Method of Small Surface Area Measurement by Kr Adsorption Can we measure the low specific surface area by using Kr?

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 0.202 nm2 and 0.162 nm2 respectively. Kr molecules are larger 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 % (=50/30400) for N₂ and 63 % (=50/80) for Kr at the same relative pressure (P/Po=0.3, N₂=30400 Pa, Kr=80 Pa). Obviously, it is easier to measure larger pressure change, and therefore, the accuracy of the measurement is better. Therefore, the lower the saturation vapor pressure at the adsorption measurement temperature, the more accurate the measurement of the low specific surface area.

Table  Parameters of the surface area and adsorptive

In 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 leak-tight system.