Karl Fischer Titration Method - Theory and Applications

1. WHAT IS THE KARL FISCHER TITRATION METHOD?

The Karl Fischer titration method is an electrochemical titration method used to determine water content in liquid and solid samples. The Karl Fischer method is suitable both for samples with high moisture content (%) (titration-volumetric) and samples with low content (ppm) (coulometric).
Karl Fischer titration method is the reference method for determining water content and gives fast and accurate results within a few minutes.

2. MEASURING RANGE OF THE KARL FISCHER TITRATION METHOD

There are two Karl Fischer titration techniques:
Volumetric Karl Fischer can determine moisture from 100 ppm -100%
Coulometric Karl Fischer can measure moisture down to 1 ppm -5%

3. PRINCIPLE OF THE KARL FISCHER METHOD
The Karl Fischer titration method is a reference method applied to many different matrices.
In general terms, the Karl Fischer titration method is a chemical analysis method based on the oxidation of sulphur dioxide by iodine in a methanolic hydroxide solution (the titration medium), after which the reaction equation is:

H2O + I2+ SO2 + CH3OH +3RN → (RNH)SO4CH3 +2(RNH)I
The Karl Fischer titration method can be performed in volumetric or coulometric mode.
With the volumetric method (volumetric Karl Fischer): the iodine-containing Karl Fischer reagent is added until the solution shows the first sign of excess iodine. The amount of iodine converted is determined from the volume of Karl Fischer reagent used for the titration.
Coulometric Karl Fischer method: the amount of iodine taking part in the reaction is generated directly in the titration cell by electrochemical oxidation of iodine until the sign of unreacted iodine appears. Faraday's law can be applied to calculate the amount of iodine generated from the required amount of electricity.

Reagents used for Karl Fischer titration:

Hydranal ™ is one of the most reliable brands in Karl Fischer titration because of the manufacturer's quality (stability, purity, accuracy) and consistent continuous renewal. You can easily find this product line of Fuluka -Honeywell in many countries around the world. In Vietnam, PacificLAB is currently the distributor of the Hydranal ™ product line and reference standards/ CRM for many years.

Reagents used for the Karl Fischer titration method

Water standard used for the Karl Fischer titration method

4. ADVANTAGES AND DISADVANTAGES OF THE KARL FISCHER TITRATION METHOD

Advantages:

The Karl Fischer method is a highly accurate, fast reference method. The titration operation is simple and economical.
- Volumetric Karl Fischer: the titration vessel is optimised to keep drift low and provides the ability to determine water content in liquid, solid and gas samples.
- Coulometric Karl Fischer: the titration vessel is made entirely of glass, ensuring extremely low drift and therefore the most accurate results possible

Disadvantages:

The method must be adjusted for each specific sample
The method uses methanol as solvent

Points to note:

The iodine content in the solvent changes over time, so a standard sample (water standard) must be used to re-determine the I2 concentration.
There are several types of burette; depending on the moisture content, the appropriate type must be selected.
Choose a suitable solvent depending on the nature of the sample
This method applies to many products: flavours, chemicals, solvents, oils, fertilisers (urea)………….

Source: www.methrohm.vn