Metrohm explains Karl Fischer Oven Method

Substances that release their water slowly or at high temperatures (for example, plastics or inorganic salts) are not suitable for a direct Karl Fischer titration.

Other samples have a low solubility in alcohols, which may involve toxic solvents to promote dissolution and/or involve extensive sample preparation.

Introducing these samples directly into the titration cell may contaminate it, and thus require exchanging the titration solution and frequent cleaning of the cell.

There are still other substances that undergo side reactions with the KF reagents (for example, ascorbic acid), which lead to false results.

All of these problems can be avoided by using the KF Oven Method.

First, the sample is heated in an oven and a carrier gas transfers the released water to the titration cell, where it is then determined by Karl Fischer titration.

Since only the water enters the KF cell and the sample itself does not come into contact with the KF reagent, unwanted side reactions and matrix effects are eliminated.

Metrohm ovens have evolved from the conventional tube oven.

However, instead of introducing the sample into the oven by means of a sample boat, a vial technique is used.

The sample is weighed directly into a small sample vial, sealed with a septum cap and transferred to the oven.

This brings several advantages: strictly reproducible conditions yield results with improved precision; manual sample preparation is minimised; no contamination of the oven or titration cell by the sample; and reagent consumption is diminished since the solvent is exchanged less frequently.

Two Metrohm ovens are available: the 860 KF Thermoprep for manual, individual samples; and the 774 KF Oven Sample Processor for automation of multiple samples.

Both are compact instruments that feature an integrated heating block, gas pump, flow meter and drying flasks.

A free monograph on Karl Fischer Analysis is available from Metrohm UK.