BET theory

The BET measurement is a term for a method of analysis for determining the size of surfaces, particularly porous solids, by gas adsorption.

Strictly speaking, this is not a measurement, but rather a method of surface chemistry, with which the mass-based specific surface is calculated from experimental data. " BET " stands for the last names of the developers of the BET model, Stephen Brunauer, Paul Hugh Emmett and Edward Teller, which in its main features the theory published in 1938 for the first time.

Description

For the BET measurement different measurement devices are used mostly multi-point BET equipment without entailing a systematic error single-point BET devices. Detail is described in the standards DIN ISO 9277 or the previously valid DIN 66131. A gas, commonly nitrogen, is passed over the material to be examined. Due to cooling, usually by liquid nitrogen (-196 ° C) can be a standard pressure measuring device below the saturation vapor pressure of the measurement gas to determine the amount adsorbed ( adsorption). Condensation would distort the measurement result, but does not occur as long as the saturated vapor pressure is not reached. Then reducing the pressure within the apparatus solves a part of the adsorbed amount of gas from the surface ( desorption). As a result, an adsorption -desorption isotherms are determined. In certain pressure ranges (often in the relative pressure range 0.05-0.3, which therefore should be with 5 measuring points also completely measured, for example ) the thereby measured amount of adsorbed or freiwerdendem gas is proportional to the surface.

The BET surface area is typically in units of square meters per gram ( m2 · g - 1). Fumed silicas for example, have a BET specific surface area of ​​200 m2 · g -1.

Documents

• DIN ISO 9277:2003-05 determination of the specific surface area of ​​solids by gas adsorption using the BET method (ISO 9277:1995 ).
• Stephen Brunauer, PH Emmett, Edward Teller: Adsorption of Gases in Multi- Molecular Layers. In: Journal of the American Chemical Society. 60, No. 2, 1938, pp. 309-319, doi: 10.1021/ja01269a023 (PDF).