APPARATUS for DETERMINATION of BOILING POINT-COMPOSITION CURVES of MISCIBLE LIQUID PAIRS E. M. COLLINS, J. 0. MORRISON, AND J. R. STONE Williams College, Williamstown, Massachusetts
The requirements of an apparatus for the determination f boiling point-composition curves of binary liquid pairs re discussed together with a study of the apparatus which eve been used for this purpose and their suitability in an xperiment of this nature in a course in physical chemstry. It is pointed out that many of the apparatus used n such an experiment do not meet these requirements. I description of an eesily constructed apparatus which reets most of these requirements is given and its operation .nd application are discussed.
A
N EXPERIMENT which is suitable for a course in physical chemistry is the study of a bmary liquid mixture by the determination of the boiling)oint curves showing the composition of the liquid and .apor phases, the residue and distillate, respectively, vhich are in equilibrium with each other a t a given boiling point. Experiments of such a nature together vith a description of the apparatus used are described )y Bennett1, a and Brown3 and are included in the aboratory manuals of Mack and F r a n ~ e ,Daniels, ~ vlathews, and William~,~and Fajans and Wiist.' >orthis experiment an apparatus is required which will )emit the collection of samples of residue and distillate n equilibrium with each other and yet one which is imple and easy to construct. Although it is well known that in determining the )oiling points of solutions the thermometer should be mounded by the solution and not by the vapor, most ~f the directions given for this experiment call for the hermometer to be suspended in the vapor. The temmature which is thus recorded is therefore lower than hat of true equilibrium between the liquid and vapor.
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G. W. BENNETT."A laboratory experiment on the boiling loint curves of non-azcotropic binary mixtures," J. C m x . E ~ u c . , 1 .
1544-9 (Scot.. 1929).
' F. J. $O&'AND G. W. BENNETP,"Laboratory experiments m the boiling point curves of binary mixtures," ibid.. 7, 133640 June, 1930). E. H. B n o m , "An app-tus for maximum and minimum miling-point study." ibid., 9, 1114 (June, 1932). 'E. MACKAND W. G. Fnmcs, "Laboratory manual of thysical chemistry," D. Van Nostrand Co., New York City. 928, pp. 78-80. s F. DANIELS,J. H. MATHEWS, AND J. W. WILLIAMS, "Expffioental physical chemistry," McGraw-Hill Book Co.. Inc., New lork City, 1929, pp. 57-61. %.FAJANSAND J. WOST, trans. by B. TOPLEY, "A textbook ~fpractical physical chemistry," E. P. Duttan Co., New York :ity. 1930, pp. 3 6 4 1 . '
To have the thermometer surrounded by the boiling solution introduces an error due to superheating unless precautions are taken to prevent it. Superheating is avoided in the apparatus of Fajans and Wust6 since it is especially constructed so that alternate layers of boiling solution and vapor bubbles are pumped automatically from a boiling chamber against the bulb of the thermometer which is contained in a second chamber. This is essentially the principle of the Cottrell pump which is often used to prevent superheating when determining the boiling points of solutions. The apparatus of Fajans and Wust, although it also embodies a convenient device for the collection of samples of distillate, nevertheless, is not particularly simple and probably could not readily be made by the average amateur glass-blower. Smyth and Engel7 in their study of vapor pressures of binary mixtures avoided superheating by internal electric heating with the thermometer immersed in the main body of the liquid. But again, their apparatus is more suitable for research than for classes in elementary physical chemistry. Young8,*obviated superheating to a large extent in his determinations of vapor pressures of binary mixtures by encasing the thermometer in a glass tube coaxial with the neck of the distilling flask. This tuhe was open a t the lower end and had a hole blown in it just below the cork which held it in position a t the upper end. The tube was adjusted so that its lower end was 3 mm. above the level of the liquid when cold, and the bottom of the thermometer bulb was about level with that of the tube. When the liquid was boiled it rose in the tuhe thus bathing the bulb of the thermometer with bubbles of liquid and vapor more nearly in equilibrium with each other and with superheating a t a minimum. This tube, furthermore, prevented the cool liquid returning from the reflux from coming near the thermometer, and the amount of liquid which condensed on the thermometer and inner walls of the tube was exceedingly small, Samples of the liquid and vapor phases must be obtained which are a t equilibrium with each other a t a ' C. P. S m n m E. W. ENGEL."Molecular orientation and partial vapor pressures of binary mixtures. I. Systems composed of normal liquids,'' J. Am. Chem. Soc.. 51, 2647 (Sept., 1929).
S. YOUNG, "Distillation principles and processes," Macmillan
& Co.. Ltd., London, 1922, p. 45. S. YOUNG."Vapor pressures and boiling points of mixed liquids. Part I," J. Chem. Soc., 81, 772 (1902).
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known boiling point. Samples of the residue may be withdrawn from the distilling flask by means of a pipet or forced out by air pressure in a device used by Mack and France* and Bennett.' It is, however, the method
of obtaining samples of the distillate which causes the greatest concern. Collection of samples from the end of an ordinary condenser in the distilling position (cf. Mack and France,' Daniels,' Bennett1) has the objection that the sample is not in true equilibrium with the residue because of the great distance from the flask and consequent time-lag. There is also opportunity for fractionation of the vapor phase as i t condenses in heating the colder portions of the apparatus. To avoid such fractionation in any apparatus i t is best to have the condenser and still-head as short as possible. Fractionating columns are therefore out of the question although Daniels6 suggests their use. Prevention of the loss of heat from the still-head by the use of heat shields helps to prevent premature condensation and subsequent fractionation of the vapor phase. Recourse to ordinary distillation causes the composition of both distillate and residue and, in consequence, the boiling point to change so rapidly that the temperature corresponding to a pair of samples is only approximate and often varies over several degrees. The use of a condenser in the reflux position, however, permits equilibrium conditions between the two phases to be maintained constant as long as de~ired.~.~,' To collect samples of distillate when a reflux condenser is used presents an interesting problem. The r d u x condenser in Brown's apparatus3 has a special inner tube with a cup a t the lower end in which the distillate may collect and from which i t may be withdrawn by means of a pipet. The apparatus of Fajans and Wust6 has a reflux condenser joined to the rest of the apparatus by a ground-glass joint about which the condenser may be swung to a distilling position for the collection of
samples. But, as previously mentioned, this apparatus is not easily constructed and as a description of Brown's apparatus had not yet been published we set about to devise one which was suitable. The result was an apparatus (Figure 1) which we believe to be quite simple but which meets the important requirements mentioned above. It is made from a 200-cc. pyrex distilling flask and a plain condenser with pyrex inner tube. It is similar to the apparatus of Y o ~ n in g ~ that ~ ~ the condenser is sealed to the distilling flask in a reflux position. The chief difference is that it has an opening a t A for the collection of samples of distillate. The opening a t A should not be greater than 4 mm. A glass capsule is attached a t A by a ground-glass connection. The making of this ground-glass joint is not difficult but a small cork stopper may be substituted. The use of the Young thermometer tnbe described above, together with boilmg chips, prevents superheating. A thermometer graduated in '/G degrees is recommended. When equilibrium has become established as evidenced by a constant temperature the capsule is removed and a few drops of the refluxing distillate are collected a t A in a test-tube. An electric heater is recommended to minimize any danger of iire. (A durable and inexpensive electric heater may be purchased under the name "Marshmallow Toaster" from the Angelus-Campfire Company.) The composition of the boiling mixture may be changed by distilling through A and, to keep the volume of the liquid constant, the addition of an approximately equivalent amount of the component toward which the residue is changing. Analyses of the samples are made by means of an Abbe refractometer. We have constructed several of these apparatus and they have been used by our class in physical chemistry for the past two years in the study of the system, benzene-methyl alcohol. This experiment has proved very popular among the students and the smooth curves (Figure 2) and good results invariably obtained have added to their satisfaction.
Mole-fraction benzene in mixture. FIGURE 2.-BOn1NG POINT-COMPOSITION CURVES FOR SYSTEM. BENZENE-MmALCOHOL
THE
