What does Karl Fischer titration measure?

It is specific to water. The method relies on a reaction in which iodine is consumed in direct proportion to the water present, in a ratio of one molecule of iodine to one molecule of water. Unlike a method that measures every volatile component, it reacts with water rather than with everything that leaves a heated sample, although a few substances such as free thiols can interfere. The instrument detects the point at which all the water has reacted and converts the amount of iodine used into a mass of water. The result is usually expressed as a percentage of the sample weight.

Why do peptides contain water after freeze drying?

Freeze drying removes most of the water but not all of it. Some remains held within the solid, and peptides are hygroscopic, so the powder also takes up moisture from the air whenever it is exposed. Charged and polar groups in the sequence attract water, and the salt form of the peptide adds to this. A lyophilised powder is therefore best understood as peptide plus counterion plus a small, variable amount of water.

Does water content affect peptide purity?

Not directly. HPLC area-percent purity is calculated from UV-absorbing peaks, and water does not absorb meaningfully at 214 nm, so it contributes nothing to the calculation. What water does change is the mass balance. A higher water content means a smaller share of the powder is peptide by weight, which lowers net peptide content without moving the purity figure. Over time, absorbed water can also contribute to degradation, which would then show up as a change in purity on a later test.

What is a normal water content for a lyophilised peptide?

Residual moisture in lyophilised pharmaceutical preparations is commonly cited as roughly 0.5 to 3 percent by weight. Peptide powders can sit outside that range, particularly if they have been exposed to humid air or stored without protection. There is no single correct value. The figure depends on the sequence, the salt form, the freeze-drying process and the handling history, which is why it is measured on the sample rather than assumed.

Karl Fischer vs loss on drying, what is the difference?

Loss on drying heats a sample and records the weight lost. That loss includes water, but also any other volatile material, such as residual solvents or volatile acids. Karl Fischer titration reacts with water chemically, so it is specific to water. For a peptide powder that may contain trace solvents, the two methods can give different answers, and the Karl Fischer figure is the one that reports water alone.

Does residual water make a peptide degrade faster?

In general, yes. Water is a reactant in hydrolysis and also gives molecules in the solid more mobility, so higher moisture is associated with faster degradation of lyophilised products. This is why low residual moisture is a standard aim for freeze-dried material. How much faster depends on the sequence and storage conditions, and a single water figure cannot predict a shelf life.

Analytical methods

Water Content and Why Karl Fischer Titration Is on the Certificate

Water does not show up on a chromatogram, but it is part of the powder you weigh. Karl Fischer titration is the method used to measure it. Here is the chemistry, the two instrument types, and the limits of the number.

Novagen Analytical Labs October 27, 2026 10 min read

A certificate reports water content because water is part of the powder and nothing else on the certificate measures it. A lyophilised peptide always holds some residual moisture. That water adds to the weighed mass, can influence how well the material keeps, and is invisible to HPLC. Karl Fischer titration is the method used to measure it, because it reacts with water specifically and with little else. The result is a mass percentage, and it is one of the numbers needed to account for what is actually in the vial.

Why a dry powder still contains water

A freeze-dried peptide looks dry. It is not water-free. Lyophilisation removes ice by sublimation and then removes some of the remaining bound water in a secondary drying step. It does not remove everything, and it is not meant to.

Two kinds of water are involved. Bound water is held within the solid, associated with polar and charged groups on the peptide and with its counterion. Surface water is picked up from the atmosphere after drying. Peptides are hygroscopic, so surface uptake begins as soon as the powder is exposed to humid air.

Residual moisture in lyophilised pharmaceutical preparations is commonly cited as roughly 0.5 to 3 percent by weight. That range is a general description, not a specification for any given peptide. A sequence rich in charged residues, a different salt form, or a vial that was opened in humid conditions can all produce a different number. The only way to know the value for a particular sample is to measure it.

What Karl Fischer titration measures

Karl Fischer titration is a chemical determination of water. It is built on the Bunsen reaction, in which iodine oxidises sulfur dioxide in the presence of water. In the Karl Fischer version, the reaction runs in an alcohol medium with a base present, and water is consumed in a fixed ratio to iodine. One mole of iodine reacts with one mole of water.

The reagent system has four working parts.

Iodine. The titrating species. Its consumption is the measurement.
Sulfur dioxide. The substance that iodine oxidises in the presence of water.
A base. It buffers the acidic products so the reaction proceeds to completion. Pyridine was the traditional choice. Imidazole has largely replaced it in modern reagents.
An alcohol solvent. Methanol is the most common. It dissolves the sulfur dioxide and takes part in the reaction chemistry.

While water remains, the iodine added is used up almost immediately. When the water is exhausted, free iodine persists in the solution. The instrument detects that change electrically, and that is the endpoint. The quantity of iodine consumed up to the endpoint, together with the 1:1 ratio, gives the mass of water.

Volumetric and coulometric titration

There are two common forms of the technique. They share the chemistry and differ in how the iodine reaches the solution.

Volumetric Karl Fischer. A burette adds a titrant containing iodine at a known concentration. The volume needed to reach the endpoint is converted to a mass of water. This form suits samples with higher water content. Published ranges for the method generally start around 0.1 percent water and extend upwards.

Coulometric Karl Fischer. The iodine is not added from a burette. It is generated electrochemically at an electrode inside the cell. The electrical charge passed is proportional to the amount of iodine generated, and therefore to the amount of water, by Faraday's law. About 10.7 coulombs correspond to one milligram of water. This form suits trace water and very small samples. Quoted working ranges vary between sources, but all place it at the low-moisture end.

The choice depends on the sample. A peptide powder of a few milligrams at around 1 to 3 percent water holds only tens of micrograms of water, which suits the coulometric form. The deciding factors are usually how much material can be consumed and how much water that material is expected to contain. A larger sample improves the precision of the measurement, but a peptide submitted for testing is often only a few milligrams.

Why specificity matters: Karl Fischer versus loss on drying

Loss on drying is a simpler idea. Weigh the sample, heat it, weigh it again. The difference is reported as moisture. Thermogravimetric analysis follows the same principle with continuous weighing as the temperature rises.

The limitation is that these methods measure weight lost, not water lost. Anything volatile at the test temperature leaves the sample and is counted. A peptide powder can retain traces of organic solvent from purification, or volatile acid from the salt form. Loss on drying would record these as moisture. Heating can also decompose some samples, which adds further weight loss that has nothing to do with water.

Karl Fischer titration avoids this because the signal comes from a chemical reaction with water. Volatile solvents that do not react with the reagent do not contribute to the result. That is the reason it is used when the question is specifically how much water is present.

The method has its own limits. Some substances react with iodine or with the reagents and produce a false water signal. Free thiol groups, for example, can be oxidised by iodine, which can inflate a result for a peptide that contains them. Samples that do not dissolve or release their water readily may need an oven attachment that drives the water into the cell as a vapour. A water figure is only as sound as the method choice behind it, and the method should be stated.

Water in the mass balance

This is the main reason the figure appears on a certificate. Everything in the vial adds to the weighed mass: peptide, counterion, water and trace solvents. These components should sum to the total. If the water content is not measured, a share of the mass is simply unaccounted for.

Consider a vial labelled 5 mg that holds 3 percent water. That is 0.15 mg of water. It is not a large amount in absolute terms. It is also not zero, and it is carried by every vial. The water sits alongside the counterion, which usually accounts for a larger share of the non-peptide mass.

The link to net peptide content is direct. The more water in the powder, the smaller the peptide share of the weighed mass. Net peptide content is measured by its own assay, and our article on net peptide content versus purity covers how that works and why it differs from chromatographic purity. The Karl Fischer figure explains part of the gap. It does not replace the content assay.

What water does and does not do to the purity figure

Water does not change HPLC area-percent purity. The calculation divides the main peak area by the total area of UV-absorbing peaks. Water does not absorb meaningfully at 214 nm, so it produces no peak and takes no part in the sum. A powder with 1 percent water and a powder with 4 percent water can return the same purity figure if their peptide-related composition is identical.

That is not a flaw in the method. Area-percent purity answers a question about composition within the peptide fraction. Water is outside that fraction. The practical consequence is that purity cannot be used to infer water content, and water content cannot be used to infer purity. They are separate measurements of separate things. The reasoning behind area-percent purity is set out in What 99 Percent Purity Means.

There is an indirect route by which water can reach the purity figure. If absorbed moisture drives degradation, the degradation products appear as new peaks on a later chromatogram. In that case the purity falls, but the cause is chemistry over time, not the water peak, because there is none.

Moisture and stability

Water takes part in the most common routes by which peptides degrade. Hydrolysis breaks peptide bonds and requires water. Deamidation of asparagine and glutamine residues proceeds through water-dependent pathways. Beyond acting as a reactant, water increases molecular mobility in the solid, and a more mobile solid reacts faster.

This is why low residual moisture is a standard aim for lyophilised products. Pharmacopoeias generally describe water determination, including Karl Fischer methods, and lyophilised products commonly carry a water specification. The specification differs between products and is set by the product, not by a universal figure. No single number is correct for every peptide.

There are limits to what a single measurement can say here. A water figure describes the moisture at the time of testing. It does not predict how long the material will last, because that also depends on temperature, light, the sequence, the salt form and how often the vial is opened. Moisture in a sealed vial is one input to stability, not a forecast of it.

What the figure describes, and what it does not

A water result describes the powder as received. It does not describe the powder after it has been stored for six months, opened repeatedly, or kept in a humid room. Because peptides are hygroscopic, the value can change once the vial is open. A sample handled in humid air before analysis can read higher than the material in the sealed vial, which is why sample handling forms part of the result.

Several limits follow from this.

The figure applies to the sample submitted, not to every vial from the same source.
It is a snapshot. Moisture content changes with exposure.
It is a composition measurement, not a safety assessment. A low water figure does not make a material safe or suitable for any use.
It says nothing about identity or purity. Those require mass spectrometry and chromatography.

Novagen is a third-party analytical laboratory. It reports what was measured in the sample submitted and nothing beyond it. Certificates are listed on the public register at novagenlaboratories.com. Questions about adding water content to a testing submission can be sent to admin@novagenlaboratories.com. Current quoted turnaround is 21 days from receipt of samples.

Reading the water line on a certificate

When a water figure is present, three questions help to read it.

First, which method produced it. Karl Fischer titration reports water specifically. A figure labelled only as moisture or loss on drying may include other volatiles. Second, whether the variant is stated, volumetric or coulometric, since that tells you something about the sample size and the range. Third, whether the figure sits beside the other components of the mass balance. Water is most informative when read with the counterion and the net peptide content.

An absent water line is not evidence of a problem. It is an absence of information. The mass in the vial may be accounted for in full, or it may not be, and the certificate does not say. For any work that depends on knowing how much peptide is present, the water figure is one of several numbers that make the arithmetic possible. The wider structure of a certificate is covered in How to Read a Peptide Certificate of Analysis.

Common questions

What does Karl Fischer titration measure?: It is specific to water. The method relies on a reaction in which iodine is consumed in direct proportion to the water present, in a ratio of one molecule of iodine to one molecule of water. Unlike a method that measures every volatile component, it reacts with water rather than with everything that leaves a heated sample, although a few substances such as free thiols can interfere. The instrument detects the point at which all the water has reacted and converts the amount of iodine used into a mass of water. The result is usually expressed as a percentage of the sample weight.
Why do peptides contain water after freeze drying?: Freeze drying removes most of the water but not all of it. Some remains held within the solid, and peptides are hygroscopic, so the powder also takes up moisture from the air whenever it is exposed. Charged and polar groups in the sequence attract water, and the salt form of the peptide adds to this. A lyophilised powder is therefore best understood as peptide plus counterion plus a small, variable amount of water.
Does water content affect peptide purity?: Not directly. HPLC area-percent purity is calculated from UV-absorbing peaks, and water does not absorb meaningfully at 214 nm, so it contributes nothing to the calculation. What water does change is the mass balance. A higher water content means a smaller share of the powder is peptide by weight, which lowers net peptide content without moving the purity figure. Over time, absorbed water can also contribute to degradation, which would then show up as a change in purity on a later test.
What is a normal water content for a lyophilised peptide?: Residual moisture in lyophilised pharmaceutical preparations is commonly cited as roughly 0.5 to 3 percent by weight. Peptide powders can sit outside that range, particularly if they have been exposed to humid air or stored without protection. There is no single correct value. The figure depends on the sequence, the salt form, the freeze-drying process and the handling history, which is why it is measured on the sample rather than assumed.
Karl Fischer vs loss on drying, what is the difference?: Loss on drying heats a sample and records the weight lost. That loss includes water, but also any other volatile material, such as residual solvents or volatile acids. Karl Fischer titration reacts with water chemically, so it is specific to water. For a peptide powder that may contain trace solvents, the two methods can give different answers, and the Karl Fischer figure is the one that reports water alone.
Does residual water make a peptide degrade faster?: In general, yes. Water is a reactant in hydrolysis and also gives molecules in the solid more mobility, so higher moisture is associated with faster degradation of lyophilised products. This is why low residual moisture is a standard aim for freeze-dried material. How much faster depends on the sequence and storage conditions, and a single water figure cannot predict a shelf life.