EXPERIMENT on a TERNARY SYSTEM of LIQUIDS ARTHUR A. VERNON AND BERTRAM BROWN Rhode Island State College, Kinpton, Rhode Island
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N INTERESTING experiment in a laboratory course in physical chemistry can be done to illustrate equilibrium in a three-component system by determining the amount of one liquid necessary to make two immiscible liquids become miscible. Experiments of this kind are described by Davison and van Kloosterl in which the amount of acetic acid necessary to make benzene-water mixtures miscible is determined and by Daniels, Mathews, and William~,~ using the system water-chloroform-acetic acid. The endpoints in these systems are rather difficult to determine, since the solutions must be made to change from clear to cloudy or cloudy to clear. The change is particularly difficultto determine in the benzene-water-aceticsystem when the percentage of acetic acid is high. It has been the experience in this laboratory that the system nitrobenzene, water and acetic acid is very well adapted for a three-component experiment. The endpoint for this system is easy to detect since the nitrobenzene is colored and heavier than water. When the container is gently rotated, the small droplets of nitrobenzine, which are present as the equilibrium point is approached, are gathered in the bottom of the flask and can be watched as they disappear. As it bas been worked out, the experiment consists in determining the amount of acetic acid necessary to make the following mixtures become miscible:
by watching for the disappearance of the nitrobenzene droplets which collect in the center on the bottom of the flask. It is important that mixtures be thoroughly shaken to make sure all the CsHsNOe has been made miscible. The amounts of acetic acid necessary a t equilibrium have been determined in this way and the results are given in Table 1. The error involved in these readings is different for each mixture as given in the table because of the cumulative effect of the error of each reading. The greatest error of any one reading was k 0 . 2 cc., and the greatest cumulative error was in the case of the mixture containing 10 cc. of nitrobenzene and seventy cc. of water. For this mixture i t was d 2 . 5 cc. of acetic acid. Thus, the worst error of individual measurements was 1.3 per cent. and the worst cumulative error was 1.52 per cent. TABLE 1
C&LNOI cc Vor. %
Hz0
rr.
Vd. %
C&COOH Vol. %
CL
These results are plotted on the triangular diagram The initial mixture of nitrobenzene and water is placed in an Erlenmeyer flask and acetic acid added from a buret while the Erlenmeyerflask is rotated gently. At first the mixture is opaque, while a t the equilibrium point the solution quickly becomes clear. Enough water is added to give the nitrobenzene-water proportions of the second mixture and the process repeated. In this way each series of mixtures is tested. As the amount of acetic acid increases, the change a t the endpoint is less sharp, but i t can be determined accurately
1 DAVIWN. A. W. AND VAN KWOSTEK.H. S., "Laboratory manual of physical chemistry," 2nd ed., John Wiley and Sons, NewYork City, 1931, p. 104. DANIELS, F., MATEIEWS J. H., AND WILLIAMS. J. W.,"Expermental physical chemistry," 2nd ed., 1929, McGraw-Hill Book Co.,New York City, p. 106.
Cff3coon
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as shown in Figure 1. It should be noted that the mixtures as outlined give three overlapping points.
This is valuable in determining how well the curve may sult in a greater error in percentage when the measurebe drawn a t the peak and gives a measure of the skill ment was made. of the experimenter. The last point which is of interest if the experiment The value of the experiment is to a large extent de- is to be used in a laboratory group is the variation with pendent upon the ease with which students can duplicate temperature of the acetic acid needed to produce miscithe results. In order to determine this, the experiment bility. Experiments showed an increase in temperature was given to a group of fifteen seniors majoring in from 25'C. to 35°C. gave equilibrium values of acetic chemistry and to a group of fourteen pre-medical senior acid two per cent. higher. This change with temperastudents. Of course, the former were more experienced ture is small enough to make i t a good experiment for in chemical technic, and thus a comparison between comparison among a group of students who are workexperienced and inexperienced students could be made. ing in conditions of varying temperature. It was found that the experiment could be performed On the basis of the information given, i t is felt that equally well by either type of student. The individual the experiment described is very satisfactory for use in sets of data gave smooth curves when plotted, but there experimental physical chemistry. was a maximum variation in the cc. of acetic acid a t SUMMARY the equilibrium points of about ten per cent. The 1. An experiment illustrating equilibrium in a cumulative variations were, however, only about this large and the iinal per cent. of acetic acid varied by a three-component system is described. little more than five per cent. These variations were 2. The correct values of each component present a t probably due to errors which occurred in measuring equilibrium is given and tests cited to show that i t is out the nitrobenzene. A small error here would re- satisfactory for students to perform in the laboratory.
