92168.pdf

120 The solid-gas interface

The adsorption of a gas or vapour can be measured by admitting a

known amount of the adsorbate into an evacuated, leak-free space

containing the outgassed adsorbent. The extent of adsorption can

then be determined either volumetrically or gravimetrically.

The volumetric method is mainly used for the purpose of

determining specific surface areas of solids from gas (particularly

nitrogen) adsorption measurements (see page 134). The gas is

contained in a gas burette, and its pressure is measured with a

manometer (see Figure 5.4). All of the volumes in the apparatus are

calibrated so that when the gas is admitted to the adsorbent sample

the amount adsorbed can be calculated from the equilibrium pressure

reading. The adsorption isotherm is obtained from a series of

measurements at different pressures.

Gas

reservoir

Liquid

nitrogen

Adsorbent

'Vacuum

system

d—)

Cathetometer

Manometer

Figure 5.4 Volumetric apparatus for measuring gas adsorption at 77 K

The gravimetric method can be used for studying both gas and
vapour sorption. Purified gas or vapour is introduced into an
evacuated apparatus in which the sorbent sample is contained in a
weighing pan, the pressure is noted and the extent of sorption is
measured directly (allowing for buoyancy) as the increase in the
weight of the sorbent sample. In the original method of McBain and
Bakr^164 a precalibrated quartz spiral is used for weighing the sample,