Edward T. Radleyl Memphis Stote University Tennessee
Vapor Pressure Determination An elementary experiment
The approximate vapor pressure of low boiling liquids, such as benzene, chloroform, ethyl acetate, and carbon tetrachloride may be estimated by measuring the increase in length of an air column trapped between two short columns of mercury in a uniform diameter glass tube, when the liquid vaporizes between them. As Damerell2 has pointed out, the volume in such cases is proportional to the length of the gas column when the length of the column is considerably greater than its cross section. The apparatus consists of a meter stick, several medicine droppers which have been drawn out into capillary pipets, a small rubber hulb with a hole in each end, a few milliliters of mercury, and a piece of constant bore glass tuhing about a meter long and 1or 2 mm i.d. The tubing may be tested for uniformity of bore by measuring the distance between two short mercury globules about 20 cm apart as they are drawn up and down the tube by means of the rubber bulb. The tubing may be bent a few centimeters from the end, to facilitate manipulation and prevent i t from accidentally rolling off the table. The experiment is performed by introducing a short column of mercury, about 1 cm long, into the measuring tube by means of one of the capillary pipets. The mercury is drawn down the tube by means of the rubber hulb attached to the opposite end until an air column about 40 cm long has entered the tube. Another short column of mercury is then introduced to serve as a seal. The mercury globules are drawn into the central portion of the tuhe and the distance between them is measured to the nearest millimeter with the meter stick. The mercury globule nearest the open end of the tube is forced to within a few millimeters of the end of the tuhe and a short column of the liquid is drawn into the tube behind the mercury. As the organic liquid is drawn into the tuhe it wets the glass. Reversal of the direction of flow causes the mercury globule to roll over a film of this liquid, thus introducing the sample into the space between the mercury globules. The amount of liquid between the globules may be increased by rolling them back and forth a few times. It is necessary that a small excess of liquid be present after equilibrium has been reached, but a considerable excess is not desirable. After introduction of the sample the tube is allowed to stand until equilibrium has been reached as evidenced by constancy of length of the air column on standing, the globules are drawn into the part of the tube where the original measurement was made, and the length of the air-
' Present addreas: Graduate student, Dept. of Chemistry, University of Missouri, Columbia, Mo. DAMERELL, V. R., J. CAEM.EDUC., 32,534 (1955).
plus-vapor column is measured. Attainment of equilibrium may be accelerated by closing the open end of the tuhe with a medicine dropper hulb in such a way that the mercury remains a t least 30 cm from the opposite end of the tube and turning the tube so that its axis is vertical with the organic liquid a t the upper end. If the tube is stroked between the thumb and forefinger in the vicinity of the upper mercury globule, the temperature in this region will be elevated, causing the liquid to evaporate more rapidly, and the dense vapors to movemore rapidly down the tube axis. The tube may then be returned to its horizontal position and allowed to stand for five minutes to establish thermal equilibrium with the atmosphere; and a measurement of the length of the air column is recorded. This process may be repeated until a constant length of air column is obtained. The barometer reading, temperature, original and final lengths of the air column, and the identity of the liquid should be recorded. The tuhe is then emptied and thoroughly flushed with dry air to remove all traces of the organic liquid. Vapor pressure determinationsat different temperatures may be made by putting a small condenser jacket around the tube and circulating water of known temperature through it or preferably by repeating the experiment a t another laboratory period when the temperature has changed or in a different room where the temperature is different. From data a t two different temperatures an approximate heat of vaporization may be calculated for the liquid. If a suitable long tuhe is available it is possible to conduct duplicate determinations simultaneously by introducing a cohimn of mercury about 2 cm long between the outside mercury columns, thus forming 2 air columns each of which can be charged with sample from its end of the tuhe. If this is attempted, each outer mercury globule must be brought to rest a t least 25 cm from the end of the tuhe prior to equilibration so that it is not ejected from the tuhe during vaporization. The vapor pressure is given by the equation:
P, = vapor pressure of the liquid
PB = barometric pressure L, = length of air column L2 = length of air-plusvapor column
'Typical results for the vapor pressure of chloroform (30°C), measured by this method were 246 mm, 252 mm, and 250 mm. Refinements such as calibrated uniform bore tubing and careful temperature control should make the method capable of high precision. Volume 37, Number I, bnuory 1960
/ 35
