Page 164 - Adsorption by Powders and Porous Solids

150 ADSORPTION BY POWDERS AND POROUS SOLID$

5-33 Basic experimental methods for the study of adsorption from solution
We may divide the experimental techniques available for the study of adsovtion
from solution into three main categories: (a) for the determination of adsoTtion
isotherms, (b) for the measurement of the energies involved, and (c) for the provision
of extra information on the properties of the adsorbed layer.

Methods for determining the amounts adsorbed
A first distinction must be made between the methods which use one sample for each
point on the adsorption isotherm (i.e. immersion methods) and those using a single
sample through which the solution of increasing concentration is allowed to flow (iae.
flow-through methods). A critical outline of most of these methods is given by
Everett ( 1986).

Immersion methods. These are the oldest and the easiest to apply with conventional
bench-type equipment but may suffer, as we shall see, from a lack of accuracy or
from a large sample consumption.
In the standard immersion method, the dry sample is immersed in the solution (see
Figure 5.15a). In a slightly different version (Rouquerol and Partyka, 1981), the
sample is initially covered with the pure solvent (protected from any contact with
ambient atmosphere) before receiving an appropriate dose of mother solution (see
Figure 5.15b).
Equilibration may take between 1 minute and more than a day, in a thermostatted
bath with continuous and slow tumbling. The suspension, still kept at the controlled
temperature, is allowed to settle, which may take one full day, or, more frequently,
the suspension is centrifuged (taking care, in the case of polymers, not to produce a
measurable concentration gradient) and the supernatant is then pipetted and analysed.
The analysis may involve differential refractometry, UV or TR spectroscopy (the
former mainly for aqueous solutions, the latter for organic solutions), organic carbon
analysis, colorimetry (for dye adsorption) or surface tension measurements. As men-
tioned in Section 5.3.1, this method has the advantage that it directly gives the
reduced surface excess amounts. Since one experiment only provides one point of the
adsorption aotherm, it is usual to undertake a number of simultaneous measurements
(each requiring a fresh sample) in order to cover the desired portion of the adsorption
isotherm.
A possible way to increase the accuracy of this immersion approach is to use the
slurry method and to analyse a weighed sample of the slurry in the bottom of the test-
tube, instead of analysing the supernatant (Nunn er al., 198 1). One then simply makes
use of Equation (5.49). the operational expression of the relative surface excess of the
solute with respect to the solvent. Here n, and n, are the total amounts of solute and
solvent in the sample of slurry (either adsorbed or in solution) and ci and c: their con-
centrations in the solution. If one uses a liquid-solid ratio large enough to avoid any
measurablechange in concentration on adsorption, then c: and ci are simply the con-
centrations in the starting solution. The measurement is accurate provided the quan-
titative analysis of the slurry, which involves measuring the total amounts of 2 and 1