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 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.
Read more about this topic: Karl Fischer Titration