Water Content by Karl Fischer

Karl Fischer titration measures water by chemistry rather than by weight loss. Water is consumed stoichiometrically in a reaction between iodine and sulfur dioxide in an alcoholic medium containing a base, one mole of iodine per mole of water, so the iodine consumed reports the water present. Volumetric titration suits samples with appreciable water; the coulometric variant, which generates iodine electrochemically at an electrode, resolves down to microgram quantities.

Lyophilised peptides typically retain low single-digit percentages of residual water, and specifications are normally written as a maximum. The figure matters because water plasticises an amorphous solid, lowering its glass transition temperature and raising molecular mobility, which in turn accelerates deamidation, hydrolysis and aggregation. Drying harder is not automatically better either, since some proteins are destabilised at very low moisture where the residual hydration shell is stripped away.

Water is also a term in the mass balance. Peptide, counterion, water and residual solvent account for the powder between them, so a high moisture result explains a low net peptide content and a shortened shelf life in the same stroke. It is the cheapest measurement that links a visible formulation observation, such as a collapsed cake, to a quantitative prediction about how the material will behave in storage.

Loss on drying is routinely reported as water content and is not the same test. It weighs everything volatile, including residual acetic acid and organic solvent, so it overstates water in exactly the samples where solvent removal was incomplete. A titration result is also a snapshot of a hygroscopic material: a vial opened repeatedly in humid air no longer matches the number printed on its certificate.