Decomposition of aspirin in the solid state in the presence of limited amounts of moisture.

When limited amounts of moisture are added to aspirin in a closed system, the model for decomposition differs from that experienced in an open system, i.e., a system at constant relative humidity. The composition of the liquid decomposition layer on the surface of the aspirin crystals will constantly change, and a model for accounting for both the change in volume and composition of the decomposition layer as a function of time has been developed. Solid-state decomposition data, when plotted as a function of time, exhibit a profile similar to the one predicted, but the position of the decomposition curve observed differs from that of the predicted. The predicted curve indicates better stability than the observed data.