Pharmaceutical Heterogeneous Systems II: Study of Hydrolysis of Aspirin in Combination with Fatty Acid Tablet Lubricants

Stearic acid USP, in combination with aspirin in tablets, capsules, and powder mixes increases the rate of salicylic acid formation over pure aspirin controls. The breakdown of aspirin in combination with stearic acid USP and reagent grade stearic and palmitic acids has been studied with regard to the relative suitability of the fatty acids in question as tablet lubricants. Commercial stearic acid was found to have a marked effect on acceleration of salicylic acid formation as compared to reagent palmitic and stearic acids which were 95% + pure (gas chromatography). In powders and tablets with a constant aspirin: fatty acid ratio (20:1) the degree of aspirin degradation reaches a maximum when the two reagent acids are mixed together at approximately the same molar ratio as found in stearic acid USP. This maximum closely parallels the minimum on the melting point curve for mixtures of stearic and palmitic acids.