Testing principle of petroleum moisture meter

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Testing principle of the petroleum moisture tester: Determining moisture by the Karl Fischer method is an electrochemical technique. The principle involves adding a water-containing sample once the Kalium Ferricyanide reagent in the electrolytic cell of the instrument reaches equilibrium. Water participates in the redox reactions involving iodine and sulfur dioxide; in the presence of pyridine and methanol, hydrogen, iodine, picolinic acid, and methyl picolinate are formed. The iodine that is consumed is regenerated through electrolysis at the anode, thereby allowing the redox reactions to continue until all the water is exhausted. According to Faraday’s laws of electrolysis, the amount of iodine produced through electrolysis is directly proportional to the amount of electricity used in the process. The reactions are as follows:
H2O + I2 + SO2 + 3C5H5N → 2C5H5N•HI + C5H5N•SO3
C5H5N•SO3 + CH3OH → C5H5N•HSO4CH3
During electrolysis, the electrode reactions are as follows:
Anode: 2I– → 2e– → I2
Cathode: I2 + 2e– → 2I–
2H+ + 2e– → H2↑
From these reactions, it can be seen that 1 mole of iodine oxidizes 1 mole of sulfur dioxide, with 1 mole of water being required for this process. Thus, it is an equivalent reaction of 1 mole of iodine with 1 mole of water; that is, the amount of electricity required to electrolyze iodine is equal to the amount needed to electrolyze water. 2×96493 coulombs of electricity are required to electrolyze 1 mole of iodine, while 96493 milli-coulombs are needed to electrolyze 1 millimole of water. 　　In the Karl Fischer titration method for determining moisture content, the process is based on electrochemical reactions: When both I2 and I- are present in the solution in the reaction cell, this reaction takes place at both the positive and negative electrodes; that is, I2 is reduced at one electrode while I- is oxidized at the other electrode, which results in an electric current flowing between the two electrodes. If there is only I- in the solution without I2 present, no current flows between the two electrodes. 　Karl Fischer reagent contains active components such as pyridine and iodine; when added in measured amounts to the reaction vessel, it undergoes the following chemical reactions with the water present in the solution to be analyzed:
I2 + SO2 + 3Base + ROH + H2O → 2Base•HI + HSO4R
Base: amine, pyridine, etc.
ROH (solvent): 2-methoxyethanol, methanol, etc.
H2O + SO2 + I2 + CH3OH + 3RN → 2RN•HI + RN•HSO4CH3
This reaction continues, consuming water and producing I-, until the end of the titration process, at which point all the water has been consumed. At this point, the presence of trace amounts of unreacted Karl Fischer reagent in the solution is necessary for I2 and I- to coexist; the solution between the two platinum electrodes begins to conduct electricity, and the endpoint is indicated by the current, at which point the titration is stopped. Thus, the moisture content in the solution is determined by measuring the volume (capacity) of the KF reagent consumed.