FAQ: Karl Fischer Reagent
Advance Preparation
How do you choose to use the volumetric or coulomb method based on sample moisture content?
The volumetric method detects moisture content in the range of 0.01% (100ppm) to 100%, with moisture values in the milligram scale;
The Coulomb method tests moisture content in the range of 0.001% (10ppm) to 10%, and moisture detection values are generally at the microgram level (10-5000μg).
Why use standard water when pure water is not recommended for the Carl Fisher moisture test?
Karl Fischer method is generally suitable for the detection of trace moisture. When using pure water to correct the Karl Fischer moisture meter, the amount of water needed to be weighed is very small, which will obviously lead to the increase of standard deviation, resulting in the increase of uncertainty;
Standard water is a mixture of water content, weighing size is much larger than pure water, so as to reduce the uncertainty factors brought by the weighing, improve the accuracy, at the same time, standard water provides formal certificate and uncertainty, the value can be traced.
There is always error in weighing standard water, how to accurately weigh?
KF water mark is generally divided into liquid water mark and solid water mark. Liquids water labels are packaged in ampoules protected by argon gas. When in use, use a 5mL or 10mL glass gas-tight syringe for post-wash differential weighing. First, use the 0.5mL water standard sample moistening needle for 3 times, and then inhale the remaining standard sample into the syringe (Note: it is recommended that the remaining part of the water label in the bottle, so as to avoid air inhalation affecting the result) for weighing.
The solid standard sample is usually packed in an amber glass bottle, which contains a certain amount of chemically bound water. When it in use, the samples are weighed by a weighing boat.
What are the pH requirements for the sample?
The best pH range of Karl Fischer reagent is 5~7.5. Too high pH will cause side reactions and consume iodine. If the pH is too low, the titration process will be very slow
Testing Procedure
Volumetric titrator titration degree and label deviation, can still use?
For single-component volumetric titrators, due to their nature and the inevitable moisture in the environment, the titrator will decline year by year to about 0.5 mg H per year2O/mL.For open bottles of reagents, due to preservation environment differences, as long as the accurate measurement of titration, according to the detection titration can continue to test.
How much lower the drift value of the reaction pool can the next step be detected?
Drift value is the volume of reagent continuously consumed by the whole closed system. The main reason for drift is that water in the environment penetrates into the reaction tank. generally speaking,
when the drift value of the instrument is stable at 10μL/min, it can be detected, when the drift value is < 20μL/min, it can be considered that the titration has reached the end point, the end of the experiment.
How to select electrolytic cell according to the actual situation in Coulomb method?
No diaphragm electrolytic cell suitable for determination:
• Hydrocarbons - chlorinated hydrocarbons
• Alcohols - Phenols (most)
• Esters - diethyl ethers
• Ketones (with special reagent) -acetamide
• Ethers and fragrances - Edible oils
• Petroleum
There are diaphragm electrolytic cells suitable for determination:
• Samples with very low water content (<50ug of water/sample)
• Samples that require very high accuracy
• Nitro compounds
• Unsaturated hydrocarbons, especially samples that are easily reduced
What is the possible reason for the instrument's failure to balance or for a long period of time?
1. The two brands of reagents have been exchanged, resulting in the different brands of reagents may have some ingredients incompatible, please thoroughly clean and dry the pipeline before changing the brand;
2.There is a problem with the sealing of the instrument or the failure of the desiccant, and the moisture in the air keeps entering the system leading to the imbalance. Please test the sealing surface, replace the gasket or check the molecular sieve, and replace the molecular sieve.
Which samples are likely to have adverse reactions in the Carl Fischer method of moisture detection?
1.Samples containing aldehydes and ketones;
2.Samples of amines;
3.Samples containing free halogens;
4.Mercaptan samples.
How to reduce the measurement error when the sample is not soluble?
1.Add cosolvents (chloroform, xylene, formamide, etc.), or directly select Aladdin specific types of Karl Fischer reagents: special aldehydes and ketones, special reagents for long chain hydrocarbons, etc.
2.Titrate in 50C or boiling methanol;
3.Use a homogenizing device;
4.Use a suitable solvent to extract the water and then titrate;
5.Use the card furnace method.
End of Test
How do I clean the indicating electrode and the generating electrode?
1.The indicator electrode is usually wiped with a paper towel is sufficient, can also use abrasive such as toothpaste to clean;
2.The electrode can be cleaned with methanol or ethanol;
3.If necessary, the indicating electrode or electrode will be washed with water first, immersed in concentrated nitric acid solution, then washed with water, and then washed away the excess water with alcohol, and dried in the oven at 60℃.
Other points for attention
• Capacity Fakal Fischer's reagent should be kept out of light, sealed, and in a cool and dry place. The titration should be calibrated before use.
• Moisture measurement instrument should be dry, and can avoid the intrusion of moisture in the air; The measurement operation should be carried out in a dry place.
• Sample pretreatment should try to ensure that the sample can be dissolved in the system, by choosing the appropriate solvent or adding solvent enhancement methods to achieve.
• If the sample is an insoluble substance, it should be through some auxiliary methods or adding auxiliary reagents, so that the moisture of the sample can be fully dispersed to the system.
• Compounds containing aldehydes and ketones will react with methanol in the common Karl Fischer reagent and interfere with moisture determination. In this case, the special Karl Fischer reagent for aldehydes and ketones should be used.