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Time:2013-06-22 10:41:56  Hits:7080Water Determination (Karl Fischer Method)----Theory (Ⅰ)

 

Water Determination (Karl Fischer Method)----Theory (Ⅰ)
 
The Water Determination Test (Karl Fischer Method) is designed to determine water content in substances, utilizing the quantitative reaction of water with iodine and sulfur dioxide in the presence of a lower alcohol such as methanol and an organic base such as pyridine, as shown in the following formulae:
 
I2+SO2+H2O+3C5H5N → 2C5H5NHI+C5H5NSO3
C5H5NSO3 + CH3OH → C5H5NHSO3OCH3
 
There are two determination methods different in iodine-providing principle: the volumetric titration method and the coulometric titration method. In the volumetric titration method, iodine required for reaction with water is previously dissolved in water determination TS, and water content is determined by measuring the amount of iodine consumed as a result of reaction with water in a sample.
 
In the coulometric titration method, first, iodine is produced by electrolysis of the reagent containing iodide ion, and then, the water content in a sample is determined by measuring the quantity of electricity which is required for the electrolysis (i.e., for the production of iodine), based on the quantitative reaction of the generated iodine with water.
 
Hereinafter in the Monographs, such a specificationnot more than 4.0% (0.5 g, back titration)indicates that when determined by weighing about 0.5 g of the sample accurately and performing back titration, the water content is not more than 4.0% of the weight of the sample.
 
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