Considerations for the determination of moisture by Karl Fischer titration
2020-08-19 13:13:01
Karl Fischer titration was proposed in 1935 by Karl Fischer. Since then, many people have carried out a more comprehensive study on the method. Based on the reaction stoichiometry, the stability of the reagents, the titration method, the determination of the endpoint, the application of various types of samples and the automation of the instrument have been continuously improved. To make this technology mature.
The Karl Fischer Moisture Analyzer has high accuracy for determining moisture and has been used as a standard analytical method in many countries to calibrate other analytical methods and analytical instruments. The Karl Fischer volumetric method is the most specific and most classical method for measuring moisture. The principle is that I2 requires a certain amount of water to participate in the oxidation of SO2. This characteristic is used to determine the moisture in the substance to be tested. When measuring the moisture in a substance, in order to obtain accurate results, you need to pay attention to the following points:
(1) Since the titration reagent easily absorbs moisture, it is required to have a good sealing property such as a burette and a titration cell. Otherwise, the moisture absorption phenomenon will cause long-term instability and serious errors in the end point.
(2) The size of the reagent titer should be determined according to the amount of water in the test solution. When measuring a test solution with a large water content, the titer should be selected to be larger; when measuring a test solution with a small water content, the titer should be selected to be smaller; the minimum reading of the burette should also be selected smaller, otherwise A large measurement error will result.
(3) Timing, stirring should be sufficient and uniform. When titrating a sample solution with a large viscosity, it is necessary to pay attention to the full and consistent agitation, including the speed of the magnetic stirrer, and the liquid level in the titration cell should be substantially the same, so that better measurement accuracy can be obtained.
(4) When injecting, prevent the syringe head from being contaminated by the outside and affect the measurement results, such as the operator's exhalation and contamination when the syringe head is wiped. At the same time, it is necessary to prevent the loss of the sample, such as the hanging drops on the injector head splashing onto the measuring cell wall or the electrode rod.
(5) A desiccator should be installed at the inlet of the reagent bottle to prevent the reagent from absorbing the moisture in the air and causing a serious measurement error due to the decrease in titer.
(6) During the titration process, a false end point sometimes occurs, that is, reaching the end point in advance, resulting in a low measurement result. Especially in the case of measuring samples with low moisture content, the effect is even greater, and even measurement is impossible. The main reason is that oxygen in the air oxidizes the iodide ions in the titration cell to iodine, thereby reducing the consumption of reagents. Sunlight also significantly promotes the oxidation of oxygen and iodide ions, so the reagents should be protected from light. In addition, the composition of the reagents and the operating environment also have a certain effect on the speed of the reaction. If the reagent contains too much sulfur dioxide, the reagent is not pure, and the water content of the reagent is too high, the end point is prone to occur.
(7) The Karl Fischer method generates sulfuric acid in the reaction of measuring water. When the concentration is higher than 0.05%, a reverse reaction may occur, which may affect the measurement result. Pyridine can be combined with the acid produced to ensure that the reaction proceeds in one direction. In the absence of methanol, water or any other active hydrogen-containing compound can react with the methanol intermediate compound, which can disturb the stoichiometry of the chemical reaction, leaving the reaction with no particular selectivity for water. Therefore, it is necessary to pay attention to the presence of sufficient amounts of pyridine and methanol in the reagent and the titration solution during the measurement.
(8) When measuring moisture, pay attention to whether or not there is a substance capable of generating water with the reagent in the sample to be measured.
(9) If the sample contains those which can be reduced by iodine, it can be removed by addition reaction with an olefin. The active chlorine contained in the ferric chloride and the reagent can be removed by pretreatment with pyridine and sulfur dioxide and methanol solutions. In recent years, with the improvement of instrument automation, the Karl Fischer method can be used to perform titration analysis more easily, quickly and reliably. For example, the AKF-2010 Moisture Analyzer of Hegong Instrument is controlled by a microprocessor, which can be connected with a balance, a printer and a computer. The computer also has relevant teaching content, which enables the user to quickly become familiar with the operation of the instrument. Various functions of the instrument
The Karl Fischer Moisture Analyzer has high accuracy for determining moisture and has been used as a standard analytical method in many countries to calibrate other analytical methods and analytical instruments. The Karl Fischer volumetric method is the most specific and most classical method for measuring moisture. The principle is that I2 requires a certain amount of water to participate in the oxidation of SO2. This characteristic is used to determine the moisture in the substance to be tested. When measuring the moisture in a substance, in order to obtain accurate results, you need to pay attention to the following points:
(1) Since the titration reagent easily absorbs moisture, it is required to have a good sealing property such as a burette and a titration cell. Otherwise, the moisture absorption phenomenon will cause long-term instability and serious errors in the end point.
(2) The size of the reagent titer should be determined according to the amount of water in the test solution. When measuring a test solution with a large water content, the titer should be selected to be larger; when measuring a test solution with a small water content, the titer should be selected to be smaller; the minimum reading of the burette should also be selected smaller, otherwise A large measurement error will result.
(3) Timing, stirring should be sufficient and uniform. When titrating a sample solution with a large viscosity, it is necessary to pay attention to the full and consistent agitation, including the speed of the magnetic stirrer, and the liquid level in the titration cell should be substantially the same, so that better measurement accuracy can be obtained.
(4) When injecting, prevent the syringe head from being contaminated by the outside and affect the measurement results, such as the operator's exhalation and contamination when the syringe head is wiped. At the same time, it is necessary to prevent the loss of the sample, such as the hanging drops on the injector head splashing onto the measuring cell wall or the electrode rod.
(5) A desiccator should be installed at the inlet of the reagent bottle to prevent the reagent from absorbing the moisture in the air and causing a serious measurement error due to the decrease in titer.
(6) During the titration process, a false end point sometimes occurs, that is, reaching the end point in advance, resulting in a low measurement result. Especially in the case of measuring samples with low moisture content, the effect is even greater, and even measurement is impossible. The main reason is that oxygen in the air oxidizes the iodide ions in the titration cell to iodine, thereby reducing the consumption of reagents. Sunlight also significantly promotes the oxidation of oxygen and iodide ions, so the reagents should be protected from light. In addition, the composition of the reagents and the operating environment also have a certain effect on the speed of the reaction. If the reagent contains too much sulfur dioxide, the reagent is not pure, and the water content of the reagent is too high, the end point is prone to occur.
(7) The Karl Fischer method generates sulfuric acid in the reaction of measuring water. When the concentration is higher than 0.05%, a reverse reaction may occur, which may affect the measurement result. Pyridine can be combined with the acid produced to ensure that the reaction proceeds in one direction. In the absence of methanol, water or any other active hydrogen-containing compound can react with the methanol intermediate compound, which can disturb the stoichiometry of the chemical reaction, leaving the reaction with no particular selectivity for water. Therefore, it is necessary to pay attention to the presence of sufficient amounts of pyridine and methanol in the reagent and the titration solution during the measurement.
(8) When measuring moisture, pay attention to whether or not there is a substance capable of generating water with the reagent in the sample to be measured.
(9) If the sample contains those which can be reduced by iodine, it can be removed by addition reaction with an olefin. The active chlorine contained in the ferric chloride and the reagent can be removed by pretreatment with pyridine and sulfur dioxide and methanol solutions. In recent years, with the improvement of instrument automation, the Karl Fischer method can be used to perform titration analysis more easily, quickly and reliably. For example, the AKF-2010 Moisture Analyzer of Hegong Instrument is controlled by a microprocessor, which can be connected with a balance, a printer and a computer. The computer also has relevant teaching content, which enables the user to quickly become familiar with the operation of the instrument. Various functions of the instrument
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