Maybe you have experienced one of the following situations in the laboratory. You need to determine the water content of a sample using Karl Fischer titration and you realize one or more of these issues:
- The sample does not dissolve in the KF reagent. No solubilizer helps, the sample is still not dissolving, and the results are far from reproducible.
- The sample reacts with the KF reagent. The titration does not stop, and there is no endpoint detected.
- The sample contaminates the titration cell and the electrode(s). Even if you replace the reagent after every measurement, the obtained results are out of specification.
There is a way to solve the above-mentioned problems. Trust me—it’s fantastic!!
The solution is the oven method or gas extraction technique.
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What is the oven method?
The oven method is a sample preparation technique used in Karl Fischer titration to analyze samples…
- that do not dissolve in KF reagents
- that do release their water, but only slowly
- that only release their water at higher temperatures
- that undergo side reactions with the KF reagent
- that contaminate the titration cell
The principle is very simple.
The sample is weighed into a headspace vial and closed with a septum cap. When placed in an oven, the water evaporates and a carrier gas (usually air or nitrogen) dried with a molecular sieve transports the released water into the titration cell, where the determination of the water content takes place. The water is separated from the sample matrix, avoiding side reactions and contamination.
The temperature of the oven is chosen according to the temperature stability of the sample. This leads to the question to which temperature the sample should be heated. What is the optimal oven temperature?
Finding the optimal oven temperature
Using a suitable oven temperature to analyze a sample is crucial to obtain the correct results. The oven temperature should be as high as possible, within reason. This guarantees a fast and complete release of the water and subsequently, short titration times. However, you should avoid choosing a temperature that is too high. Decomposition of the sample usually leads to the formation of unwanted substances that can falsify the water content. Therefore, as a rule of thumb, I recommend choosing an oven temperature 20 °C below the decomposition temperature of the sample.
But what can you do if you have no idea at which temperature your sample should be analyzed? No worries! There are several ways to find the optimal oven temperature.
One possibility is to search in the literature. The more information on temperature stability of the sample you find, the better off you will be. If you are able to find a decomposition temperature, it will help immensely to define the optimal oven temperature. Maybe you are lucky and someone else has already analyzed the same sample; then you may also find a recommended oven temperature. A good start is reading our free Application Bulletin AB-280, which lists several substances.
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If literature research does not reveal a suitable oven temperature, you must determine it yourself. How this is done depends upon the type of instrument you are using.
Some instruments offer you the possibility to run a so-called temperature gradient or temperature ramp. The sample is heated at a constant rate (e.g., 0.5 °C or 2 °C per minute) in a defined temperature range (e.g., 50 to 250 °C). At the same time, the released water is determined. In the end, the software will display a curve, showing you the released water as a function of the temperature. The following graph shows an example of such a temperature gradient curve.
The blue line corresponds to the determined water content, whereas the orange line indicates the drift value. An increasing drift signals the release of water, but it can also be a sign for decomposition, especially if the drift no longer decreases to a low level. In this graph, the drift peak at 50 °C corresponds to the blank value and free water. Between 120 and 200 °C, the drift value increases again, meaning the sample releases water. Then the drift decreases and remains low and stable up to 250 °C. There are no signs of decomposition up to 250 °C. As we do not know what would happen at temperatures above 250 °C, the optimal oven temperature for this sample is 230 °C (250 °C – 20 °C = 230 °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 (released water against temperature). If there is a temperature range where you see reproducible water contents, then you have found the optimal oven temperature.
Here is an example of a sample which started to decompose at temperatures above 106 °C (left sample vial) and thus is turning brown. An optimal temperature would therefore be 85 °C.
Sample analysis with a KF oven – step by step
After you have found the optimal oven temperature, water content determination in the sample can begin.
- First, I recommend to run a system preparation. This means running a determination, but with an empty sample vial. During this preparation step, all tubes in the system are purged with dried carrier gas, and any traces of water are removed.
- Next, you need to determine the blank value. The sample vials and the caps contain some residual moisture. With the blank determination, the amount of water contained in an empty sample vial is determined. The mean value of e.g. 3 blank value determinations is then subtracted from the water content obtained for the samples.
- Finally, you can analyze the samples.
Please keep in mind that the same parameters for the system preparation, the blank value determination, and the sample determination must be used. This is of importance if you want to measure a check standard before and/or after the sample analysis or sample series. If the optimal oven temperature for the standard is different from the one for the sample, I recommend that you determine a blank value for the standard as well.
Checking an oven system
There are special, solid water standards available to check the performance of an oven system. These water standards are perfect to inspect the complete oven system and to ensure that the evaporated water reaches the titration cell and is determined there. Such standards include a certificate stating the water content.
Using the certified value, you can calculate the recovery when determining the water content of the standard with the oven. If the recovery value is between 97–103%, everything is fine. However, if the recovery is outside this range, the oven system should be checked for leaks or water deposits. It might be that only the molecular sieve needs to be exchanged. Possibly, the reagent is exhausted and needs to be replaced.
There are other reasons which explain recovery values which are too high or too low. The reason must be found, as incorrect recovery values also mean that the determined sample water content is wrong. Have a look at our free Application Bulletin 280 for detailed information on troubleshooting an oven system.
The oven method is a simple and convenient way to analyze difficult samples. Side reactions are reduced to a minimum. The titration cell and the reagent are not contaminated with sample. In case you have to analyze a large series of samples, automation of the oven method is possible. Have a look at the available instruments for the oven method on our website!
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Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.