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Many substances release their water only at high temperatures making them inaccesible for direct Karl Fischer titration. Examples of such substances are plastics and a number of inorganic salts.
The moisture in these substances can be driven out and collected in a tube furnace at 100 °C to 300 °C, depending on the sample, and then be transferred to the Karl Fischer titration cell by purging the tube with a dried inert gas. The titration cell is filled with HYDRANAL®-Coulomat AG-Oven, which is especially suitable for use with Karl Fischer ovens as it shows high drift stability. For verifying results and control of a Karl Fischer oven, we recommend the use of HYDRANAL®-Water Standard KF-Oven 140-160°C and HYDRANAL®-Water Standard KF-Oven 220-230°C.
This method of water determination can be applied to:
- Insoluble solids that only release their inherent water at temperatures above 60 °C
(e.g. plastics, salts)
- Solids and liquids that undergo side reactions with conventional Karl Fischer titration reagents (ascorbic acid, mineral oils), assuming that the matrix is not vaporized at these temperatures and that none of the substances disassociate or degrade to products which can interfere with the Karl Fischer reaction.
Suitable carrier gases are air or nitrogen, with nitrogen preferred when the sample is sensitive to oxidization at process temperatures of 100-300 °C.
We recommend the use of HYDRANAL®-Molecular Sieve 0.3 nm to dry the carrier gas. A residual water content of less than 10 µg water / liter is thereby assured. This amount does not present a potential problem in the subsequent water determination. Carrier gases containing high amounts of water should be pre-dried using silica gel.
Commercially available Karl Fischer ovens can be set to drying temperatures of 50-300 °C. Higher temperatures are necessary for drying inorganic salts. The temperature chosen depends on the properties of the substance being investigated.
The working conditions must be optimized for each product analyzed. Of particular importance is the determination of the optimum oven temperature to remove the water. The temperature must be high enough to drive off the moisture in the sample within 10-15 minutes. The temperature must be kept low enough to prevent vaporization of the sample matrix, which could interfere in the Karl Fischer titration, but again high enough to prevent condensation in the transfer tubing.
Product range HYDRANAL reagents for Karl Fischer oven
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