Karl Fischer Coulometric Moisture Analyzer Principle and Application Range

Shanghai Jiashi Electronic Technology Co., Ltd. produces hand-held moisture meters, infrared moisture analyzers, halogen moisture meters, online infrared moisture meters, online microwave moisture meters, solid moisture meters, and liquid moisture meters.

In the national economy, petrochemical products occupy an important position. There is a wide variety of products in this category, but most of them have an important indicator that must be tested - moisture content. How to choose which method to use in the test, how to select the instrument, and how to determine its conformity is a major event in many laboratory tests. As a type of measuring instrument for measuring the moisture content in substances, there are currently many kinds of instruments such as a drying method, a Karl-Fischer (Karl-Fischer) capacity method, and a Cartesian Coulomb method.

I. INTRODUCTION There are many ways to determine the moisture content in a substance. Now, several common methods are used to make a simple comparative analysis of their economical and accuracy.

1 Drying Advantages: The instrument is low in price and high in precision.

2 Spectral, Chromatographic Advantages: Can be measured to 10-6 levels. Disadvantages: expensive equipment; high environmental requirements; long preparation time (a few hours); not conducive to product process control.

3 Cartesian capacity method Advantages: There are many types of tests, and some special substances can be determined under specific reagent conditions (such as ketones and aldehydes) relative to the Coulometric Coulomb method. Disadvantages: Only 10-4 levels can be measured under optimum conditions; consumables (reagents) are large; measurement time is long. 4 advantages of the Coulometric Coulomb method: the instrument price is medium; supplies less; can be measured to 10-6; short time, the general substance in the mastery of injection volume under the premise of the use of Boshan Shandong Analytical Instrument Factory WS series of automatic micro- Moisture analyzer (hereinafter referred to as the same industry instrument) can complete the measurement within 60 seconds, which is the best method for process control and arbitration. Disadvantages: Some substances with side reactions such as ketones and aldehydes cannot be measured.

For most substances, choosing a Karlsruhl Coulomb apparatus as a measure of moisture content for quality control is an economical and accurate method.

Second, the Karl Collen instrument principle In 1935, Karl Fischer first proposed the use of capacity analysis method for the determination of moisture, this method is GB6283 "Determination of moisture content in chemical products" in the visual test. The visual method can only measure the moisture of a colorless liquid substance. Later, it was developed into the electricity method. With the development of science and technology, the Coulomb method was then combined with the volume method to introduce the Coulomb method. This method is the test method in GB7600 "Transformer Oil Content Determination (Coulomb Method)". The current classification of visual and coulometric methods is collectively referred to as volumetric method. The Karfi method is classified into two methods: the Karl Fischer method and the Coulometric Coulomb method. Both methods have been used by many countries as standard analytical methods to correct Other analytical methods and measuring instruments.

2. The Karlsson Coulometric method for determining moisture is an electrochemical method. The principle is that when the Carter's reagent in the electrolysis cell of the instrument is in equilibrium, the aqueous sample is injected. The water participates in the redox reaction of iodine and sulfur dioxide, and in the presence of pyridine and methanol, pyridine hydroiodide and pyridine methyl sulfate are generated. The iodine consumed is generated at the anode by electrolysis, so that the redox reaction continues until the water is completely consumed. According to the Faraday's law of electrolysis, the electrolysis of iodine is proportional to the amount of electricity consumed during electrolysis. The reaction is as follows:

H2O I2 SO2 3C5H5N→2C5H5N•HI C5H5N•SO3

C5H5N•SO3 CH3OH→C5H5N•HSO4CH3

In the electrolysis process, the electrode reactions are as follows:

Anode: 2I--2e→I2

Cathode: I2 2e→2I-

2H 2e→H2↑

It can be seen from the above reaction that 1 mole of water is required for 1 mole of iodine to oxidize 1 mole of sulfur dioxide. Therefore, it is the equivalent reaction of 1 mole of iodine and 1 mole of water. That is, the electricity quantity of electrolytic iodine is equivalent to the quantity of electrolyzed water. Electrolysis of 1 mole of iodine requires 2×96,493 coulombs of electricity. Electrolysis of 1 millimole of water requires 96493 milli-coulombs of electricity.

The moisture content in the sample is calculated according to formula (1):

Where: W --- the moisture content of the sample, μg;

Q---electrolytic power, mC;

18 --- the molecular weight of water;

Third, the scope of application of Coulometric Coulomb apparatus The Coulometric Coulomb apparatus can be applied to a variety of organic and inorganic moisture determination, but due to the differences in the nature of various compounds, only in the Cartesian reagent without side effects without interference In this case, the Coulometric Coulomb method is an exclusive method. The principle is (1) the side reaction cannot produce water. (2) The sample also cannot consume iodine or release iodine. There are eight types of substances that mainly have side reactions and disturbances.

1. Salts, hydroxides, and oxides. E.g:

Na2CO3 HI→NaI CO2 H2O

Ca(OH)2 H2SO4→CaSO4 H2O

MgO HI→MgI2 H2

There are also some substances that can also cause side reactions such as: Ag2O, HgO, MnO2, PbO, PbO2 and ZnO

2. Both ketone and aldehyde compounds combine with methanol in the Carter reagent to form ketals and acetals and release water. The use of the Coulometric Coulomb method is generally not easy to determine. 3. Strong acid and strong acid will react with methanol in Carter's reagent to release water 2CH3OH H2SO4→(CH3O)SO2 H2O

CH3OH HOOCH→CH3O-OCH H2O

4. The silanol/silicone terminal silanol group and methanol in the Carter's reagent undergo a lipidation reaction to produce water.

6. Metal peroxide reacts with Carter's reagent to produce water.

7. Substances that consume iodine These substances react with iodine in the Carter reagent. Causes the determination of high moisture content, such as iron salts, ketone salts, thiosulfur salts and so on.

8. Strong oxidants The reaction of these substances in the Carter's reagent generates iodine as a simple substance, resulting in low moisture content.

In industrial products, the eight categories listed above are only a small part of a wide variety of substances, and most of them can be measured by the Coulometric Coulomb method. The following substances are typical.

Fourth, the use of Carlsberg Coulomb instrument experience and experience 1. Control of sample volume The reagent volume in the electrolysis cell of the Cartesian Coulomb apparatus is currently around 200 ml. Replacing a pool of reagent can be repeated many times. Considering many factors such as test time, reagent consumption, and product standards, the amount of sample to be injected should be controlled to be the best between 10 μg and 30 μg. When less than 10 μg of water, the deviation may increase when taking the arithmetic mean of two parallel measurement data as the reported value, and the reagent consumption is large and uneconomical when it exceeds 30 μg of water. The specific amount of sample injection depends on product standards. Levels 10-6 can be used with a 1ml sampler, and grades 10-5 can be used to decrement the sample volume one by one. At present, the national standard for the moisture content of liquid products is mg/l. Huakun instrument has a direct calculation function. When the user inputs the sample volume, the measurement result will automatically calculate the report value in units of milligrams per liter. When the inspection process of the company's designated product is completed, the injection volume is a certain value. Another function of the same industry instrument is the power-off storage function, that is, if the user enters the sample volume at one time after the inspection process, if it does not need to be changed, it will never disappear after the power is turned off, and the operation is extremely simple. It is worth mentioning that Huakun Instruments is the only instrument in marketing and user-friendly instruments that has accuracy requirements of less than 10 μg of water. The technical difficulty of the Coulometric Coulomb apparatus lies in the low end of the measuring range. The low end is accurate and the high end is definitely accurate. Using the same instrument can reduce reagent consumption as much as possible. Reduce the measurement time as much as possible, reduce the work intensity, make the work comfortable and achieve the purpose of accurate measurement.

2. Similar dissolve principle Moisture in the material does not exist in one form, common free water, dissolved water and other forms. And several forms of water exist almost simultaneously. The relatively small molecular weight of water is inevitably contained by the molecules of the petrochemical product. In contrast, free water is better measured. When the sample is injected, the rapid increase of the electrolysis rate rapidly decreases at about 30 seconds (the same time, the electrolysis speed is displayed when the instrument of the same company is measured, and the dynamic change during the measurement can be intuitively judged). The water content during this time is mainly free water in the material. However, the measurement of dissolved water takes a long time. It is necessary to rely on the force of the molecular bonds in the reagents to dissolve the substances before measuring the precipitated moisture. Huakun instrument measurement when the electrolysis rate fell below 1μg/s and did not reach the end, this time is mainly measured dissolved water (about 30 seconds). If there is more dissolved water, the simple method is to control the injection volume. Reducing the injection volume causes the force of the molecular bond in the reagent to rapidly open the sample molecular cluster in a short time so that the dissolved water is quickly precipitated. Another approach is to use the principle of similar dissolution of substances to quickly dissolve the sample. In an experiment I encountered in practice to illustrate: Determine the moisture content of vinylidene carbonate, according to product standards, the material moisture content index is 10-6, below 15 °C was colorless solid crystal, Above 15°C, it gradually dissolves into a colorless liquid with little viscosity. When the room temperature is less than 20°C, it gradually returns to a solid crystal state. When the moisture content was measured at room temperature, it was just near the critical point of the solid-liquid state transition of the material. When the first sample was injected (1 ml), the measurement time was 65 seconds, the second sample measurement time was 130 seconds, the third sample measurement time was 260 seconds, and the measurement time was sequentially increased. As a process control, this is obviously not desirable, and the longer the time, the greater the deviation from the true value of the measurement result. Domestic manufacturers and users of the product can not solve this problem, can only rely on the loss of reagents to solve. A sample pool reagent, the process is very cumbersome and uneconomical. After repeated thinking, we adopted solubilization measures based on the principle of similar dissolution of chemical products. After the reagents are equilibrated, 1 ml of tetrahydrofuran is injected first, and after re-equilibration, the sample is injected. The results of the test were 60 seconds for the first sample, and slightly longer but not longer than 70 seconds for the subsequent samples. The problem is solved. The reason is that at room temperature, Carragy's reagent alone cannot completely dissolve vinylene carbonate, and it is not able to completely deplete the water. The dissolved water of each sample is accumulated in turn, resulting in an increase in time, and the solution is solved using the principle of similar dissolution. There are not many reports on this kind of problems and they can be used as reference. This case also illustrates a problem, as long as it is a material without side effects using the Coulometric Coulomb apparatus to determine the moisture can always find a solution.

3. The main components in the Coulometric Coulometric electrolysis cell are the electrolysis electrodes. The electrolysis electrodes currently used in the country all have diaphragm electrodes. Diaphragm, also called semipermeable membrane, is a microporous permeation membrane made of ceramic material. When the membrane is used once, the Carter's reagent must remain in the micropores. Carrageenan is a kind of water-absorbing substance. Once the diaphragm is exposed to the air for a long time, it is difficult to reach the equilibrium point when it is used again. The traditional method is to dry it in an oven or dry it with a hair dryer. This wastes too much time. Our experience is to inject iodine (brown-black) reagents into the electrolysis electrode first, then use a dry suction ball to force the reagent through the membrane, neutralize the moisture in the membrane first, and squeeze the reagent into electrolysis. Pool; neither waste nor save time, without losing a good way. This method has not been seen by any manufacturer in the specification, and is proposed here for reference only.

V. CONCLUSIONS In summary, the determination of the moisture content of substances in several aspects can be summarized as follows:

1. When a single product manufacturer selects an assay method, it first looks at whether the product has side effects on the Carter reagent, and there is interference. If no interference occurs, the preferred protocol is the Coulometric Coulomb apparatus. Especially as a process control, it is both economical and accurate.

2. The integrated laboratory unit (arbitration agency) purchased a Karl Fischer volumetric instrument and a Karlsruhl Coulomb counter instrument to cover the entire product. At present, domestic instruments are not inferior to imported products in terms of their quality and reliability. The price of two units is far less than the price of one imported instrument. It is worth noting that it is necessary to purchase instruments that have a manufacturing license for measuring instruments and buy them with confidence and peace of mind.

3. At present, the quality of the same industry products in the same industry is superior, the best price performance ratio, and the lowest repair rate. Inter-company companies can also develop inspection processes for users, which are both convenient and safe to use.

4. The principle of similar dissolving principle is very valuable for the user in the application of Karlsruin Coulomb's method. As long as the substance has no side effects and no interference to the reagent, it can always find a solution. Prepare viable, economical and accurate inspection processes.

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