Determination Principles of Volumetric Moisture Analyzer and Coulometric Moisture Analyzer
Principle of Karl Fischer titration for moisture determination
The volumetric moisture analyzer calculates water content by measuring the volume of Karl Fischer reagent consumed during the reaction process.
When the Karl Fischer titration method is used to determine the moisture content, it mainly relies on the electrochemical reaction: when I2+2e ó 2I - is present in the solution of the reaction cell at the same time as I2 and I -, 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 only I - and no I2 are present in the solution, then there is no current passing between the two electrodes. Karl Fischer reagent contains active ingredients such as pyridine and iodine. When dropped into a reaction tank, it can undergo the following chemical reaction with water in the test solution:
H2O＋SO2＋I2＋3C5H5N→2C5H5N·HI＋C5H5N·SO3
C5H5N·SO3+CH3OH→C5H5N·HSO4CH3
C5H5N · HI → C5H5N · H++I - The reaction continues, continuously consuming water to generate I -, until the end point of the titration reaction and the water consumption is complete. At this point, the presence of trace amounts of unreacted Karl Fischer reagent in the solution is necessary for the simultaneous presence of I2 and I - to occur. The solution between the two platinum electrodes begins to conduct and reaches the endpoint indicated by the current, stopping titration. Thus, the water content in the solution can be calibrated by measuring the volume (capacity) of the consumed Karl Fischer reagent.
The determination principle of Karl Fischer Coulomb method (electric quantity method)
The Coulomb method moisture meter calculates water content by measuring the amount of current passing through the reaction process.
The electric quantity method is based on dissolving the sample in an electrolyte containing a special solvent with a certain amount of iodine, and the water consumes iodine. However, the required iodine is no longer titrated using calibrated iodine containing reagents, but through the electrolysis process, the iodine ions in the solution are oxidized to iodine at the anode: 2I --2e -- → I2, and the iodine produced reacts with the water in the sample. The endpoint is indicated by dual platinum electrodes. Stop electrolysis when the iodine concentration in the electrolyte returns to the original concentration. Then, according to Faraday's law of electrolysis, calculate the moisture content of the sample to be tested.
