Karl Fischer titration for water determination is probably the best known and most widely used titration method. The reaction mechanism is well explored and there is a broad offering of suitable reagents and instruments to be used. The method is described in numerous regulations and guidelines with thousands of available applications. As part of method development, the type of sample should be considered. Specifically, how to best release water from it so as to make it completely accessible for titration.
The Karl Fischer method determines only water and no other compounds because of the selective and stoichiometric reaction between the water in the sample and the iodine in the reagent. There are multiple different ways to release the water from the samples and determine the correct water content. The following preconditions and questions are important to consider before getting started:
One of the first things to consider is whether the sample can be dissolved fast and completely in the Karl Fischer solvent without any additional treatments or if addition of co-solvents (solubilizers) or any sample preparation is needed to extract the water from the sample.
To measure the total water content of a solid sample, either it must be totally dissolved, or the water completely extracted.
If only surface water should be determined it must be ensured that the sample is not getting dissolved, which usually can be achieved by adding chloroform and by a fast procedure.
If the sample is soluble in the Karl Fischer solvent, it can be added directly to the titration cell, without requiring any additional sample preparation. Some samples might need additional preparations to dissolve fast and completely in the titration cell, such as, heating, homogenizing, extended stirring or addition of co-solvents. For samples that do not dissolve in the Karl Fischer solvent or cause side reactions, sample preparation is of particular importance. In coulometry, the direct addition of a solid sample to the titration cell is not recommended. This would require opening of the coulometric titration cell, and depending on the ambient humidity and handling, up to 10 μg of water can enter alongside the sample. In coulometry samples with low water content (e.g. in the range of 100 ppm) are measured, which can then result in an error e.g of up to 10% in water content for a 1 g sample. With liquid samples, a septum is used to add the sample to the titration cell and prevent the simultaneous entry of ambient moisture. For a volumetric titration, a higher amount of sample is typically added, and therefore the relative error due to addition of ambient moisture while adding solid samples directly, is very low and can be neglected. Many liquid and solid samples do not release their water completely and rapidly in the Karl Fischer solvent. They either do not dissolve or dissolve very slowly, and are often of non-homogeneous nature. This can result in a continuous release of water during the titration of such samples, which leads to incorrect and non-reproducible results. Other samples can cause side reactions with the Karl Fischer reagents which leads as well to incorrect water content values.
After homogenizing a sample by grinding, mixing or dispersing, there are different options available to completely release water for titration:
Many samples do not dissolve in the Karl Fischer solvent, usually methanol. They need addition of a solubilizer to the methanol containing Karl Fischer solvent or there are specific Karl Fischer solvents available already containing co-solvents. Ethanol based solvents can only be used for samples that dissolve in ethanol, and in such cases, the addition of a co-solvent is not recommended.
To find the right solubilizer, first add the different possible solubilizers in separate Erlenmeyer flasks followed then by the sample. After mixing, check in which solubilizer the sample is completely dissolved and then use this solvent as a co-solvent in the Karl Fischer titration. A co-solvent overview is shown in Table 1.
It should be kept in mind, that the addition of co-solvents as mentioned above, influences the conductivity and other conditions of the system.
Add not more than 40-50 % of solubilizer to volumetric solvents, and not more than 20-30 % to the coulometric reagents to avoid indication problems. If endpoint indication or over-titration problems occur, reduce the amount of your co-solvent.
In direct titration, the sample dissolves in the titration vessel before the titration starts. For some samples that dissolve slowly in the solvent, an extended stirring time is necessary.
Internal extraction is performed in a specific heated cell at 50 °C for samples that are not soluble and release water slowly. Examples are the applications for coffee and starchy products such as wheat, flour or rice. The addition of a co-solvent like formamide can also increase the solubility. For some samples a combination of the mentioned sample preparations is needed.
Some samples need an external extraction or dissolution either because of the possible side reactions taking place, or their low water content or insolubility in the Karl Fischer solvent. With such samples, first determine the water content of the used solvent, then weigh the exact amount of the solvent in a closed flask, add the weighted sample and mix them well. Certain samples will have a slow water release, therefore they require an extented extraction or mixing time. Titration at higher temperature up to 50 °C can accelerate slow water release as well. These approaches can be combined and should be evaluated during method development to achieve reproducible results. After complete water release, add an exactly weighted aliquot into your Karl Fischer titration cell and start the titration. Once finished, subtract the water content of the solvent from your result.
See also our extensive range of technical articles and protocols for Karl Fischer titration.
As a solvent methanol can be used or one of the previously mentioned solubilizers (in Table 1) or a mixture, depending on the sample.
A more convenient method to be used for critical samples that are not soluble, cause side reactions, or are solid with low water content is the Karl Fischer oven in combination with a volumetric or coulometric titrator. However, the samples need to be thermally stable and should not decompose during heating. A temperature ramp should be done first with each sample to determine the exact heating temperature needed. The samples are then heated to this specific temperature, causing the water in them to evaporate and to be transported into the titration vessel by a constant flow of dry air or nitrogen.