Magnesium Stearate: Solving The BET Surface Area Problem

By: Martin A. Thomas Ph.D., Director of Business Development

Magnesium stearate (MgSt), a waxy, lamellar (platey) solid, is the most widely used solid excipient in pharmaceutical technology [1]. Primarily, MgSt is added to a formulation in order to lubricate powder flow and compaction [2], but it also imparts certain hardness, disintegration and dissolution characteristics to tablets. This excipient's performance depends, at least to some extent, on chemical purity, density, particle size, particle shape, and surface area. The least understood of these properties, and exactly how to measure it, is surface area-despite it being required in the USP MgSt monograph. The established USP method for Specific Surface Area includes a specified pass/fail criterion (strangely perhaps, it is not an actual surface area value). That method is General Chapter 846 based on low-temperature gas adsorption and the BET calculation [3]. Specific experimental conditions, including those for pretreatment, appear in the MgSt monograph.