The discontinuous manometric procedure

The successive steps in the determination of an adsorption-desorption isotherm are normally as follows (more detailed information on particular steps is given in Section 3.4)  

Adsorbent sample introduced into adsorbent bulb and initial mass recorded (s ee Section 3.4.2). 

Adsorbent evacuated under controlled conditions (see Section 3.4.3) and dosing volume also evacuated. 

Dose of gas (e.g. He, N2 or Ar) introduced into pre-calibrated dosing volume and p and T measured. 

Gas expanded into adsorbent bulb at room temperature and new p and T measured. 

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Steps 1 and 2 (Adsorbent weighed and evacuated) are the same as in the discontinuous manometric procedure. 

When thermal equilibrium reached (i.e. after c. 3-5 hours), the dead space volume is determined as in Steps 3 and 4 of the discontinuous manometric procedure in Section 3.3.1. In the case of low-temperature calorimetry this must be done between Steps 2 and 3, by simply connecting the adsorption bulb to the manometric equipment. 

Discontinuous procedures for determining the adsorption isotherms 67 The discontinuous manometric procedure 67... 

The discontinuous procedure is the conventional technique. It is also referred to as the point-by-point procedure. The adsorptive is introduced in successive amounts and at each stage die measurement is made only when adsorption equilibrium is attained. In this way each point on the adsorption isotherm is determined. The original BET technique (Figure 3.1) and all conventional manometric measurements (Figure 3.2) are based on the discontinuous procedure. This is still the procedure most often used with adsorption balances (Figures 3.10-3.12). In principle, it takes account of the requirement that each experimental point of the adsorption isotherm should correspond to thermodynamic equilibrium. 

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