To measure the water content of an insulating oil we use the Karl Fischer titration as the basic technique. In a Karl Fischer titration, water (H2O) reacts chemically with iodine (I2), sulfur dioxide (SO2), an organic base (C5H5C) and alcohol (CH3OH) in an organic solvent.

Here, the sample is mixed with sulfur dioxide, iodide ions, and organic bases/alcohols. Here, iodide ions are produced by electrolysis and participate in the above reactions in solution. So as long as the reaction continues there will be no trace of the individual iodide ion in the solution.

The iodide ions produced by electrolysis are completely used up in the reaction as long as water molecules are present in solution. As soon as there are no more water molecules to react, Karl Fischer reactions stop. There are two platinum electrodes immersed in the solution from the start. The presence of iodide ions in the solution depolarizes the platinum electrodes after the Karl Fischer reaction is complete. As a result of the voltage-current ratio of the electrode circuit changes. Therefore, this change indicates the end point of the Karl Fischer reaction in solution.
According to Faraday’s law of electrolysis, the amount of iodine involved in the reaction is proportional to the electricity used for electrolysis during the Karl Fischer reaction.

From, the power consumption until the end of the reaction, one can easily calculate the actual amount of iodine involved in the reaction. Again, from the first reaction equation, it is found that one mole of iodine reacts with one mole of water. That is, 127 grams of iodine will react with 18 grams of water. So from the calculated weight of iodine, we can determine the exact amount of water present in the insulating oil sample.