Specific surface area, surface area divided by weight, can be measured by BET theory, Developed by Brunner, Emmett, Teller.
Working principle
- p: pressure in the testing chamber
- p0: saturated pressure of an adsorbate, e.g. N2 or Kr, the measurement temperature
- V: adsorbed volume of adsorbate
- C: a parameter
- Vm: monolayer adsorption capacity
From Vm to calculate specific surface area
Important notes
- In principle, BET method can not calculate the surface area of pores with size less than 2 nm. Or special setting is needed to get reliable data.
- For powders with low surface area, Kr is better than N2, since Kr has lower saturated pressure.
- Sample preparation, e.g. vacuum and degassing time, has influence on the measurement accuracy.
- Measurement temperature. Because most gases and solids interact weakly, the solid material must be cooled, typically using a cryogenic liquid which is 77K.
The following is a typical surface area isotherm, where the x-axis is the relative pressure of the gas and the y-axis is its volume adsorbed onto the sample.
Data treatment is shown in above.
References