Factors affecting the test results of the Karl Fischer moisture meter
The Karl Fischer moisture meter is very useful because it is easy to use, accurate, precise, and fast. But in real detection, if you don’t pay enough attention to some of the things that affect the test results, the test won’t be accurate or precise, the test results will be wrong, and your analysis and judgment will be off.
What will affect the results of the test?
The Karl Fischer moisture meter uses the Karl Fischer reagent to figure out how much water is in a sample. One thing that affects the results is how well the Karl Fischer reagent is calibrated. The Karl Fischer reagent is sensitive to water and can easily cause side effects. There is a lot of water in the air, so it is easy for the reagent to react with it. As a result, the amount of reagent in the air keeps going down. Because of this, Karl Fischer reagent needs to be calibrated often. The Karl Fischer reagent needs to be calibrated based on the type of Karl Fischer reagent chosen and how tight the moisture meter is.
Every time, you have to calibrate the Karl Fischer reagent before testing the amount of moisture. To calibrate the amount of Karl Fischer reagent, you can use either sodium tartrate dihydrate or distilled water. Distilled water is generally selected for calibration, and 10 samples are extracted with a micro injection needle. 1 liter of distilled water is added drop by drop to the titration cup.
The mass of the added water (up to 0.1 mg) is measured using the reduction method, and the data is entered into the system. A Karl Fischer reagent is used to titrate until the end point, and the measuring device shows the results. Repeat the process several times. The Karl Fischer moisture meter will automatically record the average value if the difference between the two measurements is less than 3.5 percent.
Since methanol is both a solvent and a part of the reaction itself, the amount of methanol gradually decreases as the determination time goes up. When there isn’t enough methanol, water or any other compound with active hydrogen can react instead of the methanol intermediate compound. This makes it hard to measure the chemical reaction and makes it so that the reaction doesn’t favor water in any way. Ensure that there is enough methanol in the base solution during the test so that the base solution solvent contains more than 50% methanol.
Reagent selection for the Karl Fischer moisture analyzer
Select a titre
Karl Fischer reagent usually comes in titrations of 1 mg/ml, 2 mg/ml, and 3–5 mg/ml. The sample with the least amount of water should use the reagent with the smallest titration. Reagents used in coulometry can’t tell the difference between titres.
A solution of alcohol is mixed with iodine, sulfur dioxide, and imidazole to make the titrant. Methanol or any other easy-to-dissolve liquid can be used as the solvent. As the solvent, methanol can be used to mix the sample. About two years is how long the reagent can be kept. The titre will drop by about 0.5 mg/mL per year if the reagent is kept in a sealed bottle. Single-component reagents that are sold commercially have three concentration specifications:
- 5mg / ml, suitable for samples with a water content of 1000 ppm–100%,
- 2mg / ml, suitable for samples with a water content of 1000 ppm,
- 1mg / ml, applicable to samples with a water content of 200 ppm.
Choose based on the formula
There are reagents with one part, two parts, and special parts. The titre of a single component drops quickly, but it is cheap and comes in many different titres; The two-component reagent is good for testing some special samples, like aldehyde and ketone special reagent, because it works quickly, is expensive, and has a limited amount.
Depending on the raw material, the Karl Fischer moisture tester uses different methods, so the reagents chosen will also be different. So, when choosing, it should be based on what’s going on at the time. Don’t act without thinking, or you’ll make things worse.
Differences between the volumetric and electrical Karl Fischer moisture meter methods
- For the Karl Fischer volumetric method (titration), the following five basic steps should be carried out for each sample determination:
- To begin, add some solvent to the titration tank.
- Titrate a portion of the Karl Fischer reagent to balance it;
- Inject the tested sample; 4. Then titrate Karl Fischer reagent into the titration tank;
- Get rid of any waste liquid. Simply put, it means that every time the reagent has to be changed. This method converts the amount of water based on the amount of Karl Fischer reagent injected and the titre of the reagent. This is because the titre of Karl Fischer reagent can change at any time depending on the humidity, light, sealing, and other factors in the environment, which can cause measurement errors.
In the process of figuring out and testing, the waste liquid from the last time will be dumped, and each time the new Karl Fischer reagent has to be titrated again, which will harm the environment. The quantity of reagents used is high, the process is hard to do, and the accuracy of the measurement is low. The automatic volumetric moisture meter also needs the five basic steps above, but it also has the functions of automatic titration and automatic waste liquid discharge.
- The balance of the electrolytic cell is the Karl Fischer coulometric method (electricity method) micro-humidity tester. Only one operation step is needed, which is to inject the sample. Depending on how much water is in the sample, the instrument will use electrolysis to bring it back into balance.
After that, the amount of water will be changed based on the amount of electricity used by electrolysis, and the results will be shown digitally on the instrument, making it faster and more accurate. In the coulometric method, the reagent can be used for a long time without being changed out often. The amount of reagent used is cut down, the cost of the test is low, and the process is easy.
Common problems with Karl Fischer moisture meters
- The color of the anode electrolyte is not bright yellow; it is somewhere between brown and dark yellow.
If the color is too dark, the electrode can’t respond as well to the electrolyte. The adsorbate on the surface of the double platinum needle electrode can be cleaned off with a paper towel. Check to see if the measuring electrode is connected properly; if it is, it may not work.
- Titrate fresh anode electrolyte first, drift too high?
The titration system still has some water in it. In the drying tube, the molecular sieve and silica gel can be changed. Check to see if the connections between the electrodes and the plugs on the titration table are tight. Some loose connections can be fixed with silicone grease.
- Why is the drift in standby titration too high?
Through the diaphragm, the water in the cathode pool gets to the water in the anode pool. It is possible to change the electrolyte in the anode cell. Add a small amount of volumetric Karl Fischer reagent with one component to the cathode electrolytic cell to dry it. Keep the level of liquid in the anode higher than the level of liquid in the cathode. Clean the titration cup well and get rid of any leftover samples from the last test that are causing side reactions. Check to see if the titration system is tight.
- Is the sample’s drift value very high after titration?
The test sample reacts with the anode electrolyte in a way that is not the main reaction. This reaction makes water. Change the anode electrolyte or use other ways to prepare the sample. This happens in the combined drying furnace, which means that the water in the sample doesn’t completely evaporate or that some volatile parts of the sample have side reactions with the calfischer reagent. The temperature of the blast furnace can be changed, the time it takes to evaporate can be lengthened, or the method for pre-treating the samples can be improved.
- The time for titration is long, and it doesn’t stop.
If the control parameters are chosen incorrectly, the relative drift termination can be used as the end parameter to increase the relative drift termination value and the end point. If the anode electrolyte doesn’t conduct electricity well enough, it needs to be replaced. When the combined drying furnace is used, this is because water evaporates slowly and unevenly. The maximum time can be used to stop the blast furnace, change the temperature of the blast furnace, or lengthen the time it takes for the water to evaporate.
- Too much time before titration?
The potential of the electrolyte system is too low (350 mV), and the rate at which iodine is made is slow. This means that the polarization current can reach 5 ua. There is still water hanging on the wall of the system. This water will slowly leak out, making the pre-titration time too long.
7.7. Test results that are hard to repeat
The sample size is too small, and the sample has too little water. The amount of sample can be increased so that each injection contains between 1 mg and 2 mg of pure water. The uneven distribution of water in the sample will also cause the final result to be wrong. You can mix the sample for longer, add more sample, or do any necessary pretreatments, like crushing, dissolving, etc., to the sample. Also, preparing and adding samples in the wrong way can make it hard to get the same results repeatedly, especially if the samples are very dry.
- Why is the result of the titration so low?
If the titration stops too soon, the relative drift value can be changed so that the remaining water can continue to react. The method of adding up samples is not fair. The reduction method can avoid the error that comes from adding samples in pieces, which is especially true for samples that stick together well. In other cases, the sample cannot be dissolved in the solution to make an emulsion. At this point, the anode electrolyte can be changed or an extra solvent can be added to make it easier for the electrolyte to dissolve in the sample.
- Why are the double platinum needle electrode and electrolytic electrode black?
This means that other things get on the surface of the electrode and make it dirty, so it needs to be cleaned. Chromic acid lotion can be used to get rid of most of the oil, organic, and inorganic materials. After that, it can be cleaned with distilled water, washed several times with absolute ethanol, purged, and dried with dry air or nitrogen.
- How long does it take to set the titrant to the right level? How should the Karl Fischer titrant be calibrated?
Most of the time, this depends on how stable the titrant is and what steps are taken to keep it from coming into contact with common pollutants that lower its concentration. People usually store titrants that are sensitive to light, like iodine solution, in brown bottles, use a molecular sieve or silica gel to keep water from getting into Karl Fischer titrant, and keep strong bases, like sodium hydroxide, from breathing in carbon dioxide.
People might think that pure water is the best thing to use to calibrate Karl Fischer reagent. But water can’t be used as a standard because it changes when it’s weighed and its molecular weight isn’t large enough. Another problem is figuring out how to weigh enough water to make sure the right amount of reagents is used.
Certified standard solutions of different concentrations (0.1 mg/g (ML) to 10 mg/g (ML) of water) can be bought instead of pure water. In this way, a more accurate amount of injection can be found.
Another option is a solid sample, like sodium tartrate dihydrate, that has a known amount of water. There are two parts of crystal water in the reference material, which is exactly 15.66% water. It’s good to use because it’s a fine powder with a stable and known amount of water. Compared to pure water, which is 100% water, it only has 15.66% water.
To get a good titration, the experimenter can accurately weigh a good-sized sample. The problem with the reference material is that it is hard to dissolve in methanol, which is a solvent that is often used in Carlsberg. The substance is mixed with 40 mg, or about 15.0 ml, of methanol. Next, if the calibrated concentration value goes up, it means that the dissolution isn’t done yet, so the fresh solvent needs to be changed.
- Where are titration tanks with diaphragms or without them used?
The coulometric titration cells in the DL32 and DL39 Karschner moisture meters are either with or without a diaphragm. We recommend using a titration cell without a diaphragm in most situations because it doesn’t need to be taken care of. METTLER TOLEDO’s titration cell without a diaphragm is a revolutionary design breakthrough that makes it possible to directly measure the amount of water in oil products without using a solvent.
The titration cell with a diaphragm can be used to measure the amount of water in things that contain ketones. It can also be used to find things with a very high level of accuracy.
- How do I know when to change the molecular sieve in the Karl Fischer titrator’s drying tube?
The best way to deal with this problem is to put some blue silica gel on the top of the drying tube as a sign. When the silica gel surface turns pink, the molecular sieve needs to be changed or cleaned. The background drift value going up is also a sign that the molecular sieves need to be changed.