Karl Fischer titration is a chemical method that measures the water in a sample selectively. It is named after the German chemist Karl Fischer, who described the method in 1935. It is one of the most widely used methods for measuring the water left behind in lyophilized, that is freeze-dried, products, in other words their residual moisture.
In dry products such as lyophilized venom, residual moisture is a quality indicator that affects how stable the product will remain. This article explains the principle of the method, its volumetric and coulometric types, how samples of moisture-absorbing (hygroscopic) powders are handled and how the result is interpreted.
Why does residual moisture matter?
The aim of freeze-drying is to remove water and hold the molecules in an immobile solid structure. Even after the final stage of drying, some bound water remains in the product; the target is not zero moisture but a low level suited to the product. As residual moisture rises, the solid structure softens and aggregation and degradation reactions can speed up. The stages of the drying cycle are described in the article on lyophilization, and the effect of moisture on stability in the article on venom stability.
Principle of the method
In the Karl Fischer reaction, iodine reacts with sulfur dioxide in a medium containing an alcohol and a base, consuming water as it does so. Because the reaction proceeds in a fixed ratio with water, the amount of water in the sample is calculated from the amount of iodine consumed. The end of the titration is usually detected with an electrode, at the moment free iodine appears in the medium. Today the reagents are mostly used as ready-made, pyridine-free mixtures.
The method's greatest advantage is its selectivity: the loss-on-drying method, which relies on heating and weighing the sample, also measures other substances that evaporate along with water, whereas Karl Fischer responds only to water.
Volumetric and coulometric Karl Fischer
| Species | How iodine is supplied | When it is suitable |
|---|---|---|
| Volumetric | An iodine solution of known concentration is added from a burette | Samples with relatively high water content |
| Coulometric | Iodine is generated electrically in the cell; water is calculated from the electric charge used | Water at microgram level; small amounts of sample |
| Oven method | The sample is heated and the released water is carried to the titration cell by a dry gas | Samples that do not dissolve or that interfere with the reaction |
For lyophilized protein and peptide products, the coulometric method is often preferred because the amount of sample is usually small and the water content low. The methods are also defined in the pharmacopoeias; for example, chapters 2.5.12 and 2.5.32 of the European Pharmacopoeia and chapter 921 of the United States Pharmacopeia cover water determination.
Verifying the instrument
The reliability of the measurement depends on regular verification. In the volumetric method, the water equivalent of the titrant is determined with a standard of known water content, for example sodium tartrate dihydrate. In the coulometric method, certified standards containing a known amount of water are used to check that the instrument measures correctly. In every series of measurements, moisture leaking into the titration cell from the environment is determined with a blank measurement and subtracted from the result.
Sampling a hygroscopic powder
Lyophilized powder absorbs moisture from the air quickly. If a sample stays in open air for a few minutes, the measured value reflects the humidity of the laboratory air rather than the product. That is why:
- The container is left until it reaches room temperature before it is opened; when a cold container is opened, moisture condenses on its surface.
- Weighing and transfer are done in a glove box filled with dry nitrogen where possible, or very quickly.
- The sample is transferred directly into the titration cell through a closed system.
- Several containers from the same batch are measured to see the variation between containers.
The effect of opening and closing the container on the moisture balance of the powder is also discussed in the article on the difference between powder and liquid forms.
How is the result interpreted?
Residual moisture is usually given as a percentage by mass. A single value is not “good” or “bad” on its own; it is assessed against the type of product, its intended use and earlier batches of the same product. Stating the method (volumetric or coulometric), the sample amount and the date of measurement alongside the moisture value in an analysis report makes the result comparable. The storage conditions of the sample before measurement also affect the result: if the container was not tightly closed, even the most accurate measurement will not show the moisture the product had when it was made. How reports are read in general is explained in the article on the certificate of analysis.
