Principle of Volumetric and Coulomb Moisture Meters
The volumetric moisture meter calculates water content by measuring the volume of Karl Fischer reagent consumed during the reaction process.
When using the Karl Fischer volumetric method to determine water content, the main basis is the electrochemical reaction: when I2+2e ó 2I - is present in the solution of the reaction tank, the reaction occurs simultaneously at the positive and negative ends of the electrode, that is, I2 is reduced on one electrode and I - is oxidized on the other electrode, so there is a current passing between the two electrodes. If there is only I - and no I2 present simultaneously in the solution, there is no current flowing between the two electrodes. The Karl Fischer reagent contains active ingredients such as pyridine and iodine, which can undergo the following chemical reactions with water in the test solution when dropped into the reaction pool:
H2O+SO2+I2+3C5H5N → 2C5H5N · HI+C5H5N · SO3
C5H5N · SO3+CH3OH → C5H5N · HSO4CH3
C5H5N · HI → C5H5N · H++I - This reaction continues, continuously consuming water to generate I - until the end of the titration reaction, when the water is consumed. At this point, there is a trace amount of unreacted Karl Fischer reagent present in the solution to allow for the simultaneous presence of I2 and I -. The solution between the two platinum electrodes begins to conduct electricity, reaching the endpoint indicated by the current, and titration stops. Thus, the water content in the solution is calibrated by measuring the volume (volume) of consumed Karl Fischer reagent.
The measurement principle of Karl Fischer Coulomb method (electric quantity method)
The Coulomb moisture meter calculates water content by measuring the amount of current passed through the reaction process.
The electrochemical method is based on dissolving the sample in an electrolyte containing a special solvent containing a certain amount of iodine, and then water consumes iodine. However, the required iodine is no longer titrated using calibrated iodine containing reagents. Instead, through the electrolysis process, the iodine ions in the solution are anodized to iodine: 2I --2e -- → I2, and the iodine produced reacts with the water in the sample. The endpoint is indicated by a double platinum electrode. When the iodine concentration in the electrolyte returns to the original concentration, stop electrolysis. Then, according to Faraday's electrolysis law, calculate the moisture content of the sample to be tested.
