Figure 1 shows examples for the water vapor sorption isotherm of some auxiliary substances, determined with the SPS11 analyzer.
The microcrystalline cellulose shows the classic sorptional behavior of polysaccharides with a broad hysteresis between the sorptional- and desorptional course.
Above a critical relative humidity of 80% r.H. the saccarose sample starts to liquefy.
Lactose monohydrate adsorbs only 0.13% of water over the entire humidity range. Nevertheless within this range the sorption isotherm can be determined with a good resolution.

Figure 2 shows two examples of medical products forming a hydrate or several hydrate forms. Medicament A shows a classic stoichiometric monohydrate, starting to accumulate at 40% r.H. and again ceding the water at a relative humidity below 20% r.H.
Medicament B starts metabolizing into a hydrated form not until 90% r.H. The course of the desorption courve shows several well identifiable hydrate steps. As a characteristic of stoichiometric hydrates, there is a distinctive hysteresis between sorption- und desorption courve of these samples.

Ulrich J. Griesser, Institute of Pharmacy, University of Innsbruck, Austria and
Juergen Dillenz, Projekt Messtechnik, Ulm, Germany

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