Volmer

146

ADSORPTION

Vol. 1

temperaturę, after the system reaches an eąuilibrium state. The difference be-tween the initial and finał masses of adsorbate is estimated. The estimation of this ąuantity is often difficult because of the so-called dead-space. A helium cali-bration (helium is considered as a nonadsorbable gas) at different temperatures and pressures can help overeome this problem. In praetice, it is convenient to keep the temperaturę and the volume constant and then measure the pressure changes (6).

The latest gravimetric methods usually use mierobalances. Their principal advantage is their high sensitivity; their greatest disadvantage is the lack of suit-able corrections at higher pressures (7).

The kinetic methods use gravimetric or volumetric measurements, but the introduction of adsorbate is continuous and not step by step as in the gravimetric method.

It is essential that the adsorbate be introduced very slowly because by convention the adsorption isotherms must correspond to thermodynamic eąuilibrium. For these methods, the calculated isotherms may be considered as ąuasi-isotherms. From the practical point of view, the difference from the real eąuilibrium is usually negligible.

The commercial eąuipments for isotherm measurements have appropriate softwares for ealculation; neverthcless, the correction of nonideality in the gas phase is necessary.

In the case of liąuids or Solutions an immersion method is used for the deter-mination of the amounts adsorbed (8). The adsorbents are prepared in different ways and then they are brought into contact with liąuids or Solutions. The changes in liąuid phase are monitored by IR, UV, or some other analytical methods and the isotherms are calculated. The most known isotherms are given below:

Henry’s type

n = łt\\p

(4)

Langmuir type (9,10)

_Bp_ 1 + Bp

(5)

Volmer’s isotherm (11)

(6)

Hill-de Boer isotherm (12)

(7)

BET (Brunauer-Emmet-Teller) isotherm (13)

1 C- \ p

— +-_w —

n(l-£) n_mC n_mC

(8)