Measurement of Vapour Pressure
There are four methods available to measure vapor pressure- dynamic method, equilibrium or static method, modified dynamic method and manometric method. The manometric method is the most widely used method to measure the vapor pressure of a liquis.
Dynamic Method
The apparatus used for the dynamic method is illustrated below-
An inert gas is passed through the given liquid at a constant temperature (T). The gas saturated with the vapour of the liquid leaves the flask at the exit tube.
If V be the volume of the gas passed and m the loss in weight of the liquid, the vapour pressure is given by the expression-
Vapour Pressure = mRT/MV
where M = molecular weight of the liquid and R = gas constant. This method is particularly suited for liquids of very low vapour pressure.
Equilibrium or Static Method
A sufficient amount of the liquid whose vapour pressure is to be determined is placed in the bulb connected to a mercury manometer and a vacuum pump. All the air from the bulb is removed by working the vacuum pump and the stopcock closed. A part of the liquid evaporates. The system is then maintained at a fixed temperature for enough time so that the equilibrium is established. The difference in the levels of mercury in the manometer is equal to the vapour pressure of the liquid. By adjusting the thermostat at a different temperature, the vapour pressure of the liquid at another temperature can be determined. This method is used for liquids having vapour pressures up to one atmosphere.
Manometric method
Manometric method is comparatively an accurate method. The liquid whose vapour pressure is to be determined is taken in a lask placed in a thermostat, as shown in the Figure below. One end of the tube from the flask is connected to a manometer and the other end is connected to a vacuum pump. The liquid is frozen with the help of a freezing mixture and the space above the liquid is evacuated. In this way, the air is removed from the surface of the liquid alongwith the vapours of that liquid. The frozen liquid is then melted to release any entrapped air. Liquid is again frozen and realeased air to is evacuated. The same process is repeated multiple times to remove the air from the flask.
Now the liquid is warmed in the thermostat to that temperature at which its vapour pressure in the flask is to be determined. Diference in the heights of the columns of-Hg in liquid by manometric method the two limbs of the manometer determines the vapour pressure of the liquid. The column of mercury in the manometer facing the vapours of the liquid is depressed. The other column, which faces the atmospheric pressure, rises. Actually, the pressure on the surface of the liquid in the flask is equal to the sum of the atmospheric pressure and the vapour pressure of liquid. For this reason, the column of manometer facing the liquid is more depressed than facing the atmosphere, and it is given by the following equation.
Mathematically it can be represented as-
P + Pa∆h
Where, p is the vapor pressure of the liquid at 1 atmospheric pressure, Pa is atmospheric pressure and Δh is a difference in the heights of the Hg column in two minutes.
