Volmer

145

ADSORPTION

la	Ib „_J	w	^,,b / yjf
hi	^IVa yjt	^IVb U	IVc___J
^v J		VI

P/Po

Fig. 3. Classification of isothcrms.

HJPAC had expanded the original classification of Brunauer from five to six types, and later F. Rouąuerol, J. Rouąuerol, and K. Sing introduccd subdivisions of Types I, II, IV in the case of physical adsorption (sec Fig. 3). The different types can be ; Characterized by different sorption mechanisms (2). Liąuid phase adsorption can be divided into two types, namely, adsorption from liąuids and adsorption from Solutions.

On a theoretical basis it can be stated that the adsorption isotherms must have a Henry’s law region (3), which means that each of the isotherms in a Iow pressure or diluted region will be linear. But researchers have found materials that deviate from linearity even at very Iow pressures. This fact, among others, shows discrepancies between the theoretical and the practical treatments. There-fore, as is usual, virial coefficients are introduced. The justification of practical isotherms even in the simplest gas-solid systems is very difficult on a theoretical basis, and these isotherms cannot be calculated from fundamental principles alone. Therefore, a large variety of isotherm equations and models are in use, each of which has limitations. Hence the measurement of adsorption isotherms is the most fundamental activity in this field. The measuring methods can be divided into three groups, namely volumetric, gravimetric, and kinetic.

The principle of volumetric methods (4,5) is that a given amount of adsorbent and the adsorbable gas are brought into contact and the adsorbed amount is calculated from the accurately measured data of pressure, volume, and