An Introduction to Karl Fischer Titration and Its Applications
By: Kianshar D Research Team
Karl Fischer (1901-1958) was a German
scientist who published a method for
determination of water content in samples in
1935. The method was a trimetric based on
Bunsen reaction used for determination of
sulfur dioxide in aqueous solutions:
SO2 + I2 + 2 H2O → H2SO4 + 2 HI
He realized that if sulfur dioxide was added in the excess the same reaction could
be used for determination of water content by titration of the produced acids.
The base which he accidentally used was pyridine which " was just standing in the
rack”. In his honor This method is named Karl Fischer titration (KF titration for
short).
During decades after publishment of the Karl Fischer method both original
stoichiometry and reagents were revised.
The Karl Fischer reaction included incorrect molar ratio assuming aqueous Bunsen
reaction in which methanol functions only as a solvent.
The error was corrected by Smith, Bryant and Mitchell discovered that pyridine
acted only as a buffer substance. This led them to correct the error in Karl Fischer
reaction and present the reaction which is used today:
H2O+ I2 + [RNH]+SO3CH3- + 2 RN → [RNH]+SO4CH3- + 2 [RNH]+I-
The first German DIN standard for the Karl Fischer tra on (DIN 51777) was
released in 1956 when the tra on was still conducted manually and the end
point of the titration was indicated by persistence of brown color due to excess
amount of iodine in titration vessel. Obviously the manual method was not
suitable for colored samples. On the other hand, it was slow and time wasting.
In 1960 by inventing the piston motorized burettes the first automatic Karl Fischer
titration instrument was born and it has been developing to advanced systems
which widely used nowadays for water determination in various industries .
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Karl Fischer Titration Techniques
The determination of the water content according to Karl Fischer is nowadays
performed by
two different techniques
Volumetric in this technique the sample is dissolved in KF Solvent (usually
methanol based) and the iodine is added as a part of a KF Reagent containing
sulfur dioxide and iodine dissolved in pyridine and methanol. The endpoint is
determined potentiometrically.
Figure 1. Volumetric KF trator
Coulometric KF analysis requires only one, iodide-containing, solution. Iodine
needed for KF reaction is produced by anodic oxidation of iodide from solution
and the endpoint is detected electrochemically.
Figure 2. Coulometric Karl Fischer titrator
In both techniques the sample is transferred to a titration vessel, dissolved and
then titrated. The selection of the appropriate technique is based on the
estimated water content in the sample:
Volumetric Karl Fischer Titration is Suitable for samples where water is present as
a majorcomponent: 100 ppm - 100%
Coulometric Karl Fischer analysis is Suitable for samples where water is present in
trace amounts: 1 ppm - 5%
Volumetric or coulometric titrator can be coupled with oven when some samples
aren’t soluble in any suitable solvent and some can cause side reactions with Karl
Fischer reagents. In this case the sample is heated in the oven and the water
released from the sample is carried by a dry gas flow to a titration cell where it
reacts with KF reagent. This is a perfect solution for samples with poor solubility,
samples which can cause side reactions with solvent and for highly hydroscopic
samples when handling with the sample in the laboratory conditions can result in
falsely higher results.
Karl Fischer Titration Reagents and Applications
Depending on the purpose and environmental consciousness of the user there are
many Karl Fischer titration reagents available. Pyridine can be replaced with
stronger, less smelly base, imidazole, and methanol can be substituted by ethanol
which is a safer, nontoxic solvent. Different auxiliaries have been defined, such as
additives for fats and oils, reagents for determination of low water contents,
buffer solutions for strongly alkaline or strongly acidic samples and solubilizers for
poorly soluble substances.
Karl Fischer titration as a reliable and robust method is widely used for direct
analysis of water content in various industries. In food industry it is used for water
content determination in fruit juices, flour, honey, chips, cocoa powder, in
cosmetic industry for determination of water in soaps, shampoos, creams,
lipstick, tooth paste, in petroleum industry for all kinds of different oils, gasoline,
kerosene, and petroleum, in pharmaceutic industry for raw materials, active
substances, suppositories, tablets, ointments, oils and lyophilized substrates. It is
used for determination of water in many other substances like wool, wood,
paper, silk, zeolite, cement and even in agricultural products like saffron.
It has been practically proved that the Karl Fischer titration is the most suitable
method for water content determination in almost all imaginable samples. It is no
surprise that this titration is the most common method in all the laboratories, in
all industries which are engaged with determination of water content.
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One of the most advanced automatic Karl Fischer Titration systems is KFT50 Titrika which is the third generation of
the KF titration systems produced by Kianshar D company
Sources and Further Reading
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https://www.chemistryworld.com/opinion/karl-fischers- trator/5695.ar cle
https://www.eurofins.com/biopharma-services/product-testing/services/biopharma-producttesting-services/quality-control/raw-materials-and-excipient-testing/water-determination-bykarl-fischer/
http://www.wako-chem.co.jp/english/labchem/product/analytical/aquamicron/index.htm
https://www.mt.com/dam/MT-NA/KarlFischerHelpPage/KF_Chemicals.pdf