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Having served at Metrohm for over 15 years, I've encountered numerous queries about Karl Fischer titration. Many of these questions, stemming from professionals worldwide, revolve around instrument preparation, troubleshooting, and the oven technique. To assist you better, I've curated the 20 most recurring questions. This FAQ is divided into two parts:
- Part 1: Instrument Preparation and Handling
- Part 2: Titration Troubleshooting and Oven Technique
Summary of questions in the FAQ (click to go directly to each question):
- How can I check if the electrode is working correctly?
- How long can an electrode be stored in KF reagent?
- Can the molecular sieve be dried and reused, or should it be replaced?
- How long does conditioning normally take?
- When conditioning, many bubbles form in the coulometric titration cell with a very high drift, also when using fresh reagent. What could be the reason for this effect?
- What is the best frequency to clean the Karl Fischer equipment?
- How do I clean the Karl Fischer equipment?
- Is it also possible to use a cleaning agent like «CIF» or toothpaste to clean the double Pt electrode?
- How do I clean a generator electrode with a diaphragm?
Instrument preparation and handling
1. How can I check if the electrode is working correctly?
I recommend carrying out a volumetric or coulometric Karl Fischer titration using a certified water standard as your sample.
In volumetry, you can carry out a threefold titer determination, followed by a determination of a different standard. Then, you can calculate the recovery of the water content determination of the standard.
If you want to check a coulometric system, carry out a threefold determination with a certified water standard and calculate the recovery. If the recovery is between 97–103%, this indicates that the system and the electrode are working fine.
You can also check the colour of the working medium. Pale yellow is perfect, but dark yellow or even pale brown suggests indication problems. If this happens, then the indicator electrode needs to be cleaned. Check out questions 7 and 8 for tips on the cleaning of the indicator electrode.
2. How long can an electrode be stored in Karl Fischer reagent?
Karl Fischer electrodes are made from glass and platinum, and the reagent does not affect the electrode. Therefore you can store the electrode in the reagent for as long as you want.
3. Can the molecular sieve be dried and reused, or should it be replaced?
A molecular sieve can be dried and reused. I recommend drying it for over 24 hours at a temperature between 200–300 °C. Afterwards, let it cool down in a desiccator and then transfer it into a glass bottle with an airtight seal for storage.
4. How long does conditioning normally take?
Conditioning of a freshly filled titration vessel normally takes around 2–4 minutes for volumetry and around 15–30 minutes for coulometry.
If you're using an oven, coniditioning might take a bit longer to reach a stable drift due to the constant gas flow. I recommend stabilising the entire oven system for at least 1 hour before the first titration.
Between single measurements in the same working medium, conditioning takes about 1–2 minutes. Always make sure the original drift level is reached again.
5. When conditioning, many bubbles form in the coulometric titration cell with a very high drift, also when using fresh reagent. What could be the reason for this effect?
At the anode, the generator electrode produces iodine from the iodide-containing reagent. The bubbles you see at the cathode are the result of the reduction of H+ ions to hydrogen gas. After opening the titration cell or after filling it with fresh reagent, the conditioning step removes any moisture brought into the system, and removing the water results in an increased drift level. During conditioning, the aforementioned H2 is generated. The gas bubbles are completely normal and not a cause for concern.
Generally, the following rule applies: The more moisture present in the titration vessel, the higher the drift value will be, and the more hydrogen will form.
6. What is the best frequency to clean the Karl Fischer equipment?
There is no strict rule as to when you should clean the KF equipment. The cleaning intervals strongly depend on the type and the amount of sample added to the titration cell. Poor solubility and contamination of the indicator electrode or memory effects due to large amounts of sample are good indicators that its time to clean the equipment.
The drift can be a good indicator as well. If you observe higher and unstable drift values, I would recommend cleaning the titration cell or at least refilling the working medium.
7. How do I clean the Karl Fischer equipment?
For a mounted titration vessel, it can be as simple as rinsing with alcohol. For intense cleaning, the vessel should be removed from the titrator.
Water, solvents like methanol, or cleaning agents are all suitable for cleaning the KF equipment. Even concentrated nitric acid can be used as an oxidizing agent (in case of contaminated indicator electrodes or coulometric generator electrodes.)
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All of these options are fine, but keep in mind that the last cleaning step should always be rinsing with alcohol followed by proper drying in a drying oven or with a hair dryer at max. 50 °C to remove as much adherent water as possible.
You should never use ketones (e.g. acetone) to clean Karl Fischer equipment, as they react with methanol, release water, and might cause the drift to be too high to start any titration.
8. Is it also possible to use a cleaning agent like «CIF» or toothpaste to clean the double Pt electrode?
Normally, rinsing with alcoholic solvents and polishing with paper tissue should be enough to clean the indicator electrode. You may also use detergents, toothpaste, or the recommended polishing set offered by Metrohm. Just make sure that you rinse the electrode properly after the cleaning process to remove all traces of cleaning agent before using the electrode again.
Cleaning instructions can also be found below:
9. How do I clean a generator electrode with a diaphragm?
After removing the generator electrode from the titration vessel, dispose of the catholyte solution, then rinse the electrode with water.
Place the generator electrode upright (in an Erlenmeyer flask) and cover the connector with the protection cap to prevent corrosion.
Fill the generator electrode with some milliliters of concentrated nitric acid, and let the acid flow through the diaphragm.
Then fill the cathode compartment with water, and again allow the liquid to flow through the diaphragm.
Repeat the rinsing step with water several times to make sure that all traces of nitric acid are washed out of the diaphragm.
Please note :The nitric acid treatment is only needed if the level of contamination requires it.
Finally, pour some methanol into the generator electrode to remove the water. Repeat this step a few times to remove all traces of water.
The last step is properly drying the electrode in a drying oven or with a hair dryer at max. 50 °C.
After this cleaning procedure, the electrode is as good as new and can be used again for titrations.
Read part 2:
Frequently asked questions in Karl Fischer titration – Part 2
In Part 2, I cover the topics of KF titration troubleshooting and the Karl Fischer oven technique.
Karl Fischer Video - Metrohm Expertise
