The measurement principle of volumetric moisture meter and coulomb moisture meter
The volumetric moisture meter calculates water content by measuring the volume of Karl-Fischer reagent consumed during the reaction.
When measuring water content by Karl Fischer volumetric method, it 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 proceeds simultaneously at the positive and negative ends of the electrode, that is, on one electrode I2 is reduced, and on the other electrode I- is oxidized, so a current flows between the two electrodes. If only I- and no I2 are present in the solution, no current will flow between the two electrodes. The Karl-Fischer reagent contains active ingredients such as pyridine and iodine. When dropped into the reaction tank, the following chemical reaction will occur with the water in the solution to be tested:
H2O＋SO2＋I2＋3C5H5N→2C5H5N·HI＋C5H5N·SO3
C5H5N·SO3+CH3OH→C5H5N·HSO4CH3
C5H5N·HI → C5H5N·H++I- This reaction continues, consuming water continuously and generating I-, until the end point of the reaction titration, when the water is consumed. At this time, only when there is a trace amount of unreacted Karl Fischer reagent in the solution can I2 and I- exist at the same time. The solution between the two platinum electrodes begins to conduct electricity, and the end point is indicated by the current, and the titration stops. The water content in the solution is thus calibrated by measuring the volume (capacity) of Karl Fischer reagent consumed.
The measuring principle of Karl Fischer coulomb method (coulometric method)
The coulometric moisture meter calculates the water content by measuring the current passing through the reaction process.
The coulometric method is based on the fact that after the sample is dissolved in an electrolyte containing a special solvent containing a certain amount of iodine, the water consumes iodine. However, the required iodine is no longer titrated with a calibrated iodine-containing reagent, but through electrolysis. process, the iodide ions in the solution are oxidized to iodine at the anode: 2I---2e─→I2 The iodine produced reacts with the water in the sample. Its end point is indicated by double platinum electrodes. When the iodine concentration in the electrolyte returns to the original concentration, the electrolysis is stopped. Then according to Faraday's law of electrolysis: calculate the moisture content of the sample to be tested.
