BET isotherm

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An isotherm that takes account of the possibility that the monolayer in the Langmuir adsorption isotherm can act as a substrate for further adsorption. The BET isotherm (named after S. Brunauer, P. Emmett, and E. Teller) has the form:

V/Vmon=cz/{(1 – z)[1 – (1 – c)z]}, where z=p/p* (p* is the vapour pressure above a macroscopically thick layer of liquid on the surface), Vmon is the volume that corresponds to the surface being covered by a monolayer, V and p are the volume and pressure of the gas respectively, and c is a constant. In the BET isotherm, the isotherm rises indefinitely at high pressures (in contrast to the Langmuir isotherm). It provides a useful approximation over some ranges of pressure but underestimates adsorption for low pressures and overestimates adsorption for high pressures.

V/Vmon=cz/{(1 – z)[1 – (1 – c)z]}

Subjects: Chemistry — Physics.

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