Stoichiometry

Hydrates can be described in terms of their stoichiometry relative to the host molecule. If the hydrated crystal form contains one molecule of water per host molecule, thus having a one to one stoichiometry, then the hydrate is called monohydrate, as is realised e.g. in theophylline monohydrate. Consequently, if the hydrate contains two, three,… water molecules per host molecule, they are named dihydrate, trihydrate,… (e.g. piroxicam dihydrate, amoxicilline trihydrate, etc.). The ratio of water to host molecules does not have to be integer. Thus there exist hemihydrates (0.5 to 1, e.g. estradiol hemihydrate) or sesquihydrates (1.5 to 1, e.g. codeine phosphate sesquihydrate) or even more exotic ratios like 2/3 to one or 0.8 to one (i.e. 4:5), generally as a result of a complex but well-defined structure.