Sodium Aspirin Salts: Crystallization and Characterization

Acetylsalicylic acid (trade name aspirin) is a well-known drug with antipyretic and analgesic effects. Mixtures that benefit from better solubility of its sodium salt have been sold for almost 90 years. Currently, several drug products are available with sodium aspirin as their active ingredient, but, until now, its crystal structure remained unknown. We have investigated the multicomponent system of sodium acetylsalicylate with the following results: an anhydrate and a dihydrate of a 1:1 salt were identified together with a hemihydrate of a 2:1 salt. Crystal structures of all forms were solved by single-crystal X-ray diffraction. The structural changes upon desolvation were studied by simultaneous thermogravimetric/differential scanning calorimetry supported by X-ray powder diffraction. The crystal structures were compared to those of all up-to-date published crystal structures of aspirin salts. Both of the hydrated sodium salts belong to the same isostructural family, while the anhydrate crystallizes with a unique packing of the acetylsalicylate anions.