Phase Rule Ex~eriments with Organic Compounds I
CHARLES M. MASON, B. W. ROSEN, and R. IM.SWIFT University of New Hampshire, Durham, New Hampshire
H.
ETEROGENEOUS equilibrium is one of the more ~mportanttopics covered in all courses in physical chemistry and the most important subtopic in this field is without doubt the study of the principles and applications of the phase rule. It is customary TO c j 60
.e$50
'
g40 E 30 2 0 l l l l l l l l l l l l l l l l l l l l l 10 20 30 40 50 60 70 80 00 100 Mal % Diphenyl FIGURE 1.-SYSTEM: DIPHENYL-$-DICHLORORENZENZENE
to include in the laboratory of most physical chemistry courses an experiment on the phase rule. This is usually the determination of the temperature-composition diagram of some metallic system by a study of cooling curves of diierent mixtures of the metallic components. The systems most often recommended employ metallic components, and, though they seldom tend to supercool, sufier from several serious difficulties. Fairly high temperatures must be employed
On the other hand, most organic solids melt in the temperature range from room temperature to 200°C. which is the most desirable laboratory range. When one first considers the possible number of binary organic systems, the number available seems limitless. In oractice, however. there are certain factors which lim& the use of many organic compounds for this type of work. The compounds must be inexpensive, readily available, easily purified, non-corrosive, stable in air and upon beating, and of such physical form that they can be easily weighed and handled. Needless to say, no chemical reaction should take place between the components in any given mixture. An extensive bibliography of organic systems and their phase diagrams is given by Timmermans (1) 00 80
ki70 c e
2 no $50
'40 30 0
10
20
30 40 50 60 70 Mol % p-Nitrotoluene
80
90 100
F I G ~ 3.-SYSTEM: E m-DINITROBBNZENE-$-NITROTOLUENE
and the standard tables (2). Also a number of authors (3) have suggested various organic systems suitable for this purpose. In the present investigation a study has been made of a number of systems to determine those best adapted for student experiments in physical chemistry. All the systems recommended by others have been tried and a number of new systems were examined also. EXPERIMENTAL DETAILS
and oxidation is difficult to prevent. The number of metals melting within the temperature range of most laboratory apparatus is limited, thus giving no great variety of systems or types of systems from which to choose suitable class illustrations.
The method of experimentation was simplified as much as possible. In every case the mixtures were prepared by melting together weighed pellets of the components. This was done a t the lowest possible temperature in a wax bath and various compositions
were obtained by adding further weighed pellets of either component. The apparatus used was simply a six-inch pyrex test tube provided with a thermometer of suitable range and a loop stirrer made of No. 16 B and S gage nickel wire. It was found that the wax coating from the melting bath provided suitable insulation to slow the cooling in air down to a suitable rate. To avoid supercooling the mixtures were stirred and sometimes small crystals of the solid mixture, previously removed, were introduced just below the melting point. It was found unnecessaq to plot cooling curves. The separation temperature was easily obtained from the maximum reached after supercooling. When the final eutectic was below room temperature, the tubes had to be immersed in cold water to reach the solidification temperature.
120
v " 110
2?
100
w
8 90
C
80
1 1 1 1 1 l I I I I I I I I I I I I I l l 0
10
20 30 40 50 60 70 80 90 100 Weight % @-Naphthol FIGIJREN STEM: NAPHTHALENE-8-NAPHTHOL RESULTS
Of the systems investigated, those which were found to be satisfactory are given in Table 1.
160
v
perature was found in the usual way of maximum rise after supercooling.
TABLE 1 OBGIINIC S~9mx9 S O ~ A B.o L x BPB*FIB RULBSTDDIBB
140
Systrm
:120
9-DiehlorobenzencDiphenyl Naphthalme -Diphemy1 m-Dinitrobenrene -*-Nitrotoluene Naphthalene -Pic& add* Naphthalene -@-Naphthol Naphthalene -8-Naphthylaminc *Although pie& acid is explosive, no difficulty was encountered as long as the temperature was kept below 250°C.
a
u
m El00
E 80 60 0
20 40 60 80 100 Weight % Picric Acid ACID FIGURE N SYSTEM: NAPHTHALENPI-PICRIC
To illustrate the nature of these systems their phase diagrams are shown in Figures 1to 6. These diagrams were constructed from data actually obtained in the laboratory.
In the examination of mixtures which form solid 110 solutions, i t was necessary to modify the procedure. Two schemes were tried. F i s t that labeled "end of 100 melting" or "endpoint" represents the temperature of $ the mixture when the last portion of paste changes to 90 0 solid crystals. The other method requires a rapid cooling of the melt in a 17.5-cm. pyrex test tube under 80 C the cold water tap. The liquid solidifies a t once and 70 all is of the same composition. Before cooling this a thermometer is centered in the tube and thus is frozen 0 10 20 30 40 50 60 70 80 90 100 into the solid. This tube is then placed in a beaker of Weight % 6-Naphthylamine FIGVEE6.-SYSTEM: NA~RALENE-8-NAPHTHYLAMINE water which is slowly heated, the thermometer inside registering a difference of not over one-half a degree As a laboratory experiment the student can complete from that shown by a similar thermometer placed in the water just outside the tube. The temperatures of one system and perhaps two in the usual laboratory the two are averaged and recorded as the "thaw point" period if supplied with the pure components. as the first crystals of solid change to liquid. MixWe wish to express our thanks to Professor Harold A. tures were made up to consist of definite weight per Iddles for his help in selecting suitable organic subcent of one component, and the first separation tem- stances for trial.
v
3
8
LITERATURE CITED
(1) TIDIMERMANS, "Les solutions concentr6es." Maussent Ca., Paris, 1936. (2) (a) "International critical tables." McGraw-Hill Book Co., Inc.. New York Citv. 1928. Vol. IV.
chemische Tabellen," Julius Springer, Berlin. (5). 1923. (3) (a) FINDLAY, "Practical physical chemistry," Longmans, Green and Co., Inc., New York City, (6), 1935, p. 296. (b) VERNON. 1. CHEM.EDUC..15. 88 (1938).
