The measure water content in an insulating oil we use Karl Fisher Titration as basic technique. In Karl Fisher Titration, water (H2O) chemically reacts 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 base/alcohol. Here, iodide ions are produced by electrolysis and take part in above reactions in the solution. So there would not be any trace individual iodide ion in the solution as long as the reaction continues.
The iodide ions produced by electrolysis are totally consumed in reaction as long as the water molecules present in the solution. As soon as there will be no more water molecule to react, the Karl Fisher Reactions stop. There are two platinum electrodes immersed in the solution from the beginning. Just after the Karl Fisher reaction is over the existence of iodide ions in the solution depolarizes the platinum electrodes. As a result of the voltage-current ratio of the electrode circuit changes. So, this change indicates the end point of the Karl Fisher Reaction in the solution.
According to the Faraday law of electrolysis, the quantity of iodine participated in the reaction is proportional to the electricity consumed for electrolysis during Karl Fisher Reactions.
From, the consumption of electricity till the reaction ends, one can easily calculate the actual mass of iodine participated in reaction. Again, from the first equation of reactions, it is found that one-mole iodine reacts with one-mole water. That means 127 grams iodine will react with 18 grams of water. So from a calculated weight of iodine, we can determine the exact quantity of water presented in the sample of insulating oil.