A Simplified Isoteniscope OTTO F. STEINBACH and ARTHUR W. DEVOR Adelphi College, Garden City, New York
T .
HE measurement of vapor pressure with the isotenscope, originally designed by Menzies, will not give results of a high degree of accuracy unless sufficient time is allotted for the establishment of temperature equilibrium between the heating bath and the sample enclosed within the bulb of the isoteniscope. With proper care, so that the above condition is eliminated, exceptionally good results may be obtained. I t was this consideration that led to the present simplified design of the isoteniscope.
The main body of the isoteniscope may be made from a 50 to 100-ml. distilling flask. The lower well is blown in the flask so that the thermometer bulb will be completely submerged in the liquid sample. The manometer bulbs were made from &in. test tubes. The purpose of the bulbs is to prevent the liquid in the isoteniscope manometer from being drawn over into the trap or the flask in case too rapid change in external pressure occurs. As a matter of fact, after one has sufficient experience in adjusting the pressure of the system this difficultyseldom occurs and it is only with inexperienced students that this may happen. Hence the precautionary construction. The manometer is connected close to the top of the neck of the flask so as to reduce the dead air space to a minimum. In order to have boiling take place in the flask rather than in the manometer a large boiling stone (porous plate) is held by a stiff piece of wire of sufficient length so that i t partly projects above the liquid surface. This helps to reduce undercooling to a minimum. A few loose boiling stones may also be added. Sufficient sample is added to the isoteniscope so that the thermometer bulb is completely immersed and then sufficient liquid is placed in the U-bend of the manometer to fill i t up to the bulbs. The isoteniscope i s placed in the bath (a 2-liter heaker) which is then 22
heated until the temperature is two to three degrees below the boiling point of the sample. The water aspirator pump is then turned on, the stopcock B being open. Stopcock B is then partly closed uptil the liquid in the isoteniscope bulb begins to boil, driving out the dissolved and trapped gases. In order that the temperature of the bath and that of the liquid sample will not differ by more than one degree, an efficientstirrer should be used. The temperature of the sample is recorded and the pressure of the mercury manometer is obtained when the sample ceases to boil and the liquid levels are identical in the U-tube. Allow the bath to cool slowly, meanwhile adjusting stopcock B so that the liquid levels do not differ greatly, or otherwise air will be sucked back into the bulb. If this happens the air must be boiled out again by reducing the pressure. The stopcock A is used to retain the mercury level in the manometer so that the pressure which corresponds to the temperature of the sample may be obtained a t the operator's convenience. When i t is desired to obtain a new reading the pressure is adjusted until the manometer levels are equalized, whereupon the temperature and mercury levels are recorded. The procedure is repeated as many times as readings are required. The barometric pressure is recorded a t the beginning and end of the run. It is also advisable to record the average temperature of the exposed stem in order to calculate the stem correction and so obtain the corrected temperature. To determine the accuracy of the results obtained with the present apparatus, the vapor pressures of several liquids were measured. The temperature readings were corrected for the exposed stem but the pressures were not corrected to DOC. The latent heats of vaporization were calculated from the plot of log P and (l/T) and this was compared with the latent heat of vaporization obtained by plotting the accepted values of log P and (l/T). The graphs were plotted on separate paper in order to avoid undue influence in selecting a line to represent the required slope. Inspection of the accompanying table shows very good agreement between the accepted values of L and the experimentally determined ones. The results obtained with CClr and (CHs)2C0were obtained by separate student groups. TABLE 1
cmr error
par snmpic
~enrene Wafer Acetic acid (20-100')
Carbon tetrachloride Acetone
Expe*immlol I. 103.3 cal./g. 563 cal./g. 164.7 eal./g. 9893 cal./mol 50.31 cal./g. lZR.O eal./g.
Arrrhlrd
I.
104.1 eal./g. 868 cal./g. 160.4 eel./=. 9633 cal./mol 49.33 cal./g. 121.3 cal./&
1 '
1 2.0 2.0 2.5
It may be noted that the latent heat of vaporization per gram of acetic acid reported in the handbooks and International Critical Tables is given as 96.8 cal./g. This figure is obtained by dividingthe molar latent heat of vaporization by the molecular weight of acetic acid vapor (approximately 101) a t its boiling point. In several textbooks of uhvsical chemistry, the authors have apparently used ihe value of 96.8 cal./g. to calculate Trouton's constant and accordingly report values
around 14 for the constant. If the Trouton constant is calculated using the molar latent heat of vaporization as determined from the accepted vapor pressure data in
the table, namely 9633 cal./mole , one obtains the value 391.4'A. of 24.6 for the constant. This is, of course, distinctly in cood ameement with the values ziven for other socalled agnormal liquids. The authors thank Mr. Harold Wilson for the drawing.*
-
