An EXPERIMENT on LIQUIDVAPOR EQUILIBRIUM for a TWO-COMPONENT SYSTEM* ARTHUR A. VERNONt Rhode Island State College, Kingston, Rhode Idand
A
COMMON and useful experiment in a physical the liquid-vapor equilibrium for a two-comchemistry laboratory is one which demonstrates ponent system with a maximum boiling point. The
-
* Presented before the Division of Chemical Education at the ninety-fourthmeeting of the A. C. S., Rochester, N. Y., September 6. 193i.
t Present address: Northeastern University, Boston, Massachusetts.
ing. Then a sample from the flask should be removed with a pipet to the end of which is attached a suction bulb. Thus a sample of the liquid and vapor are available for analysis. The thermometer should then be replaced and enough solution distilled over to raise the temperature of boiling about 0.2'"C. A second sample of vapor and liquid should then be obtained as before, and the process of boiling repeated. In this way seven or eight samples may be obtained and the last should be a t the maximum boiling point. These samples are then titrated with standardized 6 N sodium hydroxide. The determinations with the low acid solution should be carried out in the same general manner except that the solution should be distilled between each sample until the boiling point has been increased by approximately 0.5'C. This method will give eight or nine samples with the last a t the maximum boiling point. The amount discarded will vary from nearly 200 cc. a t the beginning to about 15 cc. a t the end. The first few samples obtained in this distillation should be titrated with standardized 1 N sodium hydroxide while the last few samples will require base which is 4 or 5 normal. If students work in pairs, both distillations can be performed a t once, but i t is not desirable to make the experiment shorter by using less solution because superheating of the vapor must be avoided. It has been the experience in this laboratory that students tend to distil the solutions too fast with consequent superheating of the vapor, and this is the only real source of error in the experiment. In such work i t is also necessary to calibrate the thermometer with boiling water. In the figure the results are plotted of a distillation experiment which was made for purposes of check on student results. The thermometer was calibrated, superheating was avoided, and the best accuracy possible maintained Mol Per Cent. of Water throughout so that the curve is based on as accurate data as this method will produce. The maximum boiling point is a t 107.1°C. and a t a composition of 43.3 mol per cent. of water.% This is another advanmic acid. The latter solution can be obtained from the tage of the system since it brings the maximum in the formic acid available from supply houses, which is ninety per cent. by weight, by drying with anhydrous curve in the center of the graph and not to one side as copper sulfate or boric acid anhydride.' An ordinary in the case of the hydrochloric acid-water system. The test of any experiment is how well it works with distillation set-up is used, and the flask should contain a t the start approximately 400 cc. of the strong a class, and to that end this experiment was given to a acid solution for the k t distillation. For the second group of seniors majoring in chemistry. The results distillation the flask should contain about 750 cc. of were very satisfactory, and so it is felt that the experithe weak acid solution. In each case glass beads ment can be recommended for a physical chemistry laboratory course. The deviations from the true valshould be used to prevent bumping. The experiment with the high acid solution is carried ues should not be more than five per cent. out by bringing it to a boil, discarding about 15 cc. and SUMMARY then collecting a 10-cc. sample for analysis. The temperature should be read a t the start of the sampling and 1. An experiment is described which demonstrates at the end. The average of the two readings is then the liquid-vapor equilibrium for a two-component used as the average temperature of the sample. Im- system. mediately after the 10-cc. sample is collected, the flame 2. The boiling point composition diagram for the should be removed and the liquid allowed to cease boil- formic acid-water system is shown.
usual system which is recommended is the HC1-Hz0 combination. This system is easy to use but has the difficulty that only one part of the curve can be determined since solutions containing a percentage of hydrochloric acid greater than that of the maximum boiling mixture are very difficult to handle. In order to develop a suitable experiment of this type, several systems were investigated. The one which seemed most promising was the formic-acid water system, and it is the purpose of this paper to describe the experiment using these components. It is necessary to boil two solutions and approach the maximum boiling point from each side. The weak acid solution should contain about twenty per cent. by weight of formic acid while the strong acid solution should be about ninety-five per cent. by weight of for-
' WEISSBEROER AND P n o s m w ~ n ,"Organic solvents," Oxford University Press. Oxford, England, 1935.
'"InternationalCriticalTables," McGraw-HillBookCompany, New York City, 1928, Vol. 3.
