Hygroscopic analytes

Pyridine. The analytical reagent grade pyridine will satisfy most requirements. If required perfectly dry, it should be refluxed over potassium or sodium hydroxide pellets or over barium monoxide, and then distilled with careful exclusion of moisture (compare Fig. 77, 47, 2). It is hygroscopic, and forms a hydrate of b.p. 94-5°. Pure pyridine has b.p. 115-5°/760 mm. 

In vacuum oven methods for moisture determination, such as the official method of the Association of Official Analytical Chemists, lactose solutions are dried at about 100°C for 2 to 6 hr. The result in the dried product is amorphous lactose glass. Since lactose glass is very hygroscopic, the dried sample must be protected from moisture until final weighing. If a-hydrate crystals are present in the product to be analyzed, the sample is diluted with water to dissolve the crystals, since slow removal of the water of crystallization under the temperature and vacuum conditions of the moisture test unduly prolongs the moisture determination. 

The first task before beginning stress-testing studies is to gather all the relevant information about the compound. Information such as molecular structure, solubility, pA (s), known chemical instability, hygroscopicity, enantiomeric purity, etc. is important. In addition, previously established analytical methods may provide a starting point for development of more discriminating methods required for the separation of the complex mixtures which may result from stress degradation. 

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