Karl-Fischer titration can be applied to the determination of aqueous in a variety of organic and inorganic materials. Due to the differences in the properties of various compounds, they can be classified into two types, which can be directly measured and cannot be directly measured.
The main organic and inorganic compounds that can be directly determined are as follows:
Inorganic compound
(1). Organic acid salt 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)3·KSO4, 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 monools, polyols, phenols
(3). Ester carboxylate, orthoester, carbamate, inorganic acid ester
(4). Stable hydroxy compound sugar, formaldehyde, diphenylethylenedione, benzophenone, dichloroacetaldehyde
(5). Acetal, ether formal, diethyl ether
(6). Hydrocarbon saturated and unsaturated aliphatic and aromatic compounds
(7). Anhydride and acid halide acetic anhydride, benzoyl chloride
(8). Halide halides
(9). Peroxygen compound hydrogen peroxide, dialkyl peroxide
(10). Nitrogen-containing amines, amines, nitriles
(11). Sulfur-containing compound sulfide, thiocyanate, thioether, sulfonate, and disulfide urethane The main organic and inorganic compounds which cannot be directly determined are shown below.
Inorganic compound
(1). Quantitative reaction of metal hydroxides and oxides with Fischer reagents
(2). Carbonate and acid carbonate
(3). Lead acetate, basic ammonia reaction is not complete
(4). Reaction of boric acid and oxide with iodine (5). Non-quantitative reaction of chromic acid and dichromic acid
(6). Cobalt ammonia complex
(7). Copper chloride and sulfate are quantitatively reduced by HI
(8). Quantitative reaction of ferric chloride with Fischer reagent
(9). Hydrogen sulfide and sodium sulfide reaction is uncertain
(10). Hydroxylamine partially reacts with Fischer reagent
(11). Phosphomolybdic acid reaction is incomplete
(12). Quantitative reaction of methyl silanol (R3SiOH) with Fischer reagent
(13). Thiosulfate
(14). Tin dichloride is the same as above
(15). The reaction of zirconyl dichloride is incomplete
2. Organic compounds
(1). Active carbonyl compounds form acetals
(2). Reaction of peroxygen compounds with SO2 in reagents
(3). Ascorbic acid is quantitatively oxidized by iodine
(4). Mercaptan
(5). 醌 quantitative reduction by HI
(6). Diacyl peroxide is reduced by HI
(7).DimethyloLnred condensation
From the above, the following points can be drawn:
1. The Karl Fischer method is suitable for the determination of water content in many inorganic and organic compounds.
2. Due to the difference in the nature of the compounds, it can be divided into two types: direct measurement and direct measurement. Therefore, analysts are required to first consider the type of water in a compound, and if it is the latter, and use direct measurement, it will produce a large measurement error or can not be measured at all.
3. If water is to be measured in a compound that cannot be directly measured, appropriate methods must be used to eliminate various interference factors for the purpose of correct measurement.
(2). Calibration materials of the instrument
The Karl Fischer titrator is usually calibrated with a methanol-water standard solution, aqueous tartaric sodium, distilled water, saturated water toluene and the like as a standard. Aqueous sodium tartrate is a commonly used aqueous standard material with a theoretical water content of 15.66%, a heating weight loss of 15.65±0.02% at 105 °C, and a long-term exposure to air with a humidity of 20-70%. The weight gain is 0.01-0.09%. . The calibration results with toluene and pure water containing saturated water were also satisfactory. Of course, the simplest is to use a methanol-water standard solution.
(3) Sampling and sampling volume
When doing the analytical sampling, try to take a representative sample after mixing, and observe the existence of free water at the bottom of the container. When the sample is taken with a syringe, the extraction speed should not be too fast, otherwise the air may enter the syringe to form a bubble, causing an injection error. If the sample and the container are found to be opaque before analysis, or if there are tiny drops of water on the wall of the bottle, they must be analyzed by ethylene glycol extraction. The specific method is as follows: a pre-dried fine-mouth bottle is added to one-third of the sample and sealed, sealed on an industrial analytical balance to 0.1 g, and then weighed 2 to 3 times the weight of ethylene glycol to shake vigorously 15 After seeding, after static stratification, 0,25-1.0 mL of ethylene glycol was sucked through the sample layer with a syringe to determine the water content, and the original water content of the ethylene glycol was also measured. After the analysis is completed, the sample in the bottle is poured off, washed and dried, and weighed to 0.1 g on the balance. According to the difference between the above three weighings, the weight of the sample and the ethylene glycol is determined, and the sample can be obtained. Water content.
Before the injection, the syringe is first cleaned 5 to 7 times with the test solution, and then the sample size is determined according to the water content of the sample. Usually, the sample of 0.1 to 5 mL is sampled according to the syringe sample specified in Table 3. Table 3 Sample amount Reference data Sample water content (ppm) Sample amount (mL) 0-102-510-1001-2100-10000.1-1>1000<0.1 From Table 3, it can be seen that the sample with large water content is small. On the contrary, the sampling amount is large, otherwise a large measurement error will be generated. At the same time, special attention should be paid to the presence of small bubbles in the syringe during injection to prevent serious measurement errors. (IV). Determination accuracy The Karl Fischer titration method determines the range of water content of a material from a few ppm to 100%. The accuracy requirement is determined by the water content. It is generally required that the difference between the two results and the arithmetic mean should not be greater than the following values: water content (ppm) allow the difference 1-10 ± 1 ppm 10-50 arithmetic mean ± 10% > 50 arithmetic mean ± 5% in progress In the analysis, the arithmetic mean of the two measurement results was taken as the analysis result.
(5). Several reasons that affect the accuracy of measurement
In addition to the above-mentioned properties of the test sample, the method of measurement, the selection of the calibration material, the sampling method and the size of the injection amount affect the measurement accuracy, the following problems must also be noted to ensure the measurement accuracy.
1. Since the Karl Fischer titration reagent readily absorbs moisture, a better sealing system is required for the burette and titration cell (measurement cell) of the titrant delivery system. Otherwise, the end point is unstable and severely affected due to hygroscopicity.
2. The titer of the Karl Fischer reagent is determined by the amount of water in the test solution. When measuring a test solution with a large water content, the titer of the Karl Fischer reagent should be selected to be larger, so that the measurement speed can be accelerated while ensuring the measurement accuracy (<5%). However, when the water content of the test solution is small, the titer of the Karl Fischer reagent should be selected to be smaller and the minimum reading of the burette is smaller, otherwise a large measurement error will be generated. If the minimum reading of the burette is 0.01 mL and the titer of the Karl Fischer reagent is 2.5 mg/mL, a drop error of the reagent will result in a measurement error of 0.025 mg (25 ppm). If the titer of the reagent is 1.00 mg/mL, a slight error in the reagent will result in a measurement error of 0.015 mg (15 ppm).
3. The Karl Fischer titration method for determining the endpoint of water is:
(1). Visual inspection of the color change of the solution by human visual observation;
(2). By observing the ammeter deflection to a certain value and stabilize for a period of time such as 60 seconds as the end point of the titration end point method (hardware titration);
(3). Based on the permanent stop point method, also known as the deadstop end point method (deadstopend-point method), the microcomputer automatically controls the software titration three methods. The visual end point method is the simplest method for indicating the end point. The indicating system device in the titrator can be omitted, and a satisfactory measurement result can be obtained in the constant titration, but in the measurement of the substance below milliequivalent, the method is Sensitivity and accuracy are relatively poor, and generally more sensitive electrochemical methods are used. The second and third methods are electrochemical methods, which are fast, sensitive, and highly accurate, and are easy to automate. Generally, several ppm to several tens of percent of water can be measured in various samples. .
4. Delivery of titration reagents
The structure and position of the head is also a very important factor in the titration error. It is usually required to send the inner diameter of the titration head and the titration head to be very fine, in order to prevent the dripping of the titrant 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 to affect the measurement accuracy.
5. Stirring at the time of the titration should be sufficient and uniform. When titrating a sample solution with a large viscosity, it is necessary to pay attention to the sufficient and consistent stirring, including the speed of the magnetic stirrer and the height of the liquid level in the titration cell are substantially the same, so that better measurement accuracy can be obtained.
6. 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 cleaning the syringe head. Also prevent loss of sample during injection, such as hanging drops on the injector head and splashing onto the measuring cell wall or electrode rod.
7. The Karl Fischer reagent bottle inlet should be equipped with a dryer to prevent the reagent from absorbing the moisture in the air and causing a serious measurement error due to the decrease in the titer of the reagent.
8. During the Karl Fischer titration process, there is sometimes a borrowing end point, that is, reaching the end point in advance, resulting in a low measurement result. Especially in the determination of samples with low concentration of water, the effect is even greater and even impossible to measure. This is mainly because the oxygen in the air oxidizes the iodide ions in the titration cell to iodine, thereby reducing the consumption of reagents. The sunlight will also significantly promote the oxidation reaction of oxygen and iodide ions, and the reagents should be protected from light. In addition, the composition of the reagents and the operating environment have a certain effect on the speed of this reaction. For example, in the Karl Fischer reagent, the sulfur dioxide is excessive, the reagent is impure, and the water content of the reagent is too high, and the terminal advance phenomenon is likely to occur.
9. Karl Fischer method will produce sulfuric acid in the water reaction, when its concentration is higher than 0.05%, reverse reaction may occur, affecting the measurement results. Pyridine can combine with the acid produced by this reaction to ensure that the chemical reaction proceeds to one side. In the titration water, if there is no methanol coexisting, water or any other active hydrogen-containing compound can replace the methanol intermediate compound, which will disturb the stoichiometry of the chemical reaction, so that the reaction has no special selectivity for water. . Therefore, it is necessary to note whether there is sufficient amount of pyridine and methanol in the reagent and the titration solution during the measurement.
10. When using the Karl Fischer method to determine the moisture content of the sample, it is necessary to pay attention to whether the sample to be tested has a substance capable of generating water with Karl Fischer reagent. If such a substance is used, corresponding measures should be taken to obtain satisfactory results. the result of. For example, reactive aldehydes and ketones react with methanol in the Karl Fischer reagent to form acetals and ketals with water consuming iodine, so that the titration reaction has no end. Sometimes when analyzing the moisture in the ketone-containing sample, reducing the amount of methanol in the reagent and increasing the pyridine content, satisfactory results can be obtained. However, this method is not suitable for aldehyde-containing compounds, and pyridine has been used as a solvent to reduce the proportion of acetal formation, and a more reliable analysis result is obtained. Metal oxides and hydroxides can also react with HI to form water. The water in the sample can be separated by azeotropic distillation or vaporization of xylene and then measured.
11. Those who can be reduced by iodine, such as thiol and hydrogen sulfide, can be reduced by iodine to make the water analysis result higher. It can be removed by an addition reaction using an olefin.
12. The ability to oxidize iodide to iodine is itself reduced to hydroquinone. Such as the peroxide, chromate, divalent copper and ferric salt of the inorganic compound can produce such a reaction, causing an error in the measurement.
13. Some weak oxyacid salts, such as carbonates, borate mainly react with HI to form water interference assays. Inorganic acids and acid oxides do not interfere with the assay. Ammonia can form iodide by direct titration with Karl Fischer reagent, and excess acetic acid can be added before titration to eliminate this interference.
14. The active chlorine contained in the ferric chloride and the reagent, such as dichloroisocyanate, can be reduced by HI in the Karl Fischer reagent. This interference can be removed by pretreatment of the sample with pyridine and sulfur dioxide and methanol solutions. About 1% of the potassium salt of free chloroisocyanate is contained in carbon tetrachloride, and when it is contained in the range of 0.001% to 0.1%, it can be measured by treatment with used methanol and pyridine, sulfur dioxide solution.
15. Silanol and Karl Fischer reagents also have a quantitative reaction which can be prevented by dilution with high molecular weight alcohol and pyridine. The use of Karl Fischer titration to determine the moisture in a substance is an important and sensitive chemical analysis method, but in addition to a very good measuring instrument, there must be interference with the substance in the measured substance, according to the moisture content of the substance. Determine the appropriate injection volume, overcome various factors affecting the accuracy of the measurement, and carefully operate to obtain good measurement results.
Guangzhou Yafei Makeup Toiletry Co.,Ltd , https://www.yfmakeupbrush.com