Page 140 - Gas Adsorption Equilibria

126 Chapter 3

a model which takes the fictitious volume of the sorbate phase
into account, the mass in (3.5) may be considered to be the total or
absolute mass of the adsorbate. It is related to the Gibbs excess mass (3.14) by
the relation

Here is the (fictitious) density of the adsorbate. A physically reasonable
assumption is to consider to be either the density of the sorptive in its
liquid triple state or in its liquid boiling state at system’s temperature, if it
exists. For low sorptive gas densities, i. e. But for high

gas densities, i. e. we have indicating the
occurrence of a liquid phase outside the sorbent material according to vapor-
liquid-equilibrium (VLE) of the sorptive fluid, cp. also Chaps. 1, 8.

Auxiliary remarks:

1. Calibration experiments with two nearly equal and well known calibration
masses prior and after gravimetric adsorption experiments are
recommended. This can avoid systematic uncertainties in experimental data
due to (slow) shifts of the zero-point-position of the balance.

2.Interchanges of the sorbent site and the ballast or tare site of the balance
should be done from time to time. This can avoid unusual unsymmetrical
effects in the balance as they may be caused by (unavoidable) shocks of the
balance’s base over the years.

3. If calibration masses of well known volume are available, the balance can
be used to measure either the density of the sorptive gas or, via an
accurate equation of state, the gas pressure in the sorptive
atmosphere. Use of two balances in parallel for combined sorption and
pressure measurements is recommended in this case [3.1, 3.23-3.25].