Water content of peptides: Karl Fischer titration

Freeze-dried peptides are never completely dry, and most take up water from the air whenever they are exposed to it. Water content matters because it is part of the difference between the mass you weigh and the amount of peptide you have. Karl Fischer titration is the standard way to measure it. This article explains how the titration works, which form suits peptide samples, what can interfere, and how the result is used.

Principle

Karl Fischer titration, introduced in 1935, measures water through a reaction that consumes it in a fixed ratio: iodine oxidizes sulfur dioxide in the presence of water, a base and an alcohol. One mole of iodine reacts for each mole of water. The endpoint, when all the water has reacted and free iodine appears, is detected electrically. Because the reaction is specific to water, the method measures water rather than everything that evaporates.

Volumetric and coulometric

The two forms of Karl Fischer titration

Form	How iodine is supplied	Best range	Peptide use
Volumetric	A titrant of known iodine strength is added from a burette	Milligrams of water upward	Larger samples; less common for peptides
Coulometric	Iodine is generated electrically in the cell; the charge passed gives the water (10.71 coulombs per milligram)	Micrograms to a few milligrams of water	Suits the small sample sizes typical of peptides

The oven method

A peptide can be added to the titration cell directly, dissolved, or measured with an oven. In the oven method the sample is sealed in a vial and heated, and a stream of dry gas carries the released water into the cell, so the solid never touches the reagents. It suits hygroscopic solids, which pick up water during transfer, and samples that would react with the reagent or not dissolve in it.

What can interfere

• Atmospheric moisture. The biggest source of error for peptides. A hygroscopic solid gains water during weighing and transfer, so handling is fast, often in a dry box, and blanks are run.
• Thiols. Free cysteine thiols can react with iodine and read as extra water. The oven method avoids this, because only the water reaches the cell.
• Sample size. Peptide samples are small, and a few milligrams of solid with a few percent water contain only tens to hundreds of micrograms of water, close to the range where handling errors dominate.

Using the result

Water content is reported as a percentage of the sample mass. For freeze-dried peptides it is commonly a few percent and sometimes higher. Together with the counter-ion content, it accounts for most of the difference between the weighed mass and the peptide in it: the mass balance described in Net peptide content and purity. Because the solid gains water after opening, the result describes the lot as tested.

Loss on drying and thermogravimetric analysis measure all the mass a sample loses when heated, including residual solvents and volatile acids such as acetic acid. They are not specific to water, which is why Karl Fischer titration is preferred when water itself is the question.

Frequently asked questions

Why does a certificate report water separately from purity?

HPLC purity does not see water. Water is measured by its own method and, with counter-ions, explains why the peptide content of the powder is below 100%.

Does water content change after I open the container?

It can rise, because freeze-dried peptides take up moisture from the air. Keeping the container closed and letting it warm before opening limits the change.

Is loss on drying the same as water content?

No. Loss on drying counts everything volatile at the test temperature, not only water.

Sources & Further Reading

1. Fischer K. Neues Verfahren zur maßanalytischen Bestimmung des Wassergehaltes von Flüssigkeiten und festen Körpern. Angewandte Chemie 1935;48(26):394–396. doi:10.1002/ange.19350482605
2. Scholz E. Karl Fischer Titration. Chemical Laboratory Practice. Springer; 1984. doi:10.1007/978-3-642-69989-4
3. United States Pharmacopeia. General Chapter <921> Water Determination.
4. European Pharmacopoeia. 2.5.12 Water: semi-micro determination; 2.5.32 Water: micro determination.

Cosmic Peptides. (2026, October 25). Water content of peptides: Karl Fischer titration. Cosmic Peptides Research Library. Retrieved October 26, 2026, from https://cosmicpeptides.com/research/methods/karl-fischer-water-content

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  author       = {{Cosmic Peptides}},
  title        = {Water content of peptides: Karl Fischer titration},
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  year         = {2026},
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  urldate      = {2026-10-26},
  note         = {Last reviewed 2026-10-25}
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