What is the Theory of Karl Fischer measurements for water content? When analyzing for moisture content by using the Karl Fischer method, samples are titrated with Karl Fischer reagent and the end point is located with a platinum electrode. The Karl Fischer reagent is a mixture of sulfur dioxide, pyridine, and iodine in methanol. Depending on the sample type, a solvent such as methanol, methanol/chloroform or methanol/formamide (sample dependent) is added to the vessel.
Each mole of water causes the reduction of one mole (2 equivalents) of iodine. The endpoint is signaled by the appearance of excess iodine. The titration is performed in an airtight apparatus to exclude atmospheric moisture.
Volumetric KF titrators (Orion AF8 and Turbo II) require the addition of a standard of known concentrations to the vessel. The quantity of analyte in solution is determined from the amount of standard that the titration uses. The amount of standard consumed is chemically equivalent to the analyte at the end point of the titration. For the Karl Fisher reaction, the endpoint occurs at the presence of an excess of the pyridine/iodine complex. The endpoint is detected when the observed voltage drops to zero. The sample solution will change from colorless to yellow-brown. In this methodology it is necessary to calibrate the instrument with a known standard. Most commonly water is used as a standard and the instrument is calibrated by volume. However, Sodium Tartrate can also be used as a standard for calibrations by weight.
Coulometric KF titrators (Orion AF7 and AF7LC) will measure the amount of electricity needed to convert the analyte to a different oxidation state. The coulometric method uses an electrolytically generated titrant that reacts with the analyte. Here, the iodide necessary for the reaction is produced by the anodic oxidation state of iodine. During the coulometric titration a constant current is maintained. As the reagent reacts with the analyte, the electrode potential changes to maintain the constant current. Detection of the endpoint is similar to the volumetric titration. The endpoint is detected by measuring the current across the electrode. At the endpoint when there is an excess of iodine, the current in the electrode will drop. Using the magnitude of the current and the time necessary to reach the endpoint, the amount of electricity used is determined. This value is equivalent to the amount of analyte present.
The coulometric titrations are more sensitive then volumetric titrations and can determine smaller amounts of water. However, coulometric titrations are more sensitive to interference caused by atmospheric moisture or chemical side reactions caused by unsaturated hydrocarbons, esterification of acids, formation of acetals and ketals and methylation of amines. Because of the chemistry of the coulometric titrations, it is not necessary to calibrate the reagent.
What is the advantage of an Orion Turbo 2 Karl Fischer analyzer over an Orion AF8 Karl Fischer analyzer? The Orion Turbo 2 has a moisture measurement range of 50 ppm to 100% moisture. The system comes with a powerful blender to break up and blend solid samples such as food products or dry pharmaceuticals. The advantage of this unique feature is that it assures complete moisture extraction from the sample into solution with little or no sample preparation. The titrator has a fully automated fill and drain pumping system to minimize reagent handling. The Turbo II has a large 720mL vessel that allows for multiple sample analysis and easy vessel access and cleaning. Additionally, we offer a Water Jacketed Vessel for use with a circulating bath that will help warm solvent to enhance moisture extraction in difficult samples.
The Orion AF8 offers measurement range of 50 ppm to 100% moisture. It offers an automatic fill and drain function that keeps reagent handling to a minimum. The system has a unique glass to glass vessel where the sealing design always has a moisture tight seal without the use of gaskets that can be attacked by organic solvents.
What is the difference between the Orion AF7 and AF7LC? The Orion AF7 and Orion AF7LC both function on the coulometric KF methodology. The Orion AF7 features both a high and low current mode for sample analysis. In the high current mode it can titrate 20,000/microgram (20mg) of water per minute to give fast and accurate titrations. For low moistures, you have the flexibility to change to the low current mode The Orion AF7 also offers a completely sealed chemical handling system.
The Orion AF7LC is the Low-Level Coulometric Titrator and is designed to run in a low current mode for low moisture measurements in the 10 ppm range. Samples with moisture contents as low as 1 ppm can be analyzed with low coefficient of variations (CV). The system automatically monitors the anolyte and catholyte life and will provide indication when it is time to change the reagents.
When would the Orion AF7LC be beneficial? For those customers whose need optimal performance for low level moistures (10ppm), the Orion AF7LC would be beneficial. This instrument is optimized to provide performance at low level moistures as it always runs in the low current mode. For those customers who run a variety of moisture levels and are looking for a coulometric titrator, the Orion AF7 would be beneficial.
How do I develop an application for my product on my Karl Fischer Titrator? The Thermo Scientific Titration Applications Laboratory has developed full range of Karl Fisher Application methods that are available to you upon purchasing and receiving your new Karl Fischer Titrator. These methods are designed for your specific model and will provide suggested reagents, sample size, and analysis details for your specific application.
For assistance in choosing the best Orion Karl Fischer Titrator for your application, contact your local Thermo Scientific Titration sales representative or call Technical Service at 800-225-1480.
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