How do Karl Fischer Titrators work?
The Karl Fischer method reacts water stoichiometrically with iodine, sulfur dioxide and an alcohol, usually methanol, to form hydrogen iodide and a sulfate ester. The amount of water is therefore proportional to the iodine consumed. In volumetric titration a reagent containing iodine is added from a burette until all water has reacted, and the endpoint is detected by a platinum electrode pair as a rise in conductivity or a drop in potential once free iodine appears. In coulometric titration iodine is generated electrochemically at the anode, and the charge required gives the water content directly by Faraday's law, which suits very low water levels. Anhydrous conditions, dry reagents and a sealed titration cell are essential, since atmospheric moisture is the main source of error.
Which Karl Fischer Titrators variants are available?
- Volumetric Karl Fischer Titrator — A volumetric Karl Fischer titrator adds iodine-containing reagent from a burette until all water has reacted, and suits water contents from about 0.1 to 100 percent.
- Coulometric Karl Fischer Titrator — A coulometric Karl Fischer titrator generates iodine electrochemically and determines very low water levels from the charge consumed.
- Volumetric-Coulometric Karl Fischer Titrator — A volumetric-coulometric titrator combines both titration modes in one instrument and switches between them as water content requires.
- Karl Fischer Titrator with Oven Sample Changer — A Karl Fischer titrator with an oven sample changer heats samples to release water and carries it into the titration cell with dry gas.
- Karl Fischer Moisture Analyzer for Solids and Liquids — A Karl Fischer moisture analyzer measures water in both solids and liquids with automated sampling and reagent handling.
How do I choose the right variant?
Choose the technique by expected water content. Volumetric titration suits water contents from about 0.1 to 100 percent and is faster for samples with appreciable water. Coulometric titration suits very low water levels, down to micrograms of water, and is preferred for lyophilised products and anhydrous solvents. Combined instruments switch between modes. For solids that do not dissolve or react with the reagent, an oven sample changer or a headspace method releases water by heating and carries it into the cell with a dry gas, which avoids side reactions and contamination. Compare measuring range, resolution, endpoint detection, titration cell sealing, sample changer capacity and software with audit trails.
Where are Karl Fischer Titrators used?
Karl Fischer Titrators are specified in water content of drug substances, excipient moisture testing, lyophilised product water content, residual moisture in solvents, water content of tablets and capsules and moisture specification release testing.
Which standards and certifications apply?
Work with Karl Fischer Titrators is commonly governed by USP <921>, Ph. Eur. 2.5.12, JP 2.48, ICH Q2(R2), IEC 61010-1 and ISO 17025. Instrument certifications such as CE marking are confirmed per configuration and never assumed.
What are the key considerations before purchase?
Work with dry glassware and fresh reagents, and keep the titration cell sealed when not in use, because the drift rate caused by atmospheric moisture is the main indicator of system condition. Perform a drift determination before starting, and replace solvent and reagent when drift rises. Calibrate the titrant against a certified water standard such as sodium tartrate or a water-in-methanol standard, and verify with an independent standard at defined intervals. Check sealing, electrodes and drying tubes, and record drift, standardisation and results. For oven methods, verify the temperature programme and the transfer line, since incomplete release or condensation causes low and variable results.
