Magnesium stearate, a waxy, lamellar (platy) solid, is a widely used excipient in pharmaceutical technology. Primarily, magnesium stearate is added to a formulation in order to modify compaction behavior and reduce ejection forces from tablet dies and is available in a range of grades having similar or different surface areas. The NOVAtouch is ideal for specific surface area measurements according to USP <846> Method II (volumetric method) and the well-known BET calculation described therein.
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Figure 1. Magnesium Stearate commonly used in pharmaceuticals and dietary supplements.
Why Measure the Surface Area of Magnesium Stearate?
The specific surface area of any solid relates to its particle morphology including porosity, aspect ratio, and fineness, and can be indicative of its manufacturing and thermal history and its suitability in a specific application. The preferred physical form of magnesium stearate has a lamellar structure that has a low shearing force, thereby imparting a means of dry lubrication between a compacted powder and the walls of a tablet die, when properly blended with the active pharmaceutical ingredient and other excipients.
It is also largely hydrophobic, however, and can impart undesirable effects to the dissolution profile of a solid dosage form. Pharmaceutical formulations are optimized with respect to both effective lubrication and desirable Bio-availability, which can be mutually counter-productive.
The surface area of pharmaceutical-grade magnesium stearate is formally recognized as an important characteristic and its analysis is formalized in the USP Monograph “Magnesium Stearate.” The analytical method is described in USP chapter <846> together with conditions specific to magnesium stearate stated in the aforementioned monograph.
Which Instrument is Used?
For this analysis, conforming to USP<846> Method II, the NOVAtouch gas sorption analyzer is recommended. This instrument has integrated sample preparation stations for degassing the sample to prepare the surface for analysis, as described in the aforementioned methodology. Analysis of one set of samples can proceed while degassing the next set of samples.
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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.
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