The principle and operation of the new card type moisture analyzer
The instrument capable of detecting the moisture content of various organic and inorganic solids, liquids, gases and other samples is called a moisture analyzer. According to the measurement principle, physical measurement and chemical measurement can be classified into two categories. The physical measurement method commonly used weight loss method, distillation layering method, gas chromatography analysis method, etc., chemical measurement methods mainly include Karl Fischer, toluene method, etc., the international standardization organization put Karl Fischer method It is designated as an international standard for the determination of trace moisture, and our country has also adopted this method as a national standard for the determination of trace moisture.
Common weight loss method moisture meter has halogen moisture measurement, infrared moisture analyzer, microwave moisture analyzer, etc.;
Common Karl Fischer moisture analyzers mainly include a volumetric Karl Fischer moisture analyzer and a Coulometric (electricity method) Karl Fischer moisture analyzer.
Application range
Can be widely used in all industries that need to quickly measure moisture, such as medicine, plastics, chemicals, food (fish, dehydrated vegetables, meat and fish processing, noodles, flour, biscuits, moon cakes, etc.), food, feed, seeds, vegetables Seeds, tea and textiles, agriculture, forestry, paper, rubber, textile and other industries in the production and experimental process.
Card Moisture Analyzer Principle
Karl-Fischer Coulomb Moisture Analyzer Principle
In 1935, Karl Fischer first proposed a method for determining moisture using capacity analysis, which is the visual method in GB6283 "Determination of Moisture Content in Chemical Products". The visual method can only measure the moisture of a colorless liquid substance. Later, it developed into the electricity law. With the development of technology, the coulomb method and the volume method are combined to introduce the coulomb method. This method is the test method in GB7600 "Measurement of Moisture Content of Transformer Oil in Operation (Coulomb Method)". The classification visual method and the electricity method are collectively referred to as the volume method. The Cartesian method is divided into two methods: the Cartesian volume method and the Cartesian coulometric method. Both methods have been standardized by many countries as standard analytical methods for calibrating other analytical methods and measuring instruments.
2. The Karl Fischer method for the determination of moisture is an electrochemical method. The principle is that the Karl Fischer reagent in the electrolysis cell of the instrument is injected into the aqueous sample when the equilibrium is reached, the redox reaction of the water ginseng and iodine and sulfur dioxide, in the presence of pyridine and methanol, the formation of pyridine hydroiodide and pyridine thiosulfate. The consumed iodine is generated by electrolysis at the anode, so that the redox reaction continues until the water is completely exhausted. According to Faraday's law of electrolysis, the iodine generated by electrolysis is proportional to the amount of electricity consumed during electrolysis, and 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 water is required to oxidize 1 mole of sulfur dioxide by 1 mole of iodine. Therefore, it is the equivalent reaction of 1 mole of iodine and 1 mole of water, that is, the amount of electrolytic iodine is equivalent to the amount of electrolyzed water. The electrolysis of 1 mole of iodine requires 2×96493 coulombs of electricity, and the electrolysis of 1 millimol of water requires electricity of 96493 millicoulombs.
Frequently Asked Questions
The anolyte color is not bright yellow, but between brown and dark yellow
The color is too dark, which is the ability of the electrode to respond to the electrolyte. The double platinum needle electrode can be cleaned with a paper towel to remove the adsorbate on the surface; check whether the measuring electrode is properly connected; the measuring electrode may malfunction.
Pre-titration of fresh anolyte, drift too high?
There is residual moisture in the titration system. The molecular sieve and silica gel in the drying tube can be replaced; check whether the electrode interface and the plug interface of the titration stand are tight; the silicone grease can be applied at some loose interfaces.
What is the reason why the standby titration drift is too high?
The water in the cathode cell penetrates into the anode cell through the membrane. The anode pool electrolyte can be replaced; a small amount of single-component volume Karl Fischer reagent is added to the cathode electrolytic cell for drying; the liquid level of the anolyte is kept higher than the liquid level in the cathode pool; the titration cup is thoroughly cleaned, and the last time is cleared. Test the residual side reactions caused by the residual sample; check the tightness of the titration system.
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