What is Coulometric Karl Fischer Titration?
Coulometric Karl Fischer titration – the Karl Fischer method – refers to the quantitative determination of water by titration, i.e. a method by which the water contents of liquids, solids and gases can be determined. The method was developed in 1935 by the German chemist Karl Fischer at Lazăr Edeleanu GmbH.
Theoretical background of the Karl Fischer method
In Karl Fischer coulometric titration, a solution is added to a galvanically active sample and then the volume of the resulting gas is measured. The result is then compared with the standard value for the substance in question to determine the content of water. Coulometric Karl Fischer titration offers many advantages over conventional titration methods, especially when measuring small water contents in solids.
The method consists of titrating water with an anhydrous methanolic solution containing iodine, sulfur dioxide and excess pyridine as buffer solution. The maximum reaction rate occurs between pH 5.5 and 8. Accordingly, one uses basic components such as imidazole for acidic samples and acidic components such as salicylic acid for basic samples. The amount of water in the sample also influences the molar ratio. However, this only occurs from about 1 mol/l of solvent.
Practical application of the Karl Fischer method
The Karl Fischer method is based on an electrochemical procedure in which an electrical voltage is first generated between two electrodes in the sample container. This releases ions from the reagent, whereupon a redox reaction takes place in which oxygen molecules are replaced by hydrogen. The resulting electrical signal can be measured to determine the water content of the sample.
Sulfur dioxide and iodine react with each other only in the presence of water:
2 H2O + SO2 + I2 → SO42− + 2 I− + 4 H+ (in the absence of alcohols).
If methanol is present in the solution, it forms an acidic ester with sulfur dioxide, which is neutralized by the base (e.g., imidazole, hereafter referred to as “RN”):
CH3OH + SO2 + RN → (RNH)·(CH3SO3)
In the titration using iodine in methanol as the measuring solution, the methyl sulfite anion is oxidized to methyl sulfate by the iodine in the presence of water. The yellow-brown iodine is thereby reduced to the colorless iodide: (RNH)·(CH3SO3) + I2 + H2O + 2 RN → (RNH)·(CH3SO4) + 2 (RNH)·I
When no more water is present, the added iodine is no longer reduced. The brown coloration that occurs as a result is used for visual endpoint indication. In practice, electrometric indications (especially biamperometry) are preferred because they are more sensitive and accurate.
Application of Karl Fischer titration in process engineering
Coulometric Karl Fischer titration is a very accurate method for determining the water content in various materials. This technique can also be used to distinguish between similar materials. Since it is very precise and can be performed quickly, it is also frequently used in industrial process engineering.
It can be used successfully especially for quality control of chemical and pharmaceutical products. An example of this is the quality testing of fuels and solvents, where it is very important that they have a certain water content. In addition, the method is frequently used in the food sector to check the moisture content of products.
Coulometric Karl Fischer titration is also used in other areas: for example, it can be used to check the degree of corrosion of metal surfaces or to determine the water content in construction products or building materials.
Quality control and process management
Coulometric Karl Fischer titration plays a crucial role in quality control and process management in many industrial companies. By accurately measuring water content, unwanted components can be detected and eliminated in time, ensuring product quality while saving costs. In addition, it is possible to use the process to control factors such as temperature and pressure, as well as to avoid or minimize undesirable contamination effects.
Ebbecke Verfahrenstechnik also uses Coulometric Karl Fischer Titration as a useful tool for quality assurance and process optimization. Feel free to contact us for more information!
https://de.wikipedia.org/wiki/Karl-Fischer-Verfahren, retrieved: 21 January 2023, 11:5o UTC.
1. Karl-Fischer-Titration: Methoden zur Wasserbestimmung / Eugen Scholz / 2013
2. Analytiker-Taschenbuch, Band 15 / Ingo Lüderwald / 2013
Coulometric Karl-Fischer-Titration (source: Wikipedia)