Moisture meter selection

There are many commercially available moisture analyzers, which are divided into the following types according to the test method:

1. Infrared method instruments, small size, wide measurement range, poor accuracy, suitable for the determination of wood, paper and other materials with moisture content of 5% to 90%, simple structure, and low price.

2, Karl Fischer Coulomb method instruments, the main principle: the use of chemical reactions after the change in conductivity calculation, complex structure, large size, the highest measurement accuracy, suitable for determination of moisture content below 100PPm. It is generally used in the determination of chemical and pharmaceutical industries such as anion polymerization that have very strict requirements on moisture, or used in large-scale color printing plants with multiple frequencies, and is more expensive.

3, Karl Fischer volumetric method, the structure is relatively simple, moderate volume and accuracy, suitable for the determination of moisture content 10PPm ~ 10%, generally used in the water has strict requirements of chemical, pharmaceutical and packaging products such as industry, the price from Thousands of dollars to tens of thousands of dollars. For the general flexible packaging industry, when measuring the moisture content of solvents such as ethyl acetate, the Karl Fischer Volumetric Moisture Analyzer fully meets the requirements of 2 to 10 measurements per day and is economical.

Step 2: Difference between volumetric method and Coulomb method Karl Fischer's volumetric method for determination of moisture content: The Karl Fischer volumetric method for determining moisture content is mainly based on the electrochemical reaction: I2+2eó2I- is present in the solution in the reaction cell When I2 and I-, the reaction proceeds at both the positive and negative sides of the electrode, that is, I2 is reduced at one electrode and I- is oxidized at the other electrode, so that a current passes between the two electrodes. If there is only I-in solution and no I2 is present at the same time, no current passes between the two electrodes. The Karl Fischer reagent contains active ingredients such as pyridine and iodine, which are metered into the reaction cell and can react with the water in the test solution as follows: H2O+SO2+I2+3C5H5N→2C5H5N•HI+C5H5N•SO3C5H5N•SO3 CH3OH→C5H5N·HSO4CH3C5H5N HI→C5H5N·H I- The reaction continues, continuously consuming water, producing I-, until the end of the reaction titration, water consumption is complete. At this time, there was a trace amount of Karl Fischer reagent that did not react, and I2 and I- were present at the same time. The solution between the two platinum electrodes started to conduct electricity, and the current reached the end point and the titration was stopped. The moisture content in the solution is thus calibrated by measuring the volume (capacity) of the spent Karl Fischer reagent.

Karl Fischer Coulometric Method (Electrochemical Method) Measuring Principle The electric quantity method is based on the fact that the sample is dissolved in an electrolyte containing a specific solvent containing a certain amount of iodine. Water consumes iodine, but the required iodine is no longer used. The iodine-containing reagent is used for the titration, but through the electrolysis process, the iodide ions in the solution are oxidized to iodine at the anode: 2I-2e-→I2 produces iodine and reacts with the water in the sample. The end point is indicated by a double platinum electrode. When the concentration of iodine in the electrolyte returned to its original concentration, the electrolysis was stopped. Then according to Faraday’s law of electrolysis: Calculate the moisture content of the sample to be tested.

Step 3: Precautions for the optional moisture meter: For the various Karl-Fischer volumetric moisture analyzers available on the market, it is recommended that the flexible packaging manufacturers should be ranked according to cost performance. When choosing a purchase, pay attention to the following points:

1. It is best to consider automatic instruments when choosing.

The manual moisture analyzer requires the tester to visually measure the reagents in the glass burette and manually close them after reaching the end point. Delays due to different motion habits will bring unnecessary errors.

2, should consider buying a fully enclosed test system.

The exposed Karl-Fischer reagent is very easy to absorb water because of the presence of iodine, and the ethyl ester sample and methanol solvent to be tested should also avoid the situation where the moisture in the air dissolves into the resulting error.

3. The life of metering pumps should be considered.

The metering pump is a key part of the volumetric moisture analyzer. The Karl-Fischer reagent is a highly corrosive reagent. A metering pump made of fluoroplastic and other corrosion-resistant materials should be purchased as far as possible to prevent leakage and cause the instrument to leak. Retired.

4, the electrode problem.

This part belongs to the core part of the instrument. On the one hand, it should have high sensitivity. On the other hand, because it is immersed in the test cell during the test, it is best to choose a platinum electrode instrument with high quality to maintain the electrode life. In addition, in order to obtain accurate test results, in addition to the strict operation of the moisture meter in accordance with the procedures, the following issues should also be noted:

- The system is completely confined.

The connection of the Karl Fischer reagent fluid circuit must be fastened, from the reagent bottle to the metering pump to the reaction cell, otherwise reagent leakage will directly affect the test results. Another problem with its lack of containment is that the Karl Fischer reagent absorbs moisture from the test during the test, which can delay the titration endpoint. - Accurate sampling problem. When the Karl Fischer reagent is calibrated, 10 mg of water must be used, and a 10-ul sampler should be used as much as possible. This will not only be accurate and fast, but also prevent the adhesion of water droplets. Similarly, the use of methanol reagents and ethyl esters have similar problems, and attention should be paid to shortening the opening time of the reaction chamber as much as possible after handling. - Magnetic stirring speed adjustment. In the reaction tank, because the titration reagent is added locally, it is not in the same place as the electrode. Therefore, the stirring speed should be as fast as possible without forming turbulent flow. This can reach the end point as quickly as possible. - The titration rate should be set quickly and slowly. When titrating, try to shorten the test time as fast as possible, and strain slowly when approaching the end point, which can improve the measurement accuracy.

- After the day's trial is completed, be sure to empty the system of Karl-Fischer reagent, and then wash it with methanol, do not wash the system with water, because it is not easy to volatilize, will cause the next test Karl-Fischer reagent calibration false.

- Moisture analyzers should be kept away from strong magnetic fields to avoid electronic display jitter during operation and abnormal phenomena.

Manual moisture analyzers, because the Karl Fischer reagent and the methanol solvent must be metered with a glass burette, and the glass burette itself must be connected to the outside world because of the equilibrium pressure. In addition to the above aspects that need attention, when using, you should also pay attention to the following two points:

- The system is as tight as possible.

The manual moisture analyzer needs to add a U-shaped tube filled with desiccant to the suction tube and the glass burette to reduce the interference of air moisture on the test results. In the environment where the relative humidity is greater than 70%, moisture testing should not be arranged as much as possible.

- When adjusting the titration speed of the buret, it is best to adjust to 1 drop/second.

Too fast a titration will result in a large delay error at the end of the titration, while a too slow titration will prolong the testing process. The above interference will easily lead to delay in reaching the end.