Since I started working at Metrohm more than 15 years ago, I have received many questions about Karl Fischer titration. Some of those questions have been asked repeatedly from several people in different locations around the world. Therefore, I have chosen 20 of the most frequent questions received over the years concerning Karl Fischer equipment and arranged them into three categories: instrument preparation and handling, titration troubleshooting, and the oven technique. Part 1 covered instrument preparation and handling, and Part 2 will now focus on titration troubleshooting and the KF oven technique.
Summary of questions in the FAQ (click to go directly to each question):
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1. In case the drift value is 0, does this mean that the titration cell is over-titrated?
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2. Should I discard the Karl Fischer reagent immediately if it turns brown?
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3. What is drift correction, and when should I use it?
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4. My results are negative. What does a negative water content mean?
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5. My samples are not soluble. What can I do?
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6. Can all types of samples be analyzed with the oven method?
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7. How do I find the optimal oven temperature for water extraction?
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8. What is the highest possible water content that can be measured with a Karl Fischer oven?
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9. What is the maximum sample size that can be used with the oven? If I use too much sample, will the needle be blocked?
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10. What is the detection limit of the oven method, and how much sample is required to analyze a sample with 10 ppm (mg/L) water content?
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11. How do I verify an oven method?
Titration troubleshooting
1. If the drift value is 0, does this mean that the titration cell is over-titrated?
A drift of zero can be a sign that the cell might be over-titrated. In combination with the mV signal (lower than end-point criteria) and the color of the working medium (darker yellow than usual), it is a clear indicator for over-titration. However, volumetric titrations sometimes exhibit a zero drift for a short time without being over-titrated. If you have a real excess of iodine in the titration cell, the result of the next determination will most likely be erroneous. Therefore, over-titration should be avoided. There are various possible reasons for over-titration, like the sample itself (e.g., oxidizing agents which generate iodine from the working medium), the electrode (coating or invisible depositions on the Pt pins/rings), the reagent, and method parameters (e.g., the titration is rate too high), to name just a few.2. Should I discard the Karl Fischer reagent immediately if it turns brown?
Different factors can cause over-titration, however, the reagent is not always the reason behind this issue. The indicator electrode can also be the reason for overshooting the endpoint. In this case, regular cleaning of the electrode can prevent over-titration (see also questions 7 to 9 from Part 1 in this series on cleaning).
A low stirring speed also increases the risk of over-titration, so make sure the solution is well mixed. Depending on the type of reagent, the parameters of the titration need to be adjusted. Especially if you use two-component reagents, I recommend decreasing the speed of the titrant addition to avoid over-titration. Over-titration has an influence on the result, especially if the degree of over-titration changes from one determination to the next. So over-titration should always be avoided to guarantee correct results.
3. What is drift correction, and when should I use it?
I recommend using the drift correction in coulometric KF titration only. You can also use it in volumetric titration, but here the drift level is normally not as stable as for coulometric titrations. This can result in variations in the results. A stabilization time can reduce such an effect. However, compared to the absolute water amounts in volumetry, the influence of drift is usually negligible.
4. My results are negative. What does a negative water content mean?
Negative values do occur if you have a high start drift and a sample with a very low water content. In this case, the value for drift correction can be higher than the absolute water content of the sample, resulting in a negative water content.
If possible, use a larger sample size to increase the amount of water added to the titration cell with the sample. Furthermore, you should try to reduce the drift value in general. Perhaps the molecular sieve or the septum need to be replaced. You can also use a stabilizing time to make sure the drift is stable before analyzing the sample.
Karl Fischer oven
5. My samples are not soluble. What can I do?
In case the sample does not dissolve in KF reagents and additional solvents do not increase the solubility of the sample, then gas extraction or the oven technique could be the perfect solution.
The sample is weighed in a headspace vial and closed with a septum cap. Then the vial is placed in the oven and heated to a predefined temperature, leading the sample to release its water. At the same time, a double hollow needle pierces through the septum. A dry carrier gas, usually nitrogen or dried air, flows into the sample vial. Taking the water of the sample with it, the carrier gas flows into the titration cell where the water content determination takes place.
6. Can all types of samples be analyzed with the oven method?
Many samples can be analyzed with the oven. Whether an application actually works for a sample strongly depends on the sample itself. Of course, there are samples that are not suitable for the oven method, e.g., samples that decompose before releasing the water or that release their water at higher temperatures than the maximum oven temperature.
7. How do I find the optimal oven temperature for water extraction?
Depending on the instrument used, you can run a temperature gradient of 2 °C/min. This means it is possible to heat a sample from 50 to 250 °C within 100 minutes. The software will then display a curve of water release against temperature (see graph).
From such a curve, the optimal temperature can be determined. Different peaks may show blank, adherent water, different kinds of bound water, or even decomposition of the sample.
This example curve shows the water release of a sample as it has been heated between 130 and 200 °C. At higher temperatures, the drift decreases to a stable and low level.
Generally, you should choose a temperature after the last water release peak (where the drift returns to the base level) but approximately 20 °C below decomposition temperature. Decomposition can be recognized by increasing drift, smoke, or a color change of the sample. In this example, there are no signs of decomposition up to an oven temperature of 250 °C. Therefore, the optimal oven temperature for this sample is 230 °C (250 °C – 20 °C).
In case the instrument you use does not offer the option to run a temperature gradient, you can manually increase the temperature and measure the sample at different temperatures. In an Excel spreadsheet, you can display the curve plotting released water against temperature. If there is a plateau (i.e., a temperature range where you find reproducible water contents), you have found the optimal oven temperature.
8. What is the highest possible water content that can be measured with a Karl Fischer oven?
Very often, the oven is used in combination with a coulometric titrator. The coulometric titration cell used in an oven system is filled with 150 mL of reagent. Theoretically, this amount of reagent allows for the determination of 1500 mg of water. However, this amount is too high to be determined in one titration and it would lead to very long titration times and negative effects on the results. We recommend that the water content of a single sample (in a vial) should not be higher than 10 mg, ideally around 1000–2000 µg water. For samples with water contents in the higher percentage range, you should consider the combination with a volumetric titrator.9. What is the maximum sample size that can be used with the oven? If I use too much sample, will the needle be blocked?
The standard vial for the oven method has a volume of approximately 9 mL. However, we do not recommend filling the vial completely. Do not fill more than 5–6 mL of sample in a vial. We offer the possibility to customize our oven systems, allowing you to use your own vials. Please contact your local Metrohm agency for more information on customized oven systems.
For liquid samples, we recommend using a long needle to lead the gas through the sample. Solid samples and especially samples that melt during analysis require a short needle. The tip of the needle is positioned above the sample material to avoid needle blockage.
Additionally, you should use a «relative blank value», i.e., taking only the remaining air volume into account for blank subtraction. You can find more information about the relative blank and how to calculate it in Application Note AN-K-048.
10. What is the detection limit of the oven method, and how much sample is required to analyze a sample with 10 ppm (mg/L) water content?
We recommend having at least 50 µg of water in the sample, if analyzed with coulometry. However, if conditions are absolutely perfect (i.e., very low and stable drift plus perfect blank determination), it is possible to determine even lower water contents, down to 20 µg of absolute water. For a sample with a water content of < 10 ppm (mg/L), this would correspond to a sample size of at least 2 g.
11. How do I verify an oven method?
For the verification of an oven system, you can use a certified water standard for oven systems. With such a standard, you can check the reproducibility and the recovery. There are a few types of standards available for different temperature ranges.
I hope this collected information helps you to answer some of your most burning KF questions. If you have further unanswered questions, do not hesitate to contact your local Metrohm distributor or check out our selection of webinars.
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Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.
Greetings..
Please share the methodology for determination of Limit of Quantification LOQ for water content determination using volumetric KF Titrator
Is the limit of Quantification of the instrument is same as that of sample method
How to calculate limit of Quantification LOQ when using an KF Titrator with 5 mL burette and 2mg/mL of reagent and which liquid water standard is suggested to use for the study.
Please share an Application note on these concepts, if any
Many thanks in advance
What is the determination error in kf?
How it will happen?
What is the preventive action for determination error?
Hello,
For more information, please consider reading through the KF FAQ on our website.
https://www.metrohm.com/en/support-and-service/kf-faq/
Best regards,
Alyson
Hi Alyson,,
Thanks for sharing the detailed information through FAQ
Would request you to please guide on the recommendation on Blank repeatability interms of RSD (n=3) for KF Coulometer coupled with Over processor.
Thanks in advance
Good day Kumar,
There is no official recommendation.
The author of this article has said that the RSD of the blank value should not be higher than 3%, but of course it depends on the blank itself.
For a blank of 20 or 30 µg, a low RSD is more difficult to achieve than for a blank that is 150 µg or even higher.
For more information, I urge you to contact your local Metrohm dealer to discuss more details.
Best regards,
Alyson
HI Alyson,
Thanks for the FAQ on KF
Request you to suggest Metrohm recommendation on acceptance criteria for Blank determination (n=3) using KF Oven Processor with Coulometer using a 6 mL vial.
Thanks in advance.
Good day Kishore,
There is no official recommendation.
The author of this article has said that the RSD of the blank value should not be higher than 3%, but of course it depends on the blank itself.
For a blank of 20 or 30 µg, a low RSD is more difficult to achieve than for a blank that is 150 µg or even higher.
For more information, I urge you to contact your local Metrohm dealer to discuss more details.
Best regards,
Alyson
Thanks Alyson for your swift response. We are noticing blank result about 100 µg (using 6mL empty vials). Sometimes we are encountering %RSD about 25 and sometimes about 10.
Surprisingly, sometimes, one of the three vials show a comparatively low value (can be first/second/third). Couldn’t find certain reason for such random behavior.
This is one of the reasons, i enquired about any specific tips in handling and storage of vials and seals of vials using crimping tongs.
Thanks
What is allowable variation in precision study of Moisture content by KFv
Hello,
This question does not have a simple answer. We recommend that you refer to the “EDQM Annex 5: Qualification of automatic titrators”:
https://www.edqm.eu/en/d/313385
Best regards,
Alyson
Is there a guide as to what the various method parameters in the 852 Titrando system mean and how to use them to set up a method appropriately?
For example: What does the “Start conditions” pause time mean versus the “Titration parameters” extraction time?
Hello,
Here is the user manual for the 852 Titrando:
https://www.metrohm.com/en/products/8/8528/88528003.html
If you cannot find the information you are looking for, please contact your local Metrohm representative for guidance about your system.
Best regards,
Alyson
Dear Alyson,
What are the current recommendations for KF volumetric titration with extraction time ? Is it better to use “automatic drift correction” or determine a blank?
Hello Wiola,
Thank you for your questions. We will contact you because we need additional information to answer them sufficiently.
Best regards,
Alyson
Is start volume parameter or pre volume addition before the end point is acceptable in potentiometric titration. (e.g EP is 14.5ml and my start volume is 12ml.
Hello,
Though a start volume is never or rarely used in KF titration, it is theoretically possible to use a start volume in this case.
Best regards,
Alyson
Dear Alyson,
Is there a guide, in terms of percentage, of the maximum volume to be added on a KF oven vial?
We are performing a analytical method validation. For accuracy evaluation, we added 3 mL of Water Standard 1.0 into a 5 mL vial already containing 50 mg of sample. As a standard procedure, we use needle for solid samples (6.2049.050). the results showed a low %recovery against the theoretical value. We think that the root cause is the addition of a large volume into a 5 mL vial and we would like to define a maximum volume of standard to add.
Best regards
Catarina Castanheira
Hello Catarina,
No, unfortunately there is no guide. The amount of sample not only depends on the percentage of water but also on the technique used to determine the water.
In case you use coulometric KF titration, then the absolute amount of water in a sample vial should be smaller compared to when you use volumetric KF titration.
The optimal amount of water for coulometric KF titration in combination with a KF oven is approximately 300 to 5000 µg.
For the volumetric technique, the absolute amount depends on the titer of the titrant you are using. We recommend that 10 to 90% of the buret volume is used for a titration.
Regarding the spiking of sample with a standard, I would rather use a solid oven standard than a liquid standard.
Best regards,
Alyson
With regards to Titre Value, say for example using Combi Titrant 5, the expected Titre is meant to be 5.000mg/mL. This factor can then be used in the calculation to determine the percent moisture.
However, it the Combi Titrant bottle is giving a value of say 4.723mg/mL or 5.284mg/mL… is it bad to use the bottle? Or, mathematically, will the correction for the Titre satisfy the final result?
And, does the Combi Titrant need to be inverted/ mixed regularly to ensure there is not stratification of Iodine/ Water contamination in the bottle?
Hello Dan,
The titer of KF titrants is not stable and will decrease over time. Usually, a range (e.g., 4.5 to 5.5 mg/mL for a Titrant 5) is indicated on the titrant bottle, and not a specific titer value.
A new bottle of Combi Titrant 5 has a titer higher than 5 to ensure that after some time the titer is still at least 5 when you open the bottle for the first time.
As long as you carry out titer determinations regularly, titer values higher or lower than 5 are fine. Titer determinations are crucial, because as you correctly say, the titer has a significant influence on the calculation of the final result.
For the last point, we have never heard from a reagent manufacturer that a titrant bottle should be mixed from time to time.
Best regards,
Alyson