressure (Po)has been distilled at a lower pressure (PI) through a fractionating column, and the changes in composition which have resulted have been noted by direct analysis or by observing some physical property, such as the density. The exact nature of these changes may best be understood by considering the temperature-composition diagrams for the two systems in the neighborhood of the azeotropic points. These diagrams have been sketched in Fig. I , in which no attempt has been made to draw the curves to scale. The lines Cf. Forster and Withers: J. Chem. SOC.,99, 266 (191 I ) . Cf. Young: “Distillation Principles and Processes”, 46 (1922) ; Wade and Merriman: J. Chem. SOC.,99, 1004 (191I ) . 3Wade and Merriman: J. Chem. Soc., 99, 997 (1911); Merriman: Ibid., 103, 628 1
2
(1913).
DETECTION OF CONSTANT BOILING MIXTURES
645
marked V, and VI ,R, and R1, represent the compositions of vapor and liquid respectively at the pressures Po and PI, A, and AI being the corresponding azeotropic compositions. In the case of hydrochloric acid and water the constant boiling mixture A,, prepared by distillation at Po,will begin to boil under the pressure PI at a ttemperatjurerepresented by the point b and the vapor will have the composition denoted by b'. On distilling A, with a fractionating column, the distillate will therefore contain relatively less of the acid and more of the water, and if distillation be continued, the residue in the flask will attain the composition represented by c, and will be transformed into the low pressure azeotropic I 1 ' mixture AI. When this point has been reached, no further change in composition is possible so long as the pressure remains constant. Since the density of solutions of hydrochloric acid is greater than that of water, the density of the first portions of distillate will be less than that of the residue, while the density of the latter will rise, until CASE 1 at length it equals that of the new r ., constant boiling mixture. Progressive changes in density such as these, since they could not possibly occur with a single chemical individual, serve to differentiate clearly between the latt,er and a mixture of constant boiling point. The conclusions formulated in the M€TAYL COMPaS/T/ON L?€NZ€Nz preceding paragraph were then subALCOndL CASE2 jected t o experimental proof. A liter FIG. I of the azeotropic mixture A, of hydrochloric acid and water was prepared at atmospheric pressure by making a 2 0 . 2 percent solution of the acid and distilling until distillate and residue had the same composition-as determined by titration with standard sodium hydroxide-and the same density, as ascertained with a Westphal balance. This azeotropic misture was then fractionated at a pressure of about 50 mm, and the average composition of an unspecified volume of the distillate was compared at regular intervals with the composition of the residue remaining in the flask, care being taken t o collect the distillate in a filter flask immersed in ice. Since the pressure fluctuated a good deal no attempt was made to measure the temperature of distillation. The data follow.
'
646
T. R. BRIGGS
TABLE I Hydrochloric Acid and Water Fractionation of the Azeotropic Mixture under Diminished Pressure. Azeotropic Mixture A,
Pressure mm 750 50 2)
,) 1)
!l
9) 1)
Azeotropic Mixture AI
Specific Gravity Distillate Residue
Grams HC1 per cc Distillate Residue
1 . IO1
I . IO1
0.222
0.222
1.035 1.044
I . 103
0.074
0.227
I . 104
0.094
0.229
I . 108
0.130
0.238
1.110
0.164
0.243
I
.060
1.075 I ,092
I . I12
0.201
0.248
I .103
1.113
0.226
0.250
I .1 0 5
1.113
0.232
0.251
')
1.112
0,249
0.252
"
1.114
1.114 1.114
0.252
0.252
These data confirm the theory in an entirely satisfactory manner. It will be observed that the first sample of distillate collected at 50 mm contains only about 7 percent of hydrochloric acid compared with 20.2 percent in the original solution and that the low-pressure azeotropic mixture ultimately formed contains about 22.6 percent and gives rise to a distillate of equal density and composition. If we turn our attention a second time to the temperature-composition diagram, it will be seen that if the low-pressure azeotropic solution AI (the residue from the preceding run) be distilled at the original atmospheric pressure Po,fractionation again becomes possible. The mixture will begin to boil a t d, and since the composition of the vapor at the same temperature is represented by d' the distillate on fractionation will contain more hydrochloric acid than the residue in the flask. The latter will accordingly become more dilute as distillation continues, until at length the composition attains the point a and the original constant-boiling mixture A, is reproduced. This process, constituting the reverse of the distillation at 5 0 mm, was tested experimentally with the following results. It will be observed that distillation gave the expected results. The first sample of distillate is an almost saturated solution of hydrochloric acid gas in water at atmospheric pressure and the final constant-boiling mixture is identical with the one with which these experiments were started (Cf. Table 1)Regarding the two distillations as a whole, it will be seen that the residue in the flask has been caused to change through a complete cycle, which may be represented in the diagram of Fig. I by the closed path abcda. No cyclic process of this nature can possibly be carried out with a liquid which consists of a single chemical individual.
647
DETECTION OF CONSTANT BOILING MIXTCRES
TABLE I1 Hydrochloric Acid and Water Fractionation of the Low-Pressure Azeotropic Mixture at Atmospheric Pressure Pressure mm A zeotropic
Rlixture
AI
50
Specific Gravity Dist,illate Residue 1.114
1.114
Grams HC1 per cc Distillate Residue 0.252
0,252
I . 198 1. I44
I .124 1.114 1.110
I . 105 I. 103
I.I02 I. 101.
I .1 0 1
The system methyl alcohol and benzene was next studied as an example of the second general case where the boiling point of the azeotropic solution is a minimum, The pressure-composition diagram has been sketched in Fig. I. Consideration of this diagram shows that the azeotropic mixture A,, on being fractionated at diminished pressure PI, gives rise to a distillate, the composition of which is practically that of the point c and which constitutes the low pressure azeotropic mixture AI. While this distillate is being formed, the residue in the flask must necessarily grow richer in alcohol. Changes occur in composition, and therefore in density, and these are sufficient to distinguish the original mixture from a chemical individual. Methyl alcohol was purified by the method recommended by Duclaux and Lanzenberg'. The boiling point was found to be 66.5'. The alcohol was then mixed with redistilled benzene to give a solution containing about 60 percent by weight of the latter and the whole fractionated at atmospheric pressure. The distillate thus obtained was found to be the aaeotropic mixture A,, as was shown by a separate fractionation in the course of which no difference in density was observed between distillate and residue. This constant-boiling mixture was then fractionated under a pressure of about 158 mm, with the following results :Bull., 29, 35
(1921).
648
T. R. BRIGGS
TABLE I11 Methyl Alcohol and Benzene Fractionation of the Azeotropic Mixture under Diminished Pressure Pressure mm 731
.I\zeotropic Mixture A,
Distillate
Specific Gravity
0.849
Residue 0.850
0.850 0.850
0.849 0.848
I57
0.854
0.845
Azeotropic Mixture AI The specific gravity of benzene is 0.879, compared with 0.790 for alcohol.
It is evident, therefore, that the residue contains more alcohol as the distillation progresses while the distillate remains constant in composition and contains more benzene than the original constgnt-boiling mixture. Had the distillation been carried far enough so that only a small amount of residue remained, the distillate a t length would have shown an increase in alcohol content. Actually the distillation was brought to a close before this change had time to begin, but was continued until a sufficient volume of the azeotropic mixture AI was made available for the remainder of the work. The low-pressure constant-boiling mixture is relatively richer in benzene. Just as in the preceding case of hydrochloric acid and water, the changes in density during fractionation serve to distinguish the constant-boiling mixture from a single chemical individual. To prove that the distillate obtained under diminished pressure was actually the aseotropic mixture, it was fractionated under the same pressure. No changes in density occurred, as the following data show.
TABLE IV Methyl Alcohol and Benzene Fractionation of the Low-Pressure Azeotropic Mixture under Diminished Pressure Pressure mm
‘I57 I59
Specific Gravity Distil1at.e Residue
0.855 0,855
0,855 0,855
Fractionation of the low-pressure constant-boiling mixture was then carried out a t atmospheric pressure, t o complete the cycle. The results follow.
DETECTION OF CONSTANT BOILING MIXTURES
640
TABLE V Methyl Alcohol and Benzene Fractionation of the Low-Pressure Azeotropic Mixture under Atmospheric Pressure Pressure mrn
Azeotropic Mixture A,
7 40 I,
I, 71
Specific Gravity Distillate Residue
0.849 0.849 0.849 0.849
0.855 0.856
0.858 0.864
Azeotropic Mixture A, By referring to the temperature-composition diagram it is apparent that fractionation of the low-pressure azeotropic mixture A1 at atmospheric pressure Po will give rise to a distillate of which the composition is represented very closely by the point a and which is therefore identical with the original constant boiling solution A,. The residue, on the other hand, will increase in benzene content as the distillate is produced, until at length the composition of the distillate will also begin t o change toward the benzene side. In the last experiment the fractionation was not carried far enough to give rise to a progressive change in the dislillate, but it may be seen that the distillate is identical with the azeotropic mixture A, while the residue is growing slowly richer in benzene. Taken as a whole, the experiments with methyl alcohol and benzene represent a complete cycle, in which the composition of thc distillate has traced the path abcda, whereas this path was followed by the residue in the case of hydrochloric acid and water. In distinguishing between a constant-boiling mixture and a pure liquid, completion of the whole cycle is of course unnecessary. A single fractional distillation at low pressure will ordinarily suffice. If there is a difference in density between the residue and the first sample of distillate the liquid under test is a mixture, even though it can not be distinguished from a chemical individual when boiled at atmospheric pressure. It should be noted, however, that there is one case where the test will fail, for if the two constituents in the mixture are dynamic isomers between which equilibrium is reached practically instantaneously, the mixture will behave as a single individual at all pressures and no changes during distillation will be possible. The detection of constant-boiling mixtures could be carried out by applying the same general theory in a different way. The azeotropic mixture obtained at atmospheric pressure, instead of being distilled under diminished pressure, could be distilled at atmospheric pressure in a current of air. The latter would serve to lower the partial pressures of both water and acid in the vapor, and the effect would be virtually the same as if the constant boiling mixture were being distilled under diminished pressure. Fractionation would
6j o
T. R. BRIGGS
therefore take place, and would serve to distinguish the original mixture from a pure liquid. This method was not tested by actual experiment, but a third possible method, as described in the next paragraph, was tried instead. If a thermometer be placed in the vapor just above the liquid in the distilling flask and a second thermometer in the escaping vapor at the top of the fractionating column, there will be no difference in theory between the readings on the two instruments if fractionation is not occurring and the pressure is the same in two places. There will thus be little or no difference between the thermometer readings whether the liquid being distilled is a chemical individual or an azeotropic mixture. In the latter case, however, a change of pressure will cause fractionation to begin, as we have seen, and the upper thermometer as a result should read noticeably less than the lower one. This method was tried out experimentally, but while the data obtained from the thermometer readings were more or less in agreement with the preceding statement,, it proved so difficult to keep from superheating the vapor in the flask, and to maintain the pressure constant during the low pressure distillation, that the method was abandoned. It could no doubt be made to work if all the necessary precautions were taken, at least for those cases where there is a sufficient difference between the boiling point of the azeotropic mixture and the boiling points of the pure components.
I.
Summary A simple laboratory method for distinguishing between a chemical indi-
vidual and an azeotropic or constant-boiling mixture has been described. 2 . The method is based on the familiar principle that a change of pressure displaces the azeotropic composition. 3. The method has been applied experimentally to the two systems: hydrochloric acid-water, and methyl alcohol-benzene. 4. In each of these systems, a complete distillation cycle, reversible in respect of either the residue or the distillate has been demonstrated by experiment. Come11 Uniwrsily.
