Karl Fischer method, referred to as Fischer method, is a volumetric analysis method proposed by Karl Fischer in 1935 to measure moisture. The Fischer method is the most specific and accurate method for water among various chemical methods for determining the moisture content of substances. Although it is a classic method, it has been improved in recent years to improve the accuracy and expand the measurement range, and has been listed as a standard method for the determination of moisture in many substances.
There are two methods of Fischer method: titration method and coulometric method. It is suitable for the determination of water content in many inorganic and organic compounds. It is a world-recognized classical method for determining the moisture content of substances. It can quickly determine the moisture content in liquids, solids and gases. It is the most specific and accurate chemical method, and it is the world’s common industry standard analysis method. It is widely used in the principles of petroleum, chemical industry, electric power, medicine, pesticide industry and scientific research in colleges and universities.
Analysis of the principle of Karl Fischer’s method:
Determination of moisture in substances by Karl Fischer method is an important and sensitive chemical analysis method, but in addition to a very good measuring instrument, it is necessary to determine whether there are interfering substances in the substances to be determined, according to the content of moisture in the substances. Appropriate injection volume, overcoming various factors affecting the measurement accuracy, and careful operation can obtain good measurement results. 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”. Visual methods can only determine the moisture content of colorless liquid substances. Later, it was developed into the electricity method. With the development of science and technology, the coulomb counter and the volume method were combined to launch the coulomb method. This method is the test method in GB7600 “Determination of Moisture Content of Transformer Oil in Operation (Coulomb Method)”. The classification visual method and the electricity method are collectively referred to as the capacity method. The Karl Fischer method is divided into two major methods: the Karl Fischer volume method and the Karl Fischer-Coulomb method. Both methods are used as standard analytical methods in many countries to calibrate other analytical methods and measuring instruments.
The Karl Fischer Coulometric method for the determination of moisture is an electrochemical method. The principle is that when the Karnofsky reagent in the electrolytic cell of the instrument reaches equilibrium, a sample containing water is injected, and the water participates in the redox reaction of iodine and sulfur dioxide. In the presence of pyridine and methanol, pyridine hydroiodic acid and pyridine methyl sulfate are generated. The consumed iodine is produced 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 produced by electrolysis is proportional to the amount of electricity consumed during electrolysis. The response 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 needs 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 electricity of electrolyzed iodine is equivalent to the electricity of electrolyzed water, the electricity required for electrolysis of 1 millimol of iodine is 2 × 96493 millicoulombs, and the electricity required for electrolysis of 1 millimol of water is 2 × 96493 milligrams Coulomb charge.
The moisture content in the sample is calculated according to the following formula:
W=0.0933Q
In the formula: W—moisture content in the sample, μg;
Q—electrolytic electricity, mC;
Calculated according to the molecular weight of water is 18.015 g/mol;
Karl Fischer operation
The Fischer method is an iodometric method, and its basic principle is that when using iodine to oxidize sulfur dioxide, a quantitative amount of water is required to participate in the reaction:
I2 + SO2 + 2H2O → 2HI + H2SO4
The above reaction is reversible. When the concentration of sulfuric acid reaches more than 0.05%, the reverse reaction can occur. If we let the equation go in a positive direction, we need to add a suitable base to neutralize the acid formed during the reaction. Pyridine is added to the system so that the reaction proceeds to the right. Experiments show that pyridine is the most suitable reagent, and pyridine also has the effect of combining with iodine and sulfur dioxide to reduce their vapor pressure. Therefore, the reagent must be added with methanol or another solvent containing an active OH group to convert pyridine sulfate anhydride into stable methyl hydrogen sulfate pyridine.
Karl Fischer moisture measurement method is a method of measuring the moisture content of samples using methanol as the medium and Karl Fischer solution as the titrant. The method is simple in operation and high in accuracy, and is widely used in the fields of medicine, petroleum, chemical industry, pesticides, dyes, and food. It is especially suitable for samples that are easily destroyed by heat. It can measure not only free water, but also bound water. It is often used as a standard analytical method for moisture, especially trace moisture. But it is not suitable for samples containing strong reducing substances such as VC.
Karl Fischer reagent is a reagent for the determination of trace moisture in certain substances. Its components are: methanol, pyridine, iodine, and sulfur dioxide. There are two types of end point determination methods: visual method and potentiometric method.
The main components are I2, SO2, C5H5N, CH3OH
The basic principle of the Karl Fischer method is that when I2 oxidizes SO2, a quantitative amount of H2O is required.
I2+SO2+2H2O=2HI+H2SO4
This reaction is a reversible reaction. To make the reaction proceed forward, it is necessary to add an appropriate alkaline substance to neutralize the acid generated by the reaction. Pyridine (C5H5N) can meet this requirement, and adding methanol can avoid side reactions.
Karl Fischer reagent is brown due to the presence of I2. When I2, SO2 and H2O react, the brown color of I2 fades.
This method takes the appearance of brown as the end point of the titration
Karl Fischer method is a non-aqueous titration method. All containers need to be dried. If 1L Karl Fischer reagent is mixed with 6g of water during the preparation and storage process, the reagent will be invalid.
Karl Fischer reagent is a reagent for the determination of trace moisture in organic matter, so it is also called water reagent, also known as Karl Fischer reagent. The initial water reagent is mainly prepared from iodine, sulfur dioxide, methanol, and pyridine in a certain proportion. This reagent has bad odor and great toxicity, poor stability, and has a shelf life of less than three months, and is not suitable for the determination of aldehydes and ketones. It is inconvenient for users to store and use. With the advent of automatic Karl Fischer analyzers, the requirements for Karl Fischer reagents have also increased accordingly. Therefore, people have developed various types of water reagents one after another. Due to the different types of water reagents and different representation methods, when using them, be sure to look at the detailed instructions of each manufacturer.
Karl Fischer reagent:
Single group: no pyridine F≥3 500ml
Liquid A and B: No pyridine F≥3 500ml*2
A+B solution: no pyridine F≥3 500ml*2
Karl Fischer results
The principle of the Karl Fischer method for the determination of trace moisture in various substances: in the presence of water, that is, the water in the sample reacts with SO2 and I2 in the Karl Fischer reagent to produce a redox reaction.
I2 + SO2 + 2H2O → 2HI + H2SO4
But this reaction is a reversible reaction, when the concentration of sulfuric acid reaches more than 0.05%, the reverse reaction can occur. If we let the reaction proceed in a positive direction, we need to add an appropriate base to neutralize the acid formed during the reaction. It has been proved by experiments that adding pyridine to the system can make the reaction proceed to the right.
3 C5H5N+H2O+I2+SO2 → 2 pyridine hydroiodic acid+pyridine sulfate anhydride
The generated sulfuric anhydride pyridine is unstable, can react with water, consume a part of water and interfere with the determination, in order to make it stable, we can add anhydrous methanol.
Pyridine sulfate anhydride + CH3OH (anhydrous) → pyridine methyl sulfate
Overall reaction formula:
I2+SO2+H2O+3pyridine+CH3OH→2 pyridine hydroiodic acid+pyridine methyl sulfate
It can be seen from the reaction formula that 1 mol of water requires 1 mol of iodine, 1 mol of sulfur dioxide, 3 mol of pyridine and 1 mol of methanol to produce 2 mol of pyridine hydroiodic acid and 1 mol of pyridine methyl sulfate. This is theoretical data, but in fact, the amount of SO2, pyridine, and CH3OH is excessive. After the reaction is completed, the excess free iodine appears reddish-brown, which can be determined as reaching the end point.
I2:SO2:C5H5N = 1:3:10