An overview of the working principle of the coulometric moisture analyzer
The working principle of the coulometric moisture analyzer is an electrochemical method. The Karl Fischer method is divided into two methods: the Karl Fischer volumetric method and the Karl Fischer Coulomb method. Both methods are designated as standard analytical methods by many countries to calibrate other analytical methods and measuring instruments.
Working principle of coulometric moisture analyzer
The principle is that when the Karl Fischer reagent in the electrolytic cell of the coulometric trace moisture analyzer reaches equilibrium, the water-containing sample is injected, and the water participates in the redox reaction of iodine and sulfur dioxide. In the presence of pyridine and methanol, hydrogen suanpyridine and methyl Pyridinium sulfate, the consumed iodine is electrolyzed at the anode, so that the oxidation-reduction reaction continues until the water is completely exhausted. According to Faraday's law of electrolysis, the iodine produced by electrolysis is proportional to the electricity consumed during electrolysis. The response is as follows:
H2O+I2+SO2+3C5H5N→2C5H5N·HI+C5H5N·SO3
C5H5N·SO3+CH3OH→C5H5N·HSO4CH3
During electrolysis, the electrode reaction is as follows:
Anode: 2I--2e→I2
Cathode: I2+2e→2I-
2H++2e→H2↑
It can be seen from the above reaction that 1 mole of iodine oxidizes 1 mole of sulfur dioxide and requires 1 mole of water. Therefore, it is the equivalent reaction of 1 mole of iodine and 1 mole of water, that is, the electricity of electrolyzing iodine is equivalent to the electricity of electrolyzing water. The electrolysis of 1 mole of iodine requires 2×96493 coulombs of electricity, and the electrolysis of 1 mmol of water requires 96493 milliocoulombs of electricity.
The water content in the sample is calculated according to formula (1):
In the formula: W---moisture content in the sample, μg;
Q---Electrolysis power, mC;
18---molecular weight of water;
Brief Introduction of Coulometric Moisture Analyzer Method
In 1935, Karl Fischer first proposed the method of measuring moisture by volumetric analysis, which is the visual method in GB6283 "Determination of Moisture Content in Chemical Products". The visual method can only determine the water content of colorless liquid substances. Later, it developed into the electricity method. With the development of science and technology, the coulometric method was combined with the volumetric method to launch the coulomb method. This method is the test method in GB7600 "Determination of Moisture Content in Transformer Oil in Operation (Coulometric Method)". The current classification visual method and electricity method are collectively referred to as capacity method.
