Principle of Karl-Fischer Coulometric Moisture Analyzer (Hainuo Instruments)

1. In 1935 Karl-Fischer (KarlFischer) first proposed the method of measuring moisture using capacity analysis. This method is the visual method in GB6283 "Determination of Water Content in Chemical Products". The visual method can only measure the moisture of colorless liquid substances. Later, it developed into the electricity method. With the development of science and technology, the coulomb counter was combined with the volume method to introduce the coulomb method. This method is the test method in GB7600 "Determination of Transformer Oil Moisture Content in Operation (Coulomb Method)" The current classification visual method and electricity method are collectively called the volume method. The Karl Fischer method is divided into two methods: the Karl Fischer volume method and the Karl Fischer Coulomb method. Both methods have been designated as standard analytical methods in many countries, and are used to calibrate other analytical methods and measuring instruments.

2. The Karl Fischer Coulometric method is an electrochemical method for determining moisture. The principle is that when the Karl Fischer reagent in the instrument's electrolytic cell reaches equilibrium, the sample containing water is injected. Water participates in the redox reaction of iodine and sulfur dioxide. In the presence of pyridine and methanol, pyridine hydroiodate and pyridine methyl sulfate are generated. The consumed iodine is produced at the anode electrolysis, so that the redox reaction continues until the water is completely exhausted. According to Faraday's law of electrolysis, the iodine produced by electrolysis is proportional to the amount of electricity consumed during electrolysis. The reaction is as follows:

H2O + I2 + SO2 + 3C5H5N → 2C5H5N · HI + C5H5N · SO3

C5H5N · SO3 + CH3OH → C5H5N · HSO4CH3

During the electrolysis process, 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 requires 1 mole of water to oxidize 1 mole of sulfur dioxide. Therefore, it is the equivalent reaction of 1 mole of iodine and 1 mole of water, that is, the amount of electricity for electrolytic iodine is equivalent to the amount of electricity for electrolyzed water. The electrolysis of 1 mole of iodine requires 2 × 96493 coulombs of electricity, and the electrolysis of 1 millimole of water requires 96493 millicoulons.

The moisture content in the sample is calculated according to formula (1):

In the formula: W --- the moisture content in the sample, μg;

Q --- Electrolytic power, mC;

18 --- Molecular weight of water;