Classification and common principles of moisture analyzers

1. Karl Fischer moisture analyzer:

The Karl Fischer method, referred to as the Fischer method, is a capacity-splitting method for the determination of moisture proposed by Karl Fischer in 1935. The Fischer method is the most specific and accurate method for water among various chemical methods for determining the moisture content of substances. Although it is a classic method, it has been improved in recent years to improve accuracy and expand the measurement range. It has been listed as a standard method for moisture determination in many substances.

The Fischer method is an iodometric method, and its basic principle is that when using iodine to oxidize sulfur dioxide, a certain amount of water is required to participate in the reaction:

 I2 + SO2 + 2H2O = 2HI +H2SO4

The above reactions are reversible. In order to make the reaction move in the positive direction and proceed quantitatively, an alkaline substance must be added. Experiments have shown that pyridine is the most suitable reagent, and pyridine can also combine with iodine and sulfur dioxide to reduce their vapor pressure. Therefore, the reagent must be added to methanol or another solvent containing active OH groups to convert pyridine sulfate anhydride into stable pyridine methyl hydrogensulfate.

2. Infrared moisture meter:

Infrared heating mechanism: When far-infrared rays radiate to an object, absorption, reflection and transmission can occur. However, not all molecules can absorb far-infrared rays, only those polar molecules that show electricity can work. Water, organic substances, and high molecular substances have a strong ability to absorb far infrared rays. When these substances absorb far-infrared radiation energy and make their molecular and atomic vibration and rotation frequency consistent with the frequency of far-infrared radiation, it is very easy for molecules and atoms to resonate or rotate, resulting in greatly increased motion, which is converted into The heat can raise the internal temperature, so that the material can be softened or dried quickly.

The general heating method is to use heat conduction and convection, which needs to be transmitted through a medium, which is slow and consumes a lot of energy, while far-infrared heating uses heat radiation without medium transmission. At the same time, since the radiant energy is directly proportional to the fourth power of the temperature of the heating element, it not only saves energy but also has high speed and high efficiency. In addition, far-infrared rays have a certain penetrating ability. Because the heated and dried material absorbs far-infrared radiation energy at a certain depth inside and surface molecules at the same time, it produces a self-heating effect, which evaporates the solvent or water molecules and generates heat evenly, thereby avoiding The deformation and qualitative change caused by different degrees of thermal expansion keep the appearance, physical and mechanical properties, fastness and color of the material intact.

The infrared moisture analyzer is mainly determined by the infrared radiation heater and electronic balance to determine its accuracy and stability.

Infrared radiation heater: tungsten vacuum tube can radiate near-infrared rays, silicon carbide is a long-wavelength far-infrared radiation heater, and quartz glass and ceramic infrared heaters can radiate mid-infrared rays.

Infrared moisture meter is an infrared moisture meter that is heat-dried and mass-measured, which is very similar to the “drying loss method” of the recognized standard measurement method of moisture measurement standards. The “drying loss method” of the recognized standard measurement method is also called (105°C 5-hour method), (135°C 3-hour method), etc., by placing the sample in a dryer and heating and drying for a long time, To accurately measure the mass change before and after drying, so as to calculate the moisture content. To this end, it is necessary for the assay personnel to be very proficient in equipment and technology. Since the measurement takes a long time, it is difficult to quickly measure a large number of samples. Therefore, for high-accuracy determination of a variety of samples, there is no need to think of anything other than an infrared moisture meter. Although there are some other electrical and optical measurement methods, they all belong to special instruments with limited measurement objects. From the perspective of versatility, they are far inferior to infrared moisture meters.

Scope of application: It can measure food-related items such as grains, starch, flour, dry noodles, brewed products, seafood, processed fish products, processed edible meat products, seasonings, desserts, hearts, dairy products, dry foods, vegetable oils, and pharmaceuticals , ore sand, coke, glass raw materials, cement, chemical fertilizers, paper, pulp, cotton, various fibers and other industrial products.

3. Dew point moisture meter:
   The dew point moisture meter is easy to operate, the instrument is not complicated, and the measured results are generally satisfactory. It is often used for the determination of trace moisture in permanent gases. However, this method has a lot of interference, and some easy-to-cool gases, especially when the concentration is high, will condense before water vapor and cause interference.

4. Microwave moisture meter:
   The microwave moisture analyzer uses the microwave field to dry the sample, which accelerates the drying process. It has the characteristics of short measurement time, convenient operation, high accuracy, and wide application range. It is suitable for grain, paper, wood, textiles, and chemical products. The moisture determination in powdery and viscous solid samples can also be applied to the determination of moisture in petroleum, kerosene and other liquid samples.

5. Coulomb moisture meter:
   Coulometric moisture analyzers are commonly used to measure the moisture contained in gases. This method is easy to operate and responds quickly, and is especially suitable for the determination of trace moisture in gas. If it is determined by general chemical methods, it is very difficult. However, the electrolysis method is not suitable for the determination of alkaline substances or conjugated dienes.