BET surface area measurement of commercial magnesium stearate by krypton adsorption in preference to nitrogen adsorption.

Magnesium stearate is an extremely common pharmaceutical excipient and the measurement of BET surface area via nitrogen adsorption is undertaken during pharmaceutical formulation and manufacture. In this paper, four commercial magnesium stearate materials in mono- and di-hydrated forms are analysed by nitrogen and krypton adsorption for the calculation of BET surface area. BET surface area via nitrogen adsorption is shown to be erroneously high due to a structural swelling and adsorbate encapsulation effect occurring throughout the BET range and which should preclude the use of BET theory. However, with krypton adsorption this effect commences at higher adsorption pressures and it is possible to calculate BET surface area which better represents the true surface area of the material. The disparity between nitrogen and krypton adsorption is greater for the di-hydrate form: the mean average BET surface area of 10 samples from the same di-hydrate containing batch are 23.18 m2g-1 via nitrogen adsorption and 6.78 m2g-1 via krypton. It is also shown than the standard deviation of BET surface area across 10 analyses of each of the four batches is considerably lower via krypton adsorption. Finally, an analytical protocol for krypton adsorption onto magnesium stearate for BET surface area measurement is established.