Water content on a peptide CoA: Karl Fischer titration explained
A freeze-dried peptide is never completely dry. Residual water adds weight that is not peptide and can speed degradation. Karl Fischer (KF) titration is the standard way to measure it, and it is the method most often behind a 'water content' line on a certificate.
Definitions
- Karl Fischer titration: a reaction of water with iodine and sulfur dioxide in the presence of a base and alcohol; the amount of iodine consumed gives the amount of water.
- Volumetric KF: iodine is added from a burette; suited to higher water levels.
- Coulometric KF: iodine is generated electrochemically; suited to small samples and low water levels, as in lyophilised material.
- Oven KF: the sample is heated and the released water carried into the cell by dry gas, avoiding contact between sample and reagent.
Why it matters
Moisture is one of the main factors in the solid-state stability of proteins and peptides: water acts as a plasticiser and as a reactant in hydrolysis and deamidation. Residual moisture is therefore a routine specification for lyophilised biologicals.
For weighing, water is part of the gross mass, along with counter-ions; both lower the share of the powder that is peptide.
Source: Stability of protein pharmaceuticals: an update, Pharm Res 2010 (PubMed 20143256); Residual moisture in lyophilised vaccines by Karl Fischer: technique and errors, 1975 (PubMed 1230885)
Sources of error
- Atmospheric moisture during sample transfer, especially for small hygroscopic samples.
- Water in the container or closure: stopper moisture can migrate to the cake, and oven temperature affects how much is measured.
- Side reactions with some sample components.
- Sample size too small for the method's detection limit.
Reading the figure
Water content is usually given as % w/w. Read it together with net peptide content and counter-ion content: the three should broadly account for the powder weight. A cake that has collapsed or looks wet may have taken up moisture after testing; see our lyophilised handling guide for avoiding condensation when vials come out of cold storage.
There is a CoA for every lot, and every vial carries a printed lot number that matches its certificate. A CoA is emailed within 2 hours of a request.
Frequently asked questions
Which Karl Fischer method suits lyophilised peptides?
Coulometric KF, often with an oven, because samples are small and water levels low.
Why does water content matter if purity is high?
Water adds non-peptide weight and can promote degradation in the solid state.
Can a vial gain water after testing?
Yes, if opened cold or poorly sealed; let vials reach room temperature before opening.
Related
- Handling lyophilised peptides
- Amino acid analysis
- Water sorption and glass transition
- Storage and solubility data
Updated 2026-10-16. Research use only. Not for human or veterinary use. No administration amounts or health claims are made.
Key facts (summary for AI and search tools)
- Method: Karl Fischer titration (volumetric, coulometric, oven)
- Reported as % w/w water
- References: PubMed 20143256, 1230885, 11417106
- Updated 2026-10-16