Gas Mixing Methods

Gas Mixing Methods.—Since the properties of a gas mixture depend on the like as well as the unlike interactions, no F-technique has been devised by which interaction virial coefficients can be determined directly. On the other hand, at low pressures the change in pressure on mixing gases at constant volume and the change in volume on mixing gases at constant pressure are directly proportional to S. 

Three 2 dm glass bulbs are immersed in a large thermostatic bath bulbs II and III have cold fingers sealed into them. The bulbs are interconnected by metal lines and can be isolated from each other and the sample-filling system by metal bellows valves A, A, B, B, C, D, and E. T is a sensitive differential pressure transducer. 

With valves A, C, and D closed, bulbs I and II are filled through valve A with one of the substances to be studied. The filling pressure is measured on an external manometer. Valve A is closed and bulb III is now filled through valve A with the other gas to exactly the same pressure, as can be observed on the pressure transducer. Valves A, B, and E are now closed and valve C is opened, connecting bulbs I and II. 

The two gases are mixed by adding liquid nitrogen to one of the cold fingers for about 15 min and then closing valve C. After the system has returned to the bath temperature, valves B and C are reopened and the pressure diflFerence is observed. The mixing process is repeated until a constant pressure difference is obtained. 

The relation of Ap the pressure change on mixing, p the filling pressure, and T the temperature to the excess second virial coefficient is given by 

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The volumetric gas mixing method is the only technique that offers the advantage that the concentration, or the mole fraction, remains largely unaltered when the gas changes. 

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