Sources
Help build the largest human-edited directory on the web.
Submit a site - Become an editor
Wikipedia. Licensed under the GNU Free Documentation License.
Are you an expert in this subject? Join the discussion and share your knowledge at Wikipedia.org.
Karl Fischer titration
From Wikipedia, the free encyclopedia
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.
Contents |
The main compartment of the titration cell contains the anode solution plus the analyte. The anode solution consists of an alcohol (ROH), a base (B), SO2 and I-. A typical alcohol that may be used is methanol or diethylene glycol monomethyl ether, and a common base is imidazole.
The titration cell also consists of a smaller compartment with a cathode solution. This compartment has an internal Pt electrode (cathode) immersed in the solution and an external Pt electrode (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 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 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 bipotentiometric method as described above.
The Karl Fischer titration method has advantages over simple 'loss on drying' methods of moisture determination because it is specific to water. Loss on drying will detect the loss of any volatile substance.