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 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.

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 standardization, 
2. While testing, the KF Consumption should be at least 30% of burette volume, this condition is for the purpose of accurate results, 
3. 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.       

ALSO READ: SOP for Operation of Karl Fisher Titration Instrument

👉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