Determination of BET surface area by gas adsorption
Gas adsorption analysis is a common method for the determination of BET surface area, pore size distribution and total pore volume. It can be used for pore characterization between approximately 0.3 and 500 nm. Norlab offers a variety of gas adsorption instruments for research and development, quality insurance as well as bulk import and export control.
The determination of pore size distribution and pore volume is described on this website as separate methods.
The determination of specific surface areas represents a major task regarding the characterization of porous and finely-dispersed solids. Gas adsorption is the appropriate method to solve this task. If a gas gets in contact with a solid material a part of the dosed gas molecules is being adsorbed onto the surface of this material. The adsorbed amount of gas depends on the gas pressure, the temperature, the kind of gas and the size of the surface area. After choosing the measuring gas and temperature, the specific surface area of a solid material can be reliably and comparably calculated from the adsorption isotherm. Due to practical reasons the adsorption of Nitrogen at a temperature of 77 K (liquid Nitrogen) has been established as the method for the determination of specific surface areas.
Schematic measurement setup
Isotherm with highlighted range for calculation of specific surface area (BET)
Measuring method
Speaking about the BET method, actually means the analysis of isotherm data by a method developed by Brunauer, Emmett and Teller. By means of the BET equation the amount of adsorbed gas, which build up one monolayer on the surface, can be calculated from the measured isotherm. The amount of molecules in this monolayer multiplied by the required space of one molecule gives the BET surface area. Besides the adsorption of Nitrogen at 77 K, Krypton adsorption at 77 K is recommended for the determination of very small surface areas.
Mathematical principles for calculation of BET surface area
The choice of the measuring instrument depends on:
- Scope of the task (only determination of BET surface area or also the pore volume and pore size distribution)
- The range of the surface area of the materials (the determination of very small surface areas by Krypton adsorption requires a special measuring technique)
- The desired throughput (1, 2, 3 or 4 station instrument)
Instruments
The specific surface area according to the BET-Theory is a common parameter for the characterization of powders. We offer a variety of instruments tailored towards the respective fields of applications such as research and development, quality insurance as well as bulk import and export control.
- 1 – 3 analysis stations
- Options for micropore and Krypton analysis
- In-situ or external sample preparation
- 1, 2 or 4 fully independent analysis stations
- In-situ or external sample preparation
- Up to 4 completely independent analysis stations in one dewar
- Separate p0 measuring cell for simultaneous measurement
- 4 fully independent analysis stations
- Setup is configured for flow method (fast BET measurements)
- Allows coolant adjustment to temperatures of 82 – 323 K for pore analysis on different instruments of all manufacturers
- Allows micropore and BET surface analysis according to the IUPAC recommendations with Argon isotherms at 87 K