# Measuring vapour pressure in a closed system

My experiment requires the determination of vapour pressure from which I will calculate the number density (or number of moles of the molecule) later, for a closed system.

Here's my setup :

Liquid is placed in a steel container with the steel tube connecting it to a t-junction. One branch goes to the liq N2 immersed catch flask, followed by a pump. Other branch goes to a pressure transducer (Si sensor, which measures absolute pressure, linear from 0.2 atm to 4 atm).

Procedure : Liquid, in my case n-hexane, is frozen in the container using liq. N2. The gases in the container and the tubes are removed using pump. While the valves are closed, the liquid is brought to room temperature. The freeze-pump-thaw cycle is repeated twice again, to remove dissolved gases from the liquid.

Later the liquid is brought to room temperature and the system is left for few hours for equilibrium while the container is immersed in a water bath. Valve for pump is closed during this time, while the valve towards the pressure sensor is open. Temperature measurement of the water bath is done using thermocouple.

Issues : The pressure sensor reading is always more than the saturated vapour pressure of the liquid for a given temperature. This maybe due to a leak. From Raman measurements of gases in the tube it was found that some N2 is present in the system.

1. What would be a good way to determine the vapour pressure of the liquid in this closed system.

2. Would it be a good approximation to assume liquid-vapour equilibrium and use the tables of vapour pressure vs temperature to determine the corresponding value for my case.

Importantly, maximal error allowed in the determined vapour pressure value is up to 2%.

Thanks.

Pressure reading while melting n-hexane at room temperature.

Pump used is an oil pump.

• What is the residual vacuum pressure, compared to vapour pressure ? May 29 '20 at 5:30
• a) fix the leak, b) test with empty sample container, c) calibrate your thermocouple, d) how was your pressure sensor calibrated, e) why does it take a few hours to equillibrate?
– Karl
May 29 '20 at 5:34
• What about dynamic measurement, watching how boiling point decreases with decreasing of pressure ? May 29 '20 at 5:35
• By "calibration", I mean "when did you last check the calibration?" If it was more than two days or two months ago (depending on who handled the instrument in the meantime), check it again. An OC lab full of solvent vapour is not a nice environment for delicate equipment.
– Karl
May 29 '20 at 6:00
• @ankit7540 I have not said you did. It was alternative arrangement suggestion, that could or could not be applied to your case. May 29 '20 at 8:06