The Surface Area Of Magnesium Stearate — An Excipient Used In Pharmaceuticals

Magnesium stearate is widely used in the pharmaceutical industry as an excipient in tableted drugs. The physical properties of the magnesium stearate influence the tableting characteristics of the formulation as well as the in vivo degradation and dissolution of the solid drug. The United States Pharmacopia, which has already established a standard for the particle size distribution of magnesium stearate, may adopt surface area standards for this material in the near future. To assist in this endeavor, Micromeritics obtained four samples of commercially available magnesium stearate and measured the BET surface area using the Micromeritics Gemini Surface Area Analyzer.

Sample Preparation (Degas)
Before a sample can be analyzed, it is necessary to remove gas and vapors which may have adsorbed onto the surface from the ambient air. If this is not done, the surface area result can be low and non-reproducible since an indeterminate amount of the surface will be covered with these materials. This step must be done with caution; every effort must be made not to change the original surface of the sample. This sample preconditioning is usually accomplished by either applying a vacuum or purging the sample with an inert flowing gas.

Both methods usually make use of elevated temperatures to hasten the rate at which the contaminants leave the surface. Caution must be used when heating magnesium stearate because melting, dehydration, sintering, and decomposition are processes that can drastically alter the surface properties of the sample. A test protocol used to establish that excessive sample preparation temperatures have not been used is to determine the surface area of the sample at successively higher preparation temperatures. Duplication of the results at different temperatures indicates that the initial preparation conditions were satisfactory unless both analyses were performed on a completely degraded sample.