Karl Fischer Titrator (Coulometric)
Measures very small amounts of water by generating iodine electrically inside the cell and counting the charge needed to react with all the water. Suited to trace water, from ppm levels up to about 1–5%.
- Pharmaceutical
- Biotech
- Chemicals
- Petrochemicals
- Electronics / batteries
What is KF titrator (coulometric)?
A coulometric Karl Fischer (KF) titrator uses the same chemical reaction as volumetric KF: one mole of iodine reacts with one mole of water. The difference is where the iodine comes from. Instead of adding it from a burette, the instrument makes it inside the cell by oxidising iodide at a generator electrode.
The amount of iodine produced is exactly proportional to the electric charge passed (Faraday's law). The instrument measures the charge and converts it directly to micrograms of water. No titre has to be determined, because the 'reagent' is the electron, which is an absolute standard.
Coulometric KF is the technique of choice for samples with low water content, such as lyophilised products, dry solvents, gases, lubricants and some APIs. Insoluble samples or those that interfere are usually heated in a KF oven so that only the released water vapour reaches the cell.
Used for
- Water in freeze-dried (lyophilised) products and dry powders
- Water in organic solvents used for synthesis and analysis
- Trace water in APIs and excipients with low limits
- Water in packaging materials and stoppers using a KF oven
- Residual moisture in medical devices and inhalation powders
- Water in oils, lubricants and transformer oils
How it works
- 01
Conditioning
The instrument generates iodine until the anolyte is dry, then monitors the drift.
- 02
Sample injection
A weighed liquid sample is injected through the septum, or water from an oven is carried in by dry gas.
- 03
Iodine generation
Current passes through the generator electrode, producing iodine that reacts with the water.
- 04
Endpoint detection
The indicator electrode senses the first stable excess of iodine and generation stops.
- 05
Calculation
Total charge, corrected for drift, is converted to µg of water and then to ppm or %.
Advantages
- Absolute method: no titre to determine, results traceable to the coulomb
- Very sensitive: can detect a few micrograms of water
- Small samples are enough, which helps with expensive materials
- Fast for low water contents
- Easy to combine with a KF oven for insoluble or reactive samples
Limitations
- Not suitable for high water contents; large amounts of water exhaust the reagent and take a long time
- Very sensitive to atmospheric moisture, so drift must be tightly controlled
- The same side reactions as volumetric KF apply (ketones, aldehydes, oxidising and reducing agents)
- Reagent capacity is limited; the anolyte and catholyte must be replaced regularly
- Viscous or insoluble samples need special handling or an oven
2 calibration procedures
Keep every KF titrator (coulometric) in control: usage logs, calibration and maintenance schedules, and automatic blocking of overdue instruments.