BET measurement

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

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

description

Different measuring devices are used for the BET measurement, mostly multi-point BET devices as opposed to the single-point BET devices afflicted with a systematic error. This is described in detail in the DIN ISO 9277 standards or the previously valid DIN 66131. A gas (adsorptive), often nitrogen , is passed over the material to be examined (adsorbent). Due to the cooling, usually by means of liquid nitrogen (−196 ° C), the amount adsorbed can be determined with a normal pressure measuring device below the saturation vapor pressure of the measuring gas (adsorption). Condensation would falsify the measurement result, but will not take place as long as the saturation vapor pressure is not reached. Subsequent reduction of the pressure inside the apparatus releases part of the adsorbed gas quantity from the surface (desorption). This enables an adsorption-desorption isotherm to be determined. In certain pressure ranges (often in the relative pressure range 0.05 to 0.3, which should therefore also be completely measured with five measuring points, for example) the amount of gas adsorbed or released is proportional to the surface area.

The BET surface area is usually given in the unit square meter per gram (m 2 · g −1 ). Highly disperse silicas , for example, have a specific BET surface area of ​​200–800 m 2 · g −1 . Activated carbon has a specific BET surface area of ​​up to 1200 m 2 · g −1 .

See also

Individual evidence

  1. ↑ Stephen Brunauer, PH Emmett, Edward Teller: Adsorption of Gases in Multimolecular Layers . In: Journal of the American Chemical Society . tape 60 , no. 2 , February 1938, p. 309–319 , doi : 10.1021 / ja01269a023 ( PDF ). PDF ( Memento of the original from March 9, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  @1@ 2Template: Webachiv / IABot / zumbuhllab.unibas.ch
  2. ^ Burt Davis: Brunauer, Emmitt and Teller: The Personalities behind the BET Method . In: Energeia. 5, No. 6, 1994, pages 1, 4 and 5 (PDF; 1.2 MB).
  3. ↑ DIN ISO 9277: 2003-05 Determination of the specific surface area of ​​solids by gas adsorption according to the BET method (ISO 9277: 1995) .
  4. ↑ S. Ebel and HJ Roth (editors): Lexicon of Pharmacy. Georg Thieme Verlag, 1987, ISBN 3-13-672201-9 , p. 90.