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You may have questioned whether to use volumetric or coulometric titration to analyze a sample. This blog article explains the most important factors to consider in order to choose the optimal Karl Fischer titration technique. To begin, the main difference between volumetry and coulometry is discussed: the «addition» of the iodine.
Volumetry
As the name suggests, volumes are crucial in volumetric KF titration. Why is that?
The water content of the titrated sample is calculated based on the total dosed volume of the titrant up to the endpoint. This requires accurate and reproducible dosing.
Metrohm titrators offer between 10,000 and 100,000 pulses per buret volume—more than enough to guarantee accurately dosed volumes.
Coulometry
In coulometry, instead of a buret and a titrant, an electric current is used to generate the iodine. To reach the endpoint of the titration, the current releases the required amount of iodine from the iodide-containing reagent. One molecule of water consumes one molecule of iodine. This allows the calculation of the water content using the product of the titration time and the current that is necessary to reach the endpoint.
Volumetry or coulometry? That is the question
Now that the main difference between the two techniques has been explained, the decision of whether to use a volumetric titrator or a coulometer must be made. As mentioned, there are various criteria to consider.
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Water content of the sample
Generally, volumetric KF titration is used to titrate samples with a higher water content in the range of 0.1% up to 100%. Coulometry on the other hand is used to determine water content between 0.001% to 1%.
Samples with a very high water content can be diluted with a suitable solvent. Take care that the used solvent does not react with the Karl Fischer reagents.
When diluting samples, the water content of the used solvent must be determined separately. The obtained blank value is subtracted from the result of the sample determination.
Sample consistency
The consistency of the sample should be considered when choosing between coulometric titration and volumetric titration. Is it liquid, solid, or is it a gas?
In general, the following can be said:
Volumetry
Suitable to determine water content from 0.1 to 100%.
- Titration vessel can be opened for short amounts of time
- Direct addition of pastes and solid samples is possible
- Atmospheric moisture can influence results
Coulometry
Suitable to determine water content from 0.001 to 1%.
- Never open the titration cell
- Sample must be added by using a syringe
- Solids or pastes must be dissolved before sample addition
The oven method is another option to analyze solids and pastes with KFC. Instead of dissolving those samples, this method can be used.
In general, there is no need to open the titration cell to add liquids or gaseous samples. Therefore, depending on the water content, both volumetry and coulometry are suitable for the water content determination in such samples.
The sample does not dissolve
Which technique should be chosen in case the sample does not dissolve in the reagent? Complete dissolution is essential for correct results. Usually, if the sample does not completely dissolve, this means that not all of the water contained in the sample is titrated.
Elevated temperatures can help improve the solubility of a sample. Using a titration cell with a thermostat jacket is a good option to help avoid solubility problems in volumetry and coulometry studies.
Another approach is homogenization. A homogenizer can be used to homogenize a sample directly in a volumetric titration cell. At the same time, it also mixes the contents of the titration cell during titration, eliminating the need for a magnetic stirring bar.
Homogenization is very convenient for volumetric Karl Fischer titration. Since a coulometric titration cell must be tightly sealed, mounting a homogenizer in the titration cell directly is not an option. Instead, homogenization is carried out in an external container, increasing the risk that the moisture content of the sample changes during its preparation.
Another possibility is the use of so-called solubilizers. Suitable solvents (e.g., chloroform, xylene, etc.) can be added to the reagents to help dissolve the sample.
These three options can also be combined with each other.
Finally, there is a technique that works without dissolution of the sample. This is the oven method, and it can be used with either a volumetric or a coulometric Karl Fischer titrator. Read our related blog post for more information about the Karl Fischer oven method.
The sample causes side reactions
Ketones and aldehydes undergo a side reaction with the alcohol (methanol in most cases) that is a component of almost all Karl Fischer reagents. The side reaction results in the formation of water and leads to incorrect results. Reagent manufacturers offer volumetric and coulometric KF reagents that suppress this side reaction.
There are other sample types that cause side reactions. Unfortunately, there are no special reagents available on the market to suppress every side reaction possibility. Therefore, there is no general rule or recommendation regarding which KF titration method is the optimal one in this situation. This decision strongly depends on the sample and on any side reactions caused by it.
Summary
To determine the amount of water in a sample and get accurate results, it is essential to choose the optimal Karl Fischer titration technique. For some samples it is obvious which technique is best, whereas for other samples the choice is more difficult. Considering the mentioned criteria will help the user choose between volumetry and coulometry.
Your knowledge take-aways
Monograph: Water determination by Karl Fischer Titration
Application Bulletin: Utilization of the Polytron PT 1300 D (Metrohm version)
Water content determination in ketones using Hydranal™ NEXTGEN FA reagents
In Karl Fischer (KF) titration, ketones can lead to inaccurate results due to a side reaction with the alcohol component in the reagents, resulting in increased water content. This effect is particularly pronounced in coulometric KF titration. The newer Hydranal™ NEXTGEN FA reagents from Honeywell eliminate this alcohol component. Methanesulfonic acid in these reagents serves as a stabilizer, suppressing the Bunsen reaction and maintaining a 1:1 stoichiometry between water and iodine. This innovation allows for the rapid and reliable determination of water content in ketones. Compared to other KF reagents for ketones, Honeywell's Hydranal™ NEXTGEN FA reagents demonstrate significantly better suppression of side reactions.
