Water determination by Karl Fischer Reagent (KF)

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 analysing Aldehydes and ketones, do not use methanol as a media. these compounds reacts with methanol to form additional water.
• Number of iodines is equivalent to number water molecules in the reaction of iodine consumption.
• KF degrades itself with atmospheric air and moisture, since the oxidation happening to sulfur dioxide. so that the standardisation of KF should be done frequently (Daily once).
• Each ml of KF can neutralise (here react to consume) 5-6 mg of water. This will be exactly known by standardisation of KF with DST (Disodium tartrate dihydrate) or Water.
• Commercially KF reagents available in two types with respect to concentration.

1. 2 mg/ ml
2. 5 mg/ml

Calibration of KF Apparatus as per USP<921>

1. Water or DST can be used for KF standardisation, 
2. While testing, the KF Consumption should be at least 30% of burette volume, this condition is for the purpose of accurate result, 
3. Precision & accuracy of the water standard or DST can be verified

Standardisation 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 ratio of molecular weight of water divided by DST molecular weight.       

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👉This standardisation 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 it self act as a generator of iodine in the vessel containing two platinum electrodes.
• The reagent (KF for Coulometry) contains iodine in the form of Iodine Ion.
• This iodine Ion oxidise to get free iodine which reacts with water.
• No need to change the reagent for each sample.

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