Influencing factors for the detection of moisture detectors
1. Titration speed
Control the titration dose added in the Karl Fischer titration. Theoretically speaking, the faster the titrant is added, the better, and it just stops when the end point is reached, so as to accurately measure the titrant consumption.
2. pH value
The maximum reaction rate occurs between pH 5.5 and 8. In actual operation, pH > 8 or pH < 4 should be avoided. When titrating acidic or alkaline samples, buffer should be added to adjust the pH of the solution.
3. Stirring speed
Optimal stirring speed is when a small vortex is observed. If the rate is too slow, the titration will also be slow and insufficient, and overtitration may occur. If the stirring speed is too fast, air bubbles will be formed in the solution, and the air bubbles will cause erroneous measurement values.
4. Instrument
Feed the liquid before measuring the sample to ensure that there is no air in the burette and the pipeline. If the drift value of the Karl Fischer solution is too high, the molecular sieve should be replaced in time. The molecular sieve on the waste bottle containing SO2 should be cleaned with deionized water first, then dried and regenerated. Regular maintenance is required for the instrument. To clean the surface motor of the titrator, use a deionized water ultrasonic bath for a few minutes or use a chromic acid bath, and then clean it with deionized water.
5. Environmental moisture
Environmental moisture is the main cause of errors in Cassette titration. For environments with relative humidity > 80%, a dehumidifier should be installed in the room, and the experimental equipment should not be directly facing the air conditioner.
6. Titrant concentration
For titrants and solvents, ensure that they are used within the validity period of the reagents, and they must be properly stored to prevent them from absorbing environmental moisture and reducing the concentration of the reagent itself. The reagents should be stored in a low-temperature dry place, and can be placed in a dry dish.
7. Solvent
Solvents have an effect on the Cassie reaction. The study found that when the methanol content in the solvent is ≥20%, iodine and water react in a ratio of 1:1, so the amount of methanol must always be no less than the required minimum amount.
The moisture content in the solvent also has an influence on the Karl Fischer titration. When the water content in the solvent is more than 1mol/L, the titer will increase, which will affect the accuracy of the measurement results. However, this situation is rare in practice, because the moisture content of the solvent is negligible.
8. Sample
The larger the sample volume, the higher the measurement accuracy. Because of the influence of absorbed environmental moisture on the results when sampling and adding samples. It is worth mentioning that when the sample is added, avoid sticking to the wall of the cuvette, so as not to affect the measurement results and cause unnecessary errors. When the solvent is added to the reaction bottle, it should be shaken repeatedly for 3-4 times, so that it can quickly obtain a stable drift value.
9. Temperature
Karl Fischer titrants contain more than 90% methanol or ethanol, whose volume expands rapidly due to temperature changes, and the concentration decreases accordingly.
10. Side reactions between samples and Karl Fischer reagents
The side reaction that affects the titration result occurs in parallel with the Cassie reaction, and the interference of the side reaction should be eliminated before the Cassie reaction occurs.
11. Sealing
In order to prevent the moisture in the external air from affecting the accuracy of the results, the titration equipment and the reaction pool are required to have good sealing, otherwise moisture absorption will cause long-term instability and serious errors in the end point.
12. Sample injection operation
External contamination of the injector needle will affect the measurement results, and at the same time, it is necessary to prevent the loss of samples during injection.
13. Borrow the terminal phenomenon
In the process of Karl Fischer titration, the end point phenomenon sometimes occurs, that is, the end point is reached in advance, resulting in low measurement results. Especially in the determination of samples with low concentration of water content, the influence is even greater, and even the determination cannot be carried out. 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. Sunlight will also significantly promote the oxidation reaction of oxygen and iodide ions, and measures should be taken to protect the reagents from light. In addition, the composition of the reagents and the operating environment have a certain influence on the speed of the reaction.
