Page 99 - Gas Adsorption Equilibria
P. 99

2. Volumetry / Manometry                                           85


             It should be noted that for               and for
          which reflects the occurrence of a liquid phase outside the sorbent material, i.
          e. a vapor-liquid-state of the sorptive fluid.

             Auxiliary remarks:


             1.    The quantity   in Eq.  (2.1) on principle also includes the mass of
                   gas  adsorbed on the  walls of the  adsorption chamber  and possibly
                   desorbed  from  the walls of the  storage vessel upon gas expansion
                             These  quantities should  be  determined by  calibration
                   experiments without using a sorbent sample, i. e.  cp.  Chapter
                  4.  If the  surface  of the  walls  of the  adsorption chamber and  the
                   storage vessel is  small compared to the BET surface of the sorbent
                   sample, one  may  assume                i.  e.  neglect
                   compared to

             2.   The volumes             should  be determined  experimentally by
                  using a  sample,  preferably a  cylinder of  calibrated  volume
                  made of  dense  material (Ti,  Au) and  performing gas  expansion
                  experiments as described above. The amount of gas adsorbed on the
                   surface of the sample normally can be neglected.**

             3.   Waiting times for thermodynamic equilibrium in the gas adsorption
                  system are based on experimental experience which comes from
                  gravimetric measurements, cp.  Chap. 3.  No general rules for these
                  times are available today.

          2.3      Uncertainties or Errors of Measurements


             An important part of any kind of experimental  work is to consider errors
          or uncertainties of measurements [2.16 – 2.18]. To keep these down, it is
          always recommended to repeat measurements of any experimental quantity at

          **  Suppose the sample   is placed in the adsorption chamber and a certain amount of nearly
            not adsorbing  gas  (He, Ar at   has  been prepared at pressure (p*) in  the  storage
            vessel     If  after  expansion to the  adsorption  chamber the  gas  is  at pressure  (p), the
            volume    can  be  calculated via Eq. (2.2)  by
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