Karl Fischer titration method can be applied to the determination of water content in various organic and inorganic substances. Due to the differences in the properties of various compounds, they can be divided into two categories: direct measurement and non-direct measurement.
The main organic and inorganic compounds that can be directly measured are as follows:
1. Inorganic compounds
(1). Organic acid salts Na (CH3) SO4, Ba (OOCCH3) 2, K2C2O4, VO2 (OOCCH3) 2, Na2C2H4O6
(2). Inorganic acid salts NH4PO4, CaCl2, NaHSO4, Na2SO4, KF, NH4NO3, MgSO4, Na2SO4, KSCN, FeSO4, Al2 (SO4) 3KSO4, CaHPO4, NaI, CaCO3, FeF3, VO2 (NO3) 2
(3). Acid oxide SiO2, Al2O3
(4). Inorganic acids and anhydrides SO2, HI, HF, HNO3, HCN, H2SO4, HSO3, NH2
2. Organic compounds
(1). Acid carboxylic acid, carboxylic acid, amino acid, sulfonic acid
(2). Alcohol monohydric alcohol, polyhydric alcohol, phenol
(3). Ester carboxylate, orthoester, carbamate lactone, inorganic acid ester
(4). Stable hydroxyl compound sugar, formaldehyde, diphenylethanedione, benzophenone, dichloroacetaldehyde
(5). Acetal, ether formal, diethyl ether
(6). Hydrocarbon saturated and unsaturated aliphatic and aromatic compounds
(7). Acid anhydride and acid halide acetic anhydride, benzoyl chloride
(8). Halides
(9). Hydrogen peroxide, dialkyl peroxide
(10). Nitrogen compounds amine, amine, nitrile
(11). Sulfur-containing compounds sulfide, thiocyanate, sulfide, sulfonate, sulfurate, and the main organic and inorganic compounds that cannot be directly measured are shown below.
1. Inorganic compounds
(1). Quantitative reaction of metal hydroxide and oxide with Fischer reagent
(2). Carbonate and acid carbonate
(3). Lead acetate, incomplete ammonia reaction
(4). Reaction of boric acid and oxide with iodine (5). Non-quantitative reaction of chromic acid and dichromic acid
(6). Cobalt ammonia complex
(7). Quantitative reduction of copper chloride and sulfate by HI
(8). Quantitative reaction of ferric chloride and Fischer reagent
(9). The reaction of hydrogen sulfide and sodium sulfide is uncertain
(10). Partial reaction of hydroxylamine and Fischer reagent
(11). Incomplete reaction of phosphomolybdic acid
(12). Quantitative reaction of methylsilanol (R3SiOH) with Fischer reagent
(13). Thiosulfate
(14). Tin dichloride Ibid.
(15). The reaction of zirconium dichloride is incomplete
2. Organic compounds
(1). Active carbonyl compounds form acetals
(2). Peroxide compound reacts with SO2 in reagent
(3). Quantitative oxidation of ascorbic acid by iodine
(4). Mercaptan
(5). Quantitative reduction of quinone by HI
(6). Reduction of diacyl peroxide by HI
(7). DimethyloLnred condensation
The following points can be drawn from the above:
1. Karl Fischer water measurement method is suitable for the determination of the water content in many inorganic compounds and organic compounds.
2. Due to the difference in the properties of compounds, it can be divided into two categories: direct measurement and non-direct measurement. Therefore, when analyzing water in a compound, analysts are required to first consider which type it belongs to. If it is the latter, and direct measurement is used, it will cause a large measurement error or cannot be measured at all.
3. If you want to measure water in a compound that cannot be directly measured, you must use appropriate methods to eliminate various interference factors to achieve the purpose of correct determination.
(2). Calibration material of the instrument
Karl Fischer titrators usually use methanol-water standard solution, aqueous sodium tartrate, distilled water, saturated water toluene and other substances as standards to verify the reliability of the method. Water-containing sodium tartrate is a commonly used water-containing standard substance, with a theoretical water content of 15.66%, a weight loss of 15.65 ± 0.02% when heated at 105 ° C, and long-term exposure to air with a humidity of 20 to 70%, and a weight gain of 0.01 to 0.09% . The calibration results with toluene and pure water containing saturated water are also satisfactory. Of course, the simplest method is to use methanol-water standard solution.
(3) Sampling and sampling volume
When doing analysis and sampling, the representative sample after mixing should be taken as much as possible, and the presence of free moisture at the bottom of the container should be observed. When using a syringe to draw a sample, the extraction speed should not be too fast, otherwise air may enter the syringe to form bubbles, causing sample injection errors. Before the analysis, if it is found that the sample and the container have opalescence, or when tiny water droplets are deposited on the bottle wall, it must be analyzed by ethylene glycol extraction. The specific method is as follows: add a third of the sample to the pre-dried fine-mouthed bottle and seal it tightly, weigh it to 0.1 g on an industrial analytical balance, and then weigh in 2 to 3 times the weight of ethylene glycol and shake vigorously 15 After seeding, static layering, using a syringe to draw 0,25 ~ 1.0mL ethylene glycol through the sample layer, determine its water content, and also determine the original water content of ethylene glycol. After the analysis is completed, the sample in the bottle is poured out, washed and dried, and weighed to 0.1 g on the balance.According to the difference between the three weighings above, the weight of the sample and ethylene glycol can be determined to obtain the sample. Water content.
Before injecting the sample, first clean the syringe with the test solution for 5 to 7 times, and then determine the size of the sample according to the water content of the sample. Usually, the sample of <0.1 to 5 mL is drawn according to the sample volume specified in Table 3. Table 3 Sampling volume reference data Sample water content (ppm) Sampling volume (mL) 0-102-510-1001-2100-10000.1-1> 1000 <0.1 From Table 3, it can be seen that the material with large water content has a small sampling volume On the contrary, the sampling volume should be large, otherwise a large measurement error will occur. At the same time, pay special attention to whether there are small bubbles in the syringe during injection to prevent serious measurement errors. (D). Measurement accuracy The Karl Fischer titration method measures the water content of a substance in a wide range from a few ppm to 100%. The accuracy requirement is determined according to the water content. It is usually required that the difference between the two results and the arithmetic mean of the parallel determination should not be greater than the following values: water content (ppm) Allowable difference 1-10 ± 1 ppm 10-50 arithmetic mean ± 10%> 50 arithmetic mean ± 5% in progress During the analysis, the arithmetic mean of the two determination results is taken as the analysis result.
(5) Several reasons that affect the measurement accuracy
In addition to the above-mentioned determination of the nature of the sample, the method of determination, the selection of the calibration substance, the sampling method and the size of the injection volume affect the measurement accuracy, it is necessary to pay attention to the following issues in order to ensure the measurement accuracy.
1. Because Karl Fischer titration reagent is easy to absorb moisture, it is required that the titration tube and titration cell (measurement cell) of the titrant delivery system adopt a better sealing system. Otherwise, long-term instability and serious errors at the end point will be caused by hygroscopic phenomena.
2. The titer of Karl Fischer reagent is determined according to the water content of the test solution. When measuring a test solution with a large water content, the titre of Karl Fischer reagent should be selected to be larger, so that the measurement speed can be accelerated under the premise of ensuring the measurement accuracy (<5%). However, when the water content of the test solution is small, the titre of Karl Fischer reagent should be selected to be smaller and the minimum reading of the burette is smaller, otherwise a larger measurement error will occur. If the minimum reading of the burette is 0.01mL and the titer of Karl Fischer reagent is 2.5mg / mL, the error of one drop of the reagent will produce a measurement error of 0.025mg (25ppm). If the titre of the reagent is 1.00mg / mL, a little error of the reagent will produce a measurement error of 0.015mg (15ppm).
3. Karl Fischer titration method for determining the end point of water is:
(1). The visual method of relying on human vision to observe the color change of the solution;
(2). The permanent stop endpoint method (hardware titration) that depends on observing the ammeter deflection mutation to a certain value and stable for a period of time such as 60 seconds as the end point of titration (hardware titration);
(3). Based on the permanent stop end point method, also known as the dead stop end point method (deadstop end-point method), microcomputer automatic control software titration three methods. The visual end point method is the easiest way to indicate the end point, and the indicator system device in the titrator can be omitted, and a satisfactory measurement result can be obtained in constant titration, but in the determination of substances below milligram equivalent, this method Sensitivity and accuracy are relatively poor, and generally more sensitive electrochemical methods are used. The second and third methods are both electrochemical methods, which have the advantages of being fast, sensitive, and highly accurate, and easy to realize automation.They can usually measure a few ppm to tens of percent of moisture in various samples. .
4. Delivery of titration reagents
The structure and position of the head is also a very important factor in titration error. It is usually required that the inner diameter of the sending titration head and the titration head are made very fine, the purpose is to prevent the titration agent from hanging and ensure the measurement accuracy. When the titration head is inserted into the sample solution, a chemical reaction may occur at the liquid boundary of the titration head and affect the measurement accuracy.
5. Stirring should be full and even during the titration. When titrating sample solutions with higher viscosity, more attention should be paid to the full and consistent stirring, including the consistency of the speed of the magnetic stirrer and the height of the liquid level in the titration tank.
6. When injecting the sample, it is necessary to prevent the syringe head from being polluted by the outside world and affecting the measurement results, such as the operator exhaling and the contamination when wiping the syringe head. At the same time, it is necessary to prevent the loss of the sample during injection, such as hanging drops on the syringe head and splashing on the measurement cell wall or the electrode rod.
7. A dryer should be installed at the inlet of the Karl Fischer reagent bottle to prevent the reagent from absorbing moisture in the air and reducing the titer of the reagent to cause serious measurement errors.
8. In the process of Karl Fischer titration, sometimes the end point phenomenon occurs, that is, the end point is reached early, resulting in a low measurement result. Especially when measuring samples with low concentration of water content, the influence is greater, and even the measurement cannot be performed. This is mainly because the oxygen in the air oxidizes the iodine ions in the titration cell to iodine, thereby reducing the consumption of reagents. Sunlight will also obviously promote the oxidation reaction of oxygen and iodide ions, and the reagents should be protected from light. In addition, the composition of the reagent and the operating environment have a certain influence on the speed of this reaction. Such as Karl Fischer reagent excessive sulfur dioxide, reagent impure, too high water content of the configuration reagent, etc. are prone to end point advance phenomenon.
9. Karl Fischer method will produce sulfuric acid in water reaction. When its concentration is higher than 0.05%, reverse reaction may occur, affecting the measurement result. Pyridine can be combined with the acid produced by this reaction to ensure that the chemical reaction proceeds in one direction. In the titration and measurement of water, if there is no methanol coexisting, water or any other compound containing active hydrogen can replace the methanol intermediate compound to react, which will disturb the stoichiometry of the chemical reaction and make this reaction have no special selectivity for water . Therefore, it is necessary to pay attention to whether there is sufficient amount of pyridine and methanol in the reagent and titration base solution during the measurement.
10. When using the Karl Fischer method to determine the water content of the sample, pay attention to whether there is a substance in the sample that can be generated with the Karl Fischer reagent. If such substances should be taken corresponding measures to be satisfied the result of. For example, active aldehydes and ketones react with methanol in Karl Fischer reagent to produce acetals and ketals and water consumes iodine, making the titration reaction without an end point. Sometimes when analyzing moisture in ketone-containing samples, reducing the amount of methanol in the reagent and increasing the pyridine content can give satisfactory results. However, this method is not suitable for aldehyde-containing compounds. Someone once used pyridine as a solvent to reduce the ratio of acetal formation, and obtained more reliable analysis results. Metal oxides and hydroxides can also react with HI to form water, and the water in the sample can be separated and extracted by xylene azeotropic distillation or vaporization carrying method, and then measured.
11. Those who can be reduced by iodine, such as mercaptan and hydrogen sulfide, can be reduced by iodine to make the water analysis result higher. It can be removed by addition reaction with olefin.
12. Those who can oxidize iodide to iodine will be reduced to hydroquinone. Such as inorganic compound peroxide, chromate, divalent copper and ferric iron salts can produce such a reaction, making the measurement error.
13. Some weak oxo acid salts, such as carbonate and borate, mainly react with HI to generate water and interfere with the determination. Inorganic acids and acidic oxides do not interfere with the determination. When ammonia is directly titrated with Karl Fischer reagent, nitrogen iodide will be formed, and excess acetic acid can be added before titration to eliminate this interference.
14. Ferric chloride and the active chlorine contained in the reagents, such as dichloroisocyanate can be reduced by HI in Karl Fischer reagent. This interference can be eliminated by pretreatment of pyridine and sulfur dioxide and methanol solution. Carbon tetrachloride contains about 1% of free chlorine isocyanate potassium salt. When the content is in the range of 0.001% to 0.1%, it can be measured by the treatment of used methanol, pyridine, and sulfur dioxide solution.
15. Silanol and Karl Fischer reagents also have quantitative reactions. This interference can be prevented by dilution with high molecular alcohol and pyridine. The Karl Fischer titration method is an important and sensitive chemical analysis method, but in addition to a very good measuring instrument, there must be no interfering substances in the measured substance, according to the moisture content of the substance Determine the appropriate sample injection amount, overcome various factors that affect the measurement accuracy, and operate carefully to obtain good measurement results.
These Kitchen Gadgets are carefully designed and created to make every kitchen professional, yet fun and fashionably colorful.The highly versatile spatulas can be used as a Scraper Spatula, Icing Spatula, Baking Spatula, Jar Spatula, BBQ Silicone Spatular and even as a Mixing Spoon.Looking at the above scenario and qualities of the Realsheen Spatula Set, there is no doubt you will get your money's worth when you consider purchasing your Baking Tools for use in your kitchen.To see just how much these Kitchen Utensils could make your life easier and your kitchen more colorful, ORDER YOURS NOW! And why not add these great baking supplies to your gift or wedding registry?As a Bakeware Set it would always make for the perfect gift!
Baking Spatula
Baking Spatula,Baking Tools,Silicone Baking Spatulas,Food Garde Baking Spatula
RealSheen Lifestyle Enterprise Limited , https://www.realsheen.com