Kr adsorption is commonly used to determine the low specific surface area. Why can we measure the low specific surface area by using Kr?
The cross sectional area of Kr and N2 are 0.202nm2 and 0.162nm2 respectively. Kr molecules are lager by about 25%. It is not suitable for the low specific surface area measurement.
The reason of using Kr is due to its adsorption temperature and vapor pressure. The gas adsorption amount in the volumetric method is calculated from the difference between the number of dosed gas molecules and number of unadsorbed gas molecules at the equilibrium pressure. In other words, by assuming both molecular sizes are equal and pressure of adsorption (molecule number) is 50 Pa, it is necessary to measure the pressure change of 0.16% (=50/30450) for N2 and 38% (=50/130) for Kr at the same relative pressure (P/Po=0.3, N2=30400Pa, Kr=80Pa). Obviously, it is easier to measure larger pressure change, and therefore, the accuracy of the measurement is better. From this reason, the lower the saturation vapor pressure at the adsorption measurement temperature, the more accurate to measure the low specific surface area.
| Adsorptive | Temp.／K | Vapor pressure／Pa | Cross sectional area／nm2 | Range／m2 |
|---|---|---|---|---|
| N2 | 77.4 | 101325 | 0.162 | 1< |
| Ar | 77.4 | 26664 | 0.166 | 0.1～10 |
| Kr | 77.4 | 267 | 0.202 | 0.01～1 |
| Xe | 77.4 | 0.23 | 0.232 | ca. 1cm2 |
| CO2 | 273.2 | 3490 k | 0.195 | Coal, Activated carbon |
Table Parameters of the surface area and adsorptive
In the actual cases, it is not so easy to improve the measurement accuracy as described. To measure the low specific surface area by using Kr, the apparatus is required to have a high vacuum pump, additional low pressure sensor, and highly leak tight system.
