A DEVICE FOR MEASURING VAPOR PRESSURES STANLEY TANNENBAUM Reaction Motors, Incorporated, Denville, New Jersey
THEapparatus most frequently used for the measurement of vapor pressures above room temperature is the isoteniscope. A number of interesting varieties have been described in the literature.' Where great accuracy is not required, it may be desirable to use a simpler apparatus. A device which has the added feature of being useful for measurements on compounds which will react with air is described in this article. This vapor pressure unit shown in the figure is similar to the one previously devised by Stock2for use with airreactive materials. However, this unit does not require the use of a separate manometer nor is it necessary that the sample be distilled into it. The figure shows the unit assembled for use with an air-reactive compound. The sample has been sealed into a tube and joined to the apparatus at. D. The break seals shown can he obtained from most scientific glass supply companies. Clean mercury has been added to the U-tube section through A to a suitable height. The apparatus is then connected to a vacuum pump and the mercury thoroughly degassed by warming the U-section and tapping the glass vigorously with a piece of rubber pressure tubing. When degassing is complete, the mercury is cooled and break seal-1 is broken by lifting the iron cylinder and bringing it down sharply with the aid of a high flux horseshoe magnet. The test material is then carefully degassed, the sample chilled to a point where its vapor pressure is negligible, and the apparatus sealed first at capillary C and then at B. The unit is then immersed in the temperature bath so that the bath liquid is higher than capillary C, and a eeries of temperature versus vapor pressure readings are made. At the end of the experiment, a hose fitted with a pinchclamp is attached at E. With the pinchclamp closed, break seal-2 is broken and air (or inert gas) allowed to enter the tube slowly until the mercury stops bubbling. The tube is then cut open and the mercury recovered. The apparatus can be used for only a single experiment; however, it is easy and inexpensive to construct. If it is modified by placing stopcocks at points B and C, it can be used repeatedly, but attention should be given to the possibility of leaks at these locations. The size,
' WEISSBERGER,A,, "Physical Methods of Organic Chemistry, Part I." Second edition. Interscience Publishers, Inc., New York, 1949, pp. 122-175. 'STOCK,A., "Hydrides of Boron and Silicon." Cornell University Press, Ithaca, N. Y., 1933, p. 192.
VOLUME 35, NO. 1. JANUARY, 1958
and therefore the range, of the manometer is limited hy the height of the temperature bath used. The apparatus shown in the figure was used to measure vapor pressures from 0 to 200 mm. mercury. The accuracy of the measurements depends primarily on the method used to read the heights of the mercury and on the constancy of the temperature bath. For student measurements, a simple cathetometer for reading the mercury levels and a bath which can he maintained to +O.l°C. should be adequate. Both 4 solids and liquids can be handled equally well. . series of readings made for increasing temperatures and one for decreasing temperatures should be averaged.
