Note on the use of trace moisture analyzer:
At present, in the chemical, pharmaceutical and other industries, the Karl-Fischer Moisture Analyzer trace moisture analyzer is generally used for the detection of free water or crystal water in raw materials and some finished products. In the inspection of a large number of imported and domestically produced instruments and inspectors from various industries, the exchange of questions related to the use of the Karl-Fischer Moisture Analyzer was proposed.
1, security protection
The Karl Fischer reagent is mainly a solution consisting of iodine, sulfur dioxide, pyridine and methanol. Among them, sulfur dioxide and pyridine are extremely volatile, which is very harmful to the human body, and should be carried out under good ventilation conditions during operation. Especially when changing reagents, pay attention to exhausting to prevent harmful gases from inhaling the body. Wear protective glasses and latex gloves to prevent harmful reagents from spilling on your eyes and hands. Once the reagents splash on your eyes and your hands should be rinsed immediately with running water, the patient should be sent to hospital for treatment.
However, the actual situation is that some operators have insufficient understanding of the hazard of the reagents. Without any protective measures, the reagents can be poured into and poured out at random, and the odor of the house is ignored. The problem of self-protection awareness needs to be strengthened.
2. Application of reagents
The Karl Fischer reagent requires a high degree of freshness. Pay attention to the date of manufacture when purchasing the Karl Fischer reagent, and buy it as needed. And to protect from light, in order to extend the shelf life.
At present, there is a pyridine-free Karl-Fischer reagent, which solves the problem of pungent odor of the pyridine-containing reagent, but the mutation of the pyridine-containing Karl-Fischer reagent end point is more obvious in the measurement, and the color of the reagent to the end point is It is slightly brownish yellow. According to experience, it can be predicted by the naked eye that the end point is coming, but the mutation of the pyridine-free Karl Fischer reagent end point is not obvious, and the color of the reagent to the end point is dark brown.
The choice between the two depends on the moisture content of the sample and the accuracy requirements of the sample. Pyridine reagents containing pyridine are recommended for samples with low water content and high detection accuracy. Otherwise, a pyridine-free Karl Fischer reagent is used.
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 point, ethylene glycol methyl ether can be used instead of methanol to obtain a more constant titration volume, and the moisture of certain ketone and aldehyde chemical products can be determined without any special technique.
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.
In the methanol water titration (commonly known as blank titration), if the color in the reaction bottle gradually changes from colorless to dark brown, the instrument still has no end point, and it should be considered that the Karl Fischer reagent has failed, that is, the reagent should be replaced.
3, electrode pollution and maintenance
The electrode is a key component of the moisture analyzer . The contamination of the electrode surface can directly lead to the decrease of sensitivity. Some electrodes are used for the analysis of oily samples for a long time. After the surface of the electrode is contaminated by oil, the sensitivity is lowered, which makes the judgment of the electrode to the end point dull. The Karl-Fischer reagent is too much, and the solution color is dark when the end reaction is performed. At this time, the electrode must be cleaned. Although the contaminants on the electrodes are not visible to the naked eye, slow response can be observed, which directly affects the measurement accuracy. Therefore, the electrode must be cleaned after a period of use, but a considerable number of operators have not considered this problem.
When the sensitivity is lowered, the electrode is seriously contaminated. You can wipe the electrode with a little bit of acetone, but you must be careful and wait until the acetone has evaporated. Or immerse the electrode in dilute nitric acid solution for 24 hours, then take it out, rinse with water, and wipe the filter paper. It can also be washed with potassium dichromate solution for 1 minute to activate the electrode. In the special case, if the sample is waiting for analysis, the cleaning electrode time is not allowed. In this case, the electrode contamination event can be solved urgently. Gently rub the ends of the electrode with a very fine sandpaper, and wipe the filter paper to make it work.
If the instrument is not used for a period of time, the Karl Fischer reagents in the pump tube and the liquid path should be completely drained to avoid clogging of the pipeline due to crystallization caused by the evaporation of the reagent; the Karl-Fischer reagent in the reaction flask should also be After the discharge, the electrode is wiped clean. In the detection of the instrument, it is often found that the end of the measurement of the operator's sample, the power supply is off, and the maintenance and maintenance of the instrument and the accuracy of the measurement should be closely related to the problem.
4, Karl Fischer reagent titer (commonly known as water equivalent) calibration
Whether the calibration of the Karl-Fischer reagent's titer is accurate or not is directly related to the accuracy of the sample determination. The environmental conditions of the test are different. The sealing performance of the whole set of the device has a great influence on the variation of the Karl-Fischer reagent titer. Especially for samples that require high measurement accuracy.
The calibration of the titer should in principle be carried out before the daily sample test. The titer can be calibrated with standard materials with a certain water content. Some standard materials are liquid and encapsulated in ampoules. Each time one is consumed, the accuracy is high and the cost is high: some standard materials are solid, have high accuracy, and have high requirements for preservation of standard substances. Once it gets wet, it is troublesome.
The simplest and most practical is the pure water calibration. The amount of water is accurately removed by a micro-syringe. The CSY series micro-moisture analyzer generally takes 10 to 30 microliters of water for calibration, repeats several times in succession, and takes the average value to determine the titer of the Karl-Fischer reagent. However, some laboratory inspectors do not fully understand the meaning of Karl Fischer reagent titer calibration, or irresponsible inertia, even a bottle of reagent from the start of use to a titration calibration until the reagent After that, the sample test in a few months always uses a calibration value of the titer, which obviously has a large error.
The titer of the Karl Fischer reagent is gradually changed with the extension of the use time, and the titer error is also increased with respect to the first day. It can be seen that the initial titration is used to calculate the measured value for several months, and the error to the measured value is large. Therefore, the calibration of the Karl Fischer reagent should be carried out frequently. The time interval for the calibration of the Karl-Fischer reagent titer should be reasonably determined according to the ambient temperature, humidity and the sealing performance of the test chamber, as well as the requirements of the test. To ensure the accuracy of the measurement. The accuracy of the sample measurement is related to the quality of the product.
5 Conclusion
The above discussion shows that the Karl Fischer Moisture Analyzer is a classical method for the determination of trace moisture. Safety protection and proper use and maintenance are important for the accuracy of the measurement.
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