Measuring principle of Karl Fischer volumetric moisture determination
The volumetric Moisture Meter calculates the water content by measuring the volume of Karl Fischer reagent consumed in the reaction process.
Karl Fischer volumetric method to determine the water content is mainly based on the electrochemical reaction: I2+2eó2I- When I2 and I- exist in the solution of the reaction cell at the same time, the reaction is carried out at the positive and negative ends of the electrode at the same time, that is, on one electrode I2 is reduced and I- is oxidized on the other electrode, so a current flows between the two electrodes. If there is only I- but no I2 in the solution, no current will flow between the two electrodes. The Karl Fischer reagent contains active ingredients such as pyridine and iodine, which can react with the water in the solution to be tested as follows when it is metered and dropped into the reaction cell:
H2O＋SO2＋I2＋3C5H5N→2C5H5N·HI＋C5H5N·SO3
C5H5N·SO3+CH3OH→C5H5N·HSO4CH3
C5H5N·HI→C5H5N·H++I- This reaction continues to consume water and generate I-, until the end of the reaction titration, the water is consumed. At this time, the solution has a small amount of Karl Fischer reagent that has not reacted, so that I2 and I- exist simultaneously. The solution between the two platinum electrodes begins to conduct electricity, and the end point is indicated by the current, and the titration is stopped. The water content in the solution is thus calibrated by metering the consumed Karl Fischer reagent volume (capacity).
Measuring principle of Karl Fischer coulometric method (coulometric method)
The coulometric Moisture Meter calculates the water content by measuring the amount of current passed through the reaction process.
The coulometric method is based on dissolving the sample in the electrolyte of a special solvent containing a certain amount of iodine, and the water consumes iodine, but the required iodine is no longer titrated with a calibrated iodine-containing reagent, but is obtained through electrolysis. Process, the iodide ion in the solution is oxidized to iodine at the anode: 2I---2e─→I2 The iodine produced reacts with the water in the sample. The endpoint is indicated with a double platinum electrode. When the iodine concentration in the electrolyte returns to the original concentration, stop the electrolysis. Then according to Faraday's law of electrolysis: calculate the moisture content of the sample to be tested.
