Karl Fischer titration

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.

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Coulometric titration

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 (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.

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 (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.

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:
  • Solids
  • Liquids
  • Gases
• 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.

See also

• Titration
• Moisture analysis