An ebulliometer for measurements of vapour pressure at low temperatures: the vapour pressures and the critical state of perfluoromethylcyclopentane

The vapour pressure of cyclohexane over the whole fluid range determined using comparative ebulliometry

“…Although the high-pressure ebulliometers have been used before, (2,3) they are described here because no details were given previously and the working pressures were much less than those used in the present work. The sample ebulliometer was constructed from stainless-steel tubing (46.3 mm o.d., 4.1 mm wall, 575 mm length) welded to a base of thickness 8 mm and to a side arm (25.4 mm o.d., 3.2 mm wall) leading to an integral condenser.…”

“…Consequently, the length of the thermometer well was reduced from 45 cm to 32 cm, which increased the permissible sample volume to v = 265 cm 3 with only a small increase in the total volume to V = 605 cm 3 . Hence, the modified ebulliometer could be loaded to an overall density of 0.44 times the liquid density at the lowest temperature which was comfortably greater than the likely critical density.…”

“…The re-entrant well (12.7 mm o.d., 2.8 mm wall, 75 mm length) in the base plate was originally used with a cartridge heater but, subsequently, a mica band heater of resistance 410 was clamped to the lower 4 cm of the cylindrical wall of the ebulliometer. The re-entrant well, which had been cross threaded on the surface exposed to the sample to assist bubble formation, therefore became notionally redundant, but it may well have been important in achieving smooth boiling down to a pressure of 5 kPa, which is unusually low for an ebulliometer without bubble caps (2,3) or a Cottrell pump. (8) No other heating was used, although the ebulliometer was well insulated by Kaowool 100 mm in thickness.…”

“…and it was clear that it would be necessary to add the sample progressively as the temperature was increased, with all the attendant problems of cross contamination of sample and reference fluids each time the apparatus was depressurized, (3) unless the ebulliometer was modified. Consequently, the length of the thermometer well was reduced from 45 cm to 32 cm, which increased the permissible sample volume to v = 265 cm 3 with only a small increase in the total volume to V = 605 cm 3 .…”

“…(1)(2)(3) The sample and a reference fluid are boiled in separate ebulliometers at a common pressure, the two condensation temperatures are measured, and the vapour pressure of the sample at its condensation temperature is deduced from the known p(T ) behaviour of the reference fluid. Although comparative ebulliometry at pressures up to 1.6 MPa was recently reported, (3) the great majority of vapour pressures measured by comparative ebulliometry have been restricted, by the use of glass boilers, to pressure below 200 kPa and to below 1 MPa even with metal apparatus. (2,4,5) Here we have extended greatly the range of comparative ebulliometry with measurements of the vapour pressure of cyclohexane over virtually the whole fluid range from the triple point (279.446 K, 5.388 kPa) to just below the critical point (553.60 K, 4080.3 kPa).…”