Standard volumetric gas adsorption apparatus

In all volumetric methods, the principle underlying the determination is the same. Hie powder is heated under vacuum to drive off any adsorbed vapors. The pressure, volume and temperature of a quantity of adsorbate [nitrogen gas usually although krypton is used for low surface areas (5 1 m2 g-1)], are measured and the amount of gas present is determined. Traditionally this is recorded as cm3 at standard temperature and pressure (STP) although some prefer moles. 

The adsorbent (powder) is then brought into contact with the adsorbate and, when constant pressure, volume and temperature conditions show that the system has attained equilibrium, the amount of gas is again calculated. The difference between the amount of gas present initially and finally represents the adsorbate lost from the gas phase to the adsorbate phase. The accurate determination of the amount of gas nnadsorbed at equilibrium depends upon the accurate determination of the dead space or the space surrounding the adsorbent particles. The dead space is determirud by expansion measurements using helium, whose adsorption can be assumed to be negligible. 

Estimation of the quantity of unadsorbed gas is complicated by the fact that part of the dead space is at room temperature and part at the temperature of the adsorbate. 

Since the amount adsorbed represents the differerKe between the amount admitted to the dead space and the amount remaining at equilibrium, it can only be evaluated with confidence when the quantities are of unlike magnitude. To achieve this, the apparatus is designed so as to minimize dead space volume. 

The volumes of the burettes (in mL) are predetermined by filling with mercury and weighing prior to assembly. This needs to be repeated several times for the necessary accuracy. 

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