Classification of moisture analyzer Moisture analyzers are mainly used by laboratories to control the quality of raw materials before production and after production, and some are to control the quality of production. Therefore, the moisture analyzer is mainly used for the detection of moisture content and quality control of samples in various industries. Then the moisture analyzers commonly used in our laboratory are mainly divided into two categories: heating method and Karl Fischer method: 1 heating method moisture analyzer Heating moisture analyzer has infrared moisture analyzer, halogen moisture analyzer 1.1, infrared / halogen moisture analyzer 1. Testing principle: (1) The sample does not contain volatile substances other than water. (2) Samples that are insoluble or poorly soluble in organic solvents 2, physical method moisture analyzer main test: Powdered, granulated, viscous, fluid-like substances such as seeds, rapeseed, flour, paper, medicine, soil, wastewater, pigments, dehydrated vegetables, etc. 3. The difference between infrared moisture analyzer and halogen moisture analyzer: Infrared: Uniform heating, traditional heating method, suitable for testing temperature sensitive samples, such as samples with higher sugar content Halogen: Fast heating, uniform heating, high efficiency, energy saving and environmental protection 2, Karl Fischer moisture analyzer Karl Fischer moisture analyzer is divided into two methods: volumetric method and coulometric method. 1 Introduction: The Karl Fischer method, referred to as the Fisher method, is a method for analyzing the volume of water proposed by Karl Fj Scher in 1935. The Fisher method is the most specific and accurate method for determining the water content of a substance. Although it is a classical method, it has been improved in recent years, improving accuracy and expanding the measurement range. It has been listed as the standard method for moisture determination in many substances. 2. Chemical reaction Karl Fischer Moisture Determination is a redox titration method in non-aqueous solution. The basic principle of titration is that iodine oxidation of sulfur dioxide requires a certain amount of water to participate in the reaction. The chemical reaction equation is as follows: I2+SO2+2H2O → 2HI+H2SO4(2-1) I2+SO2+H2O+3RN+R1OH → 2RNHI+RNSO4R1 (2-2) The Karl Fischer reagent contains molecular iodine and is dark brown. When a reagent or sample containing water is added, a methylsulfate compound (RNSO4R1) is formed by a chemical reaction to make the solution yellow, and the end point can be judged visually. That is, it turns from light yellow to orange. However, the visual error is large and it is troublesome when measuring colored substances. Most of the national standards stipulate the "permanent stop method" to determine the end of the Karst reaction. The principle is: insert a double platinum electrode into the reaction solution, and add a fixed voltage between the two electrodes. If water exists in the solvent. , there will be no electrical pair in the solution, the solution is not conductive, when the reaction reaches the end point, there are I2 and I-electric pairs in the solution, namely: 2I- = I2+2e (2-3) Therefore, the conductivity of the solution suddenly increases, and the current value between the double platinum electrodes provided with the applied voltage suddenly increases, and the stability is determined by setting an information value in advance, and the end point of the titration can be judged. The titration is automatically stopped to calculate the water content of the sample by consuming the volume of the Karl Fischer reagent. 3. Composition of Karl Fischer Iodine I2 Sulfur dioxide SO2 Solvent methanol Organic base pyridine (imidazole) 4. Several factors affecting Karl Fischer Anhydrous methanol is used as a solvent for the sample and has a wide range of applications. General organic compounds, saturated or unsaturated hydrocarbons, and general inorganic compounds, acidic oxides, and partially organic and inorganic salts are suitable. However, some ketone and aldehyde samples could not be reacted with methanol. If the reaction is found to be uninterrupted, there is no end point, and the reaction should be carried out continuously, the question of whether there is a side reaction should be considered. When a side reaction occurs, it takes only a few minutes to react, but it continues. At this time, ethylene glycol methyl ether can be used instead of methanol. Compounds that react with iodine cannot be tested with Karl Fischer. 5. Features of the Karl Fischer Moisture Analyzer (1) The sensitivity and specificity of the reagent to water are high, which is an ideal method for moisture determination. (2) It is suitable for the determination of moisture in materials that are easily decomposed by heat or in samples containing volatile components. (3) Karl Fischer's reagent itself can be used as an indicator, and the endpoint can also be indicated by permanent stop titration. (4) A redox titration method belonging to a non-aqueous solution, so that it is not applicable to a substance which can react with iodine. (5) It is greatly affected by the humidity of the air, so the air humidity is high and the rainy weather is not suitable. (Attention to control temperature and humidity) 6. Reagent selection and calibration Only by knowing exactly the concentration of the KF reagent we use can we determine the water content of the sample by the volume and sample volume of the spent titrant. There are several reasons for the changes in the concentration of Karl Fischer reagent: *The chemical stability of the titrant is very low. Among them, iodine and sulfur dioxide are very active substances, which are easy to react with other substances. * When the titrant has a chance, it absorbs the moisture in the air and lowers its concentration because the methanol in it is highly water-absorptive. * Because 90% of the solvent in the solvent is methanol or ethanol, the density of the alcohol in the KF reagent is sensitive to changes in temperature. A slight increase in temperature will cause a sharp drop in the concentration of the reagent, and the temperature will increase by 1 degree Celsius. 0.1%. * Even if the container containing the reagent is sealed to a very good extent, a reaction may occur between the components in the reagent. Calibration of Karl Fischer reagent *Dihydrate sodium tartrate calibration It is a titrated primary standard for KF reagent. Under normal conditions, the material contains 15.66% water, which is stable and does not absorb water or water. However, sodium tartrate dihydrate can only be slowly dissolved in methanol and its solubility is very small. Therefore, it should be ground into a very fine powder before use. It should be mixed for 2 to 3 minutes (to ensure complete dissolution) before titration, and dissolved in 40 ml of methanol. 140 mg sodium tartrate dihydrate (room temperature). Pay attention to changing the solvent frequently *Standard aqueous solution calibration Weigh accurately 1.0 to 1.5 grams of water standard, determine its mass by reverse weighing method, and titrate to the end point. The amount of water standard used is generally appropriate to consume 2 to 5 ml of Karlelberg. However, water standards are more expensive and suitable for budget-rich laboratories. *Deionized water calibration Calibrating titrant concentrations with deionized water requires extensive practice and precise experimental procedures to achieve reproducible and accurate results. Since the amount of deionized water is very small (10 μl to 20 μl), the accidental error is large. Therefore, the disodium tartrate or water standard is recommended to calibrate the concentration of the titrant. Karl Fischer reagent selection 1Select by titration Generally, the Karl Fischer reagents sold on the market have titers of 1 mg/ml, 2 mg/ml, and 3-5 mg/ml. Samples with lower water content should be selected as smaller reagents. The coulometric reagent does not distinguish between titers. The titrant contains iodine, sulfur dioxide, and imidazole, and is dissolved in an alcohol solution. The solvent may be methanol or a solvent in which the sample is easily dissolved may be mixed with methanol as a solvent. The reagent can be stored for about two years. If the reagent is stored in a sealed bottle, the titer is reduced by approximately 0.5 mg/mL per year. Commercially available single component reagents are available in three concentration specifications: 5 mg/mL for samples with a water content of 1000 ppm to 100%, 2 mg/mL for samples with a water content < 1000 ppm, 1 mg/mL for samples with a water content < 200 ppm. 2 according to the formula selection Single component, two-component, special reagent, single component titer declines fast, but cheap, and the titer range is large; two-component reagents have fast reaction speed, high price, limited titer; special reagents, test some Special samples, such as aldehyde and ketone special reagents; 7. Karl Fischer moisture analyzer measurement process Measuring range: The water content in the range of 1 mg to 100% can be measured by the amount of sampling. Depending on the size of the titration vessel, 20 to 200 ml of methanol can be added thereto. Methanol is water-containing, so we have to pre-titrate with KF reagent to achieve a stable value. This "dry" methanol is then used as the working medium and solvent. The sample was added to the "dried" methanol in the same manner as the pre-titration method. The water content of the sample can be calculated from the consumption of the KF reagent. After the first sample has been titrated, the second sample can be added directly and the solution in the cuvette can be applied repeatedly. Of course, the samples to be tested cannot react with each other. The formula for calculating the moisture content of a sample is: For the insoluble methanol sample, some cosolvents can be added to help the sample dissolve for better test results. Cosolvent: Solvent proportion sample Formamide Max. 50 % KFV Max. 20 % KFC Sugar, salt Chloroform Max. 70 % Max. 30 % Oil, fat Long chain alcohol Max. 50 % Max. 30 % Oil, fat Xylene, toluene Max. 70 % Max. 30 % Asphaltene-containing crude oil 9. Volumetric method and coulomb method Volumetric method * Dissolve the sample to be tested in anhydrous methanol first, and use the Fisher test to determine the water content in the sample solution. The excess amount of the test solution can indicate the end point: the color is yellowish → light reddish brown , or permanent stop titration. Coulomb method The Karl Fischer method for the determination of moisture is an electrochemical method. The principle is that the Karl Fischer reagent in the electrolytic cell of the instrument is injected into the aqueous sample when the equilibrium is reached, the water participates in the redox reaction of iodine and sulfur dioxide, and in the presence of pyridine and methanol, pyridine hydroiodide and pyridine thiosulfate are formed. 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↑ As can be seen from the above reaction, that is, 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. The moisture content of the sample is calculated according to formula (1): Where: W---the moisture content in the sample, μg; Q---electrolytic power, mC; 18---the molecular weight of water; The coulometric titrant (Fischer's test solution) does not contain iodine, but the double platinum electrode is electrolyzed in an iodine solution containing iodide ions. When all the water is consumed, there will be an excess of iodine on the electrode, and the titrator will stop the end point. * Mainly used for the determination of trace moisture, therefore, coulometric titration is less used in the determination of drug moisture. What sample is suitable for determination by calciner A sample that releases moisture at high temperatures and is insoluble in conventional Karl Fischer reagents. There is a side reaction with the Karl Fischer reagent, which is to be in direct contact with the surface, but this ensures that the sample does not decompose at high temperatures or releases chemicals that react with the reagent. Dry gas should be used as much as possible for the carrier gas, as some compounds may oxidize during heating. Moisture Analyzer Selection Guide First, Karl Fischer moisture analyzer 1. The principle of detection is: (1) There is no water formation in the side reaction. (2) The sample also does not consume iodine or release iodine. 2. Capacity analysis and determination. The specific application range is mainly organic and inorganic compounds. (1) Inorganic compounds Inorganic compounds such as organic acids, inorganic acids, acid oxides, inorganic acids and acid anhydrides. (2) Organic compounds An organic compound such as an acid, an alcohol, an ester, a stable hydroxy compound, an acetal, an ether, a hydrocarbon anhydride, an acid halide, a halide, a peroxy compound, a nitrogen-containing compound, or a sulfur-containing compound. 3. The specific application range of the Coulomb method Hydrocarbons, oils, alcohols, halogenated hydrocarbons, phenols, lipids, ethers, and the like. Second, infrared / halogen moisture analyzer 1. Testing principle: (1) The sample does not contain volatile substances other than water. (2) Samples that are insoluble or poorly soluble in organic solvents 2, physical method moisture analyzer main test: Powdered, granulated, viscous, fluid-like substances such as seeds, rapeseed, flour, paper, medicine, soil, wastewater, pigments, dehydrated vegetables, etc. 3. The difference between infrared moisture analyzer and halogen moisture analyzer: Infrared: Uniform heating, traditional heating method, suitable for testing temperature sensitive samples, such as samples with higher sugar content Halogen: Fast heating, uniform heating, high efficiency, energy saving and environmental protection In summary: The advantage of using a heating moisture analyzer is that it does not require the use of other auxiliary reagents, and the installation and operation are relatively simple, but the accuracy and efficiency of the test results are not as good as the Karl Fischer moisture analyzer. The Karl Fischer moisture analyzer has a faster and more accurate detection characteristics than the heating moisture analyzer. The range of detection that can be used is also very wide, and the Karl Fischer should be selected according to the moisture content of the sample and the characteristics of the sample. Moisture analyzer type, when the moisture content of the sample is less than 0.5%, and does not contain the above-mentioned restricted substances, or when detecting the moisture value of the gas, the Coulometric moisture analyzer should be preferred, such as the V320 automatic Karl Fischer Hugh Coulomb tester. Otherwise choose a S-300 type capacity method fully automatic Karl Fischer moisture analyzer. 3. According to the sample properties described above, select the appropriate Karl Fischer moisture analyzer physical moisture analyzer Fourth, the accuracy value: Select the appropriate accuracy value according to the testing standards required by the industry or the moisture value reached by the production requirements sample. Water accuracy value: 0.1%, 0.05%, 0.01%, 0.001%, 0.003% Fifth, the temperature (physical method): the sample is different, the required temperature value is also different, optional between 160-270. 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If the pH is too high, the sample is too alkaline, etc., it will also cause side reactions, that is, continuous reaction, and no end point appears. At this time, if the pH value is too high, the pH value can be adjusted by using a buffer solution, and if the alkali is too high, the toluic acid and the salicylase can be added to alleviate the alkaline solution, but acetic acid cannot be used. 