Karl Fischer titration is a classic titration method in analytical chemistry that uses coulometric
or volumetric titration to determine trace amounts of water in a sample. It was invented
in 1935 by the German chemist Karl Fischer.
Coulometric titration The main compartment of the titration cell
contains the anode solution plus the analyte. The anode solution consists of an alcohol,
a base, SO2 and I2. A typical alcohol that may be used is methanol or diethylene glycol
monoethyl ether, and a common base is imidazole. The titration cell also consists of a smaller
compartment with a cathode immersed in the anode solution of the main compartment. The
two compartments are separated by an ion-permeable membrane.
The Pt anode generates I2 when current is provided through the electric circuit. The
net reaction as shown below is oxidation of SO2 by I2. One mole of I2 is consumed for
each mole of H2O. In other words, 2 moles of electrons are consumed per mole of water.
B·I2 + B·SO2 + B + H2O → 2BH+I− + BSO3 BSO3 + ROH → BH+ROSO3−
The end point is detected most commonly by a bipotentiometric method. A second pair of
Pt electrodes are immersed in the anode solution. The detector circuit maintains a constant
current between the two detector electrodes during titration. Prior to the equivalence
point, the solution contains I- but little I2. At the equivalence point, excess I2 appears
and an abrupt voltage drop marks the end point. The amount of current needed to generate I2
and reach the end point can then be used to calculate the amount of water in the original
sample. Volumetric titration
The volumetric titration is based on the same principles as the coulometric titration except
that the anode solution above now is used as the titrant solution. The titrant consists
of an alcohol, base, SO2 and a known concentration of I2.
One mole of I2 is consumed for each mole of H2O. The titration reaction proceeds as above,
and the end point may be detected by a bipotentiometric method as described above.
Advantage of analysis The popularity of the Karl Fischer titration
is due in large part to several practical advantages that it holds over other methods
of moisture determination, including: High accuracy and precision - typically within
1% of available water, e.g. 3.00% appears as 2.97 - 3.03%.
Selectivity for water Small sample quantities required
Easy sample preparation Short analysis duration
Nearly unlimited measuring range Suitability for analyzing:
Solids Liquids
Gases
Independence of presence of other volatiles Suitability for automation
Linearity - single-point calibration, no calibration curves necessary
In contrast, loss on drying will detect the loss of any volatile substance.
The major disadvantage is that the water has to be accessible and easily brought into methanol
solution. Many common substances, especially foods such as chocolate, release water slowly
and with difficulty, and require additional efforts to reliably bring the total water
content into contact with the Karl Fischer reagents.
References ^ Fischer, Karl. "Neues Verfahren zur maßanalytischen
Bestimmung des Wassergehaltes von Flüssigkeiten und festen Körpern". Angew. Chem. 48: 394–396.
doi:10.1002/ange.19350482605. Literature
Water determination by Karl Fischer Titration by Peter A. Bruttel, Regina Schlink, Metrohm
AG See also
Titration Moisture analysis
References
External links EMD Chemicals AQUASTAR Tech Notes