- Karl Fischer titration
Karl Fischer titration is a classic
titrationmethod in analytical chemistrythat uses coulometric or volumetric titration to determine trace amounts of waterin a sample. It was invented in 1935 by the German chemist Karl Fischer.
The main compartment of the titration cell contains the
anodesolution plus the analyte. The anode solution consists of an alcohol (ROH), a base (B), SO2 and I2. A typical alcohol that may be used is methanolor diethylene glycol monomethyl ether, and a common base is imidazole.
The titration cell also consists of a smaller compartment with an (anode) 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.
The end point is detected most commonly by a
bipotentiometricmethod. 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 in order to reach the end point can then be used to calculate the amount of water in the original sample.
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 (ROH), base (B), SO2 and a known concentration of I2.
One mole of I2 is consumed for each mole of H2O. The titration reaction proceeds as above.
The end point may be detected by a
bipotentiometricmethod 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
* Selectivity for water
* Small sample quantities required
* Easy sample preparation
* Short analysis duration
* Nearly unlimited measuring range (1ppm to 100%)
* Suitability for analyzing:
* Independence of presence of other volatiles
* Suitability for automation
The most important advantage of Karl Fischer titration method over conventional loss on drying (LOD) thermal methods of moisture determination is its specificity for water. Loss on drying will detect the loss of "any" volatile substance.
*Water determination by Karl Fischer Titration by Peter A. Bruttel, Regina Schlink, [http://www.metrohm.com Metrohm AG] , Order Number [http://www.metrohm.com/applications/lit/mono/order/karl-fischer-order.html 8.026.5003] (Free Monograph)
* [http://www.emdchemicals.com/analytics/literature/KF_Titration_Basics.pdf Karl Fischer Titration Basics] - EMD Chemicals AQUASTAR Tech Notes
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