The titrimetric determination of water by the Karl Fischer method depends on the reaction that takes place quantitatively between water and a reagent consisting of sulfur dioxide and iodine in anhydrous pyridine and usually methanol. The reaction is carried out in a suitable solvent such as methanol or acetic acid.
Water Determination Reference USP General Chapter <921>
Simple Reaction: Water against KF Reagent (Karl Fischer reagent)
ROH The solvent is generally methanol.
Methanol is the common solvent used as media.
- When analyzing Aldehydes and ketones, do not use methanol as a medium. these compounds react with methanol to form additional water.
- The number of iodines is equivalent to the number of water molecules in the reaction of iodine consumption.
- KF degrades itself with atmospheric air and moisture since the oxidation happens to sulfur dioxide. so that the standardization of KF should be done frequently (Daily once).
- Each ml of KF can neutralize (here react to consume) 5-6 mg of water. This will be exactly known by standardization of KF with DST (Disodium tartrate dihydrate) or Water.
- Commercially KF reagents are available in two types with respect to concentration.
- 2 mg/ ml
- 5 mg/ml
Calibration of KF Apparatus as per USP<921>
- Water or DST can be used for KF standardization,
- While testing, the KF Consumption should be at least 30% of burette volume, this condition is for the purpose of accurate results,
- The precision & accuracy of the water standard or DST can be verified
Standardization of KF Reagent :
Weigh the DST( previously dried for 3 hours at 105 degrees) about 0.2 g and titrate with KF.
Formula:
This 0.1566 comes from the ratio of the molecular weight of water divided by DST molecular weight.
👉This standardization can be done with water (Take pure water only)
Formula :
Here 0.1566 is not involved, since we are standardizing with water. so water molecular wt divided by water molecular wt. gives 1.
Note :
Take the sample to consume at least 10-30% of the burette volume so as to get accurate and reproducible results.
Second advanced method for lower levels of Water determination in the range of 5 ppm to 5.0%.
- This method is called water content by Coulometry.
- Here there is no Burette. The KF Reagent itself acts as a generator of iodine in the vessel containing two platinum electrodes.
- The reagent (KF for Coulometry) contains iodine in the form of an Iodine Ion.
- This iodine Ion oxidizes to get free iodine which reacts with water.
- No need to change the reagent for each sample.
ALSO READ: Use of Methanol in KF Titration
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