- What is the difference between one- and two-component volumetric titration?
- How to select working medium for volumetric titration?
- How to select water standard for verification of a coulometric titration setup?
- What if the moisture of a water standard titrated on a coulometric instrument is always too high?
- What if the titer of a volumetric reagent is determined as too low and solution is dark at the end of the titration?
- What are the advantages of Coulometric AGR-Oil?
- How to use Coulometric CGR generator solution?
- What is a Quveon reagent for coulometric titration with an oven?
What is the difference between one- and two-component volumetric titration?
Karl Fischer analysis requires four components for the reaction with water: iodine, base (typically, imidazole), sulfur dioxide and methanol. In one-component titration, all of these components are present in the reagent (Composit T5/Composit T2). In two-component titration, these components are separated into two solutions: titrant (iodine in methanol) and solvent (imidazole and sulfur dioxide also dissolved in methanol). One-component reagents are more popular because they are more convenient and don't require specific solvent to work. Regular dry methanol works just fine. Two-component titration is a little more tedious and can be messy if some titrant is spilled. On the other hand, titration endpoints are much sharper and the titrant holds its titer for longer. If a similarly sharp endpoint is desired with one-component titration, Methanol Sharp can be used as the working medium.
How to select working medium for volumetric titration?
In the ideal world, methanol would be the only solvent needed for one-component KF titration. However, there are plenty of samples that don't easily dissolve in methanol or react with it producing or consuming water. All of this makes standard titration with methanol impossible. In these situations, specialized solvents/working media are used. Below is a brief list of the most common samples and the corresponding solvents that should be used.
- Petrochemicals and fuels: Medium C (one-component), Solvent C (two-component), Solvent CX (both)
- Biodiesel or plant oils: Solvent D or Solvent CX
- Pharmaceutical intermediates: Medium C (one-component) or Solvent C (two-component)
Always remember that several options should be tested to find the optimal solvent. This is especially true for biofuels and plant oils. Their composition can change from batch to batch depending on the plants used for their manufacturing. This can decide whether Solvent D or Solvent CX should be used.
How to select water standard for verification of a coulometric titration setup?
Coulometric titration instruments need to be regularly verified against water standards to make sure they produce correct values. This is also necessary when titrating materials that poorly conduct electricity (such as transformer oil). As the titration cell gets filled up with samples, its electrical resistance goes up and it may start to shift the results of titration. Water standards are used to determine if the instrument is still producing accurate results or the solution must be changed.
The conventional wisdom says that the standards should have water content similar to that of the commonly tested samples. For example, if water content of the sample is typically 0.005% then you should use Water Standard 0.1 mg/g (0.01%). This logic has one problem in the case of very dry samples. Extremely dry standards are difficult to work with: once the ampule is open it has to be immediately isolated from the air with parafilm and samples of the standard must be drawn with a calibrated syringe. Even with these precautions, after one or two samples the ambient moisture seeps into the ampule and starts to shift the results.
With this in mind, a much safer alternative is Water Standard 1.0 mg/g (0.1% water). At the end of the day, the goal of the measurement is to show that the titrator produces correct results, and this standard will serve that purpose just fine. It's not as sensitive to ambient moisture and is more forgiving to handling errors. And it will still show if the solution needs to be changed.
The typical procedure is to collect approximately 0.5 ml of Water Standard 1.0 with a calibrated syringe, weigh it on an analytical balance with 1 mg accuracy and inject the sample into the titration cell. Make sure the drift has been determined right before the measurement. Then weigh the empty syringe and record the weight difference in milligrams. Once the titration is completed, divide the determined amount of water in milligrams by the weight difference (weight of the standard). The result should be as close to 1.0 as possible. Or, to be completely accurate, to the value specified in the Certificate of Analysis that is always included with the standard.
What if the moisture of a water standard titrated on a coulometric instrument is always too high?
Sometimes measurements of water standards produce results that are consistently too high. When this happens, there are multiple possible reasons: from handling errors to bad reagent to instrument problems. First check how much higher the results typically are. Are they 5% or 20% over the value in the standard's certificate of analysis? If you are seeing this problem with very dry standards (for example, Water Standard 0.1 mg/g) try to use standards with a higher water content, for example, Water Standard 1.0 mg/g. If the error goes down (say, from 20% to 2%) then most likely the reason is a handling problem. Dry standards are extremely sensitive to ambient moisture and absorb water from the air immediately after opening. Always cover the opened ampule with parafilm and only use one or two samples from each ampule.
If switching the standard hasn't changed the error, a deeper investigation is needed. If you are seeing the issue on several titrators using the same reagent then the problem is likely with the reagent. At this point, you should contact the manufacturer with the description of the problem and lot number of the reagent in question. If changing to a different lot solves the problem then the cause is definitely in the reagent and manufacturer's involvement is warranted.
However, if you see the problem on only one instrument then most likely there is a hardware problem. Most often, it's the indicator electrode. When it's not working properly, it detects end of titration with a delay and the titrator continues to generate iodine that is no longer needed. As a side effect of this issue, the solution turns darker at the end of titration. In this case, the electrode needs to be replaced.
What if the titer of a volumetric reagent is determined as too low and solution is dark at the end of the titration?
To determine titer of the volumetric reagent, you normally use Water Standard 10.0 mg/g (1% water). If the determined value is too low (anywhere below 4.5) and the solution ends up darker than normal at the end of the titration, it means that too much iodine has been added to the titration cell.
There are two most likely reasons: faulty indicator electrode and a problem with the dosing pump. A faulty indicator electrode doesn't detect the endpoint in time and the pump continues adding the reagents even though it's no longer needed. This results in the dark solution caused by excess of iodine. A defective pump is likely to always add the same excess, so the incorrectly determined titer should be roughly the same between runs. A problematic electrode is more likely to produce different values with each try. In either case, the defective component must be replaced.
What are the advantages of Coulometric AGR-Oil?
Mixing anode solution with xylene with the ratio of 60:40 is an acceptable method referenced in ASTM D4928.
However, it has several notable drawbacks that lab managers should be aware of:
The recommended ratio of Karl Fischer reagent and xylene should never be exceeded. If the content of xylene is even slightly over 40%, stoichiometry of the electrochemical reaction can shift and will produce incorrect results. Excessive addition of xylene can also cause overheating of the solution.
It’s difficult to produce exactly the same content of xylene in the mixture from batch to batch. This can negatively impact reproducibility of results.
Manually mixed solution has relatively low water capacity. In practice, it means that the solution in the titration cell has to be changed more frequently.
To verify whether the solution is still adequate, water is added at regular intervals to check for consistency of results. This process further reduces the remaining water capacity of the reagent mixture.
Quveon has developed WaveTrace™ Coulometric AGR-Oil - a reagent specifically formulated for testing of crude oil, fuels, transformer oils and other similar materials. The content of xylene is always correct, which eliminates any issues with result consistency from batch to batch. In addition, water capacity of the reagent is significantly higher than that of the 60:40 mix. Testing shows that 100 ml of Coulometric AGR-Oil can be used to titrate over 500 mg of water. This means that the titration cell would fill up long before the reagent loses its ability to produce accurate results.
When Coulometric AGR-Oil is used, there is no longer a need to manually mix xylene and risk inconsistent results. Reagents are changed far less frequently and give reproducible results with every measurement.
How to use Coulometric CGR generator solution?
Traditionally, generator solutions have been packaged in ampules, since they tend to be sensitive to moisture and can be unstable when stored over time. Quveon has developed a novel formulation of generator solution: WaveTrace Coulometric CGR. It's designed to be stable over long periods of time when stored in regular bottles. This allows its packaging in small glass bottles instead of the expensive ampules and gives Quveon the opportunity to offer this generator solution at very affordable price levels.
In addition to its stability, Coulometric CGR is more viscous and prevents diffusion of the main vessel reagent into the generator compartment. The new generator solution is manufactured with a slight excess of iodine and appears dark brown when first added to the titration cell. The solution can be added to the generator compartment with a syringe or a glass pipette. As soon as the instrument is turned on, the solution becomes light-yellow or colorless and is then ready for titration.
As with any titration involving a diaphragm, it’s important to keep it clean and free of contamination. The condition of the diaphragm determines how long the cell needs to be conditioned before it’s dry and ready for titration. For cleaning, you can add 10-20 ml of chloroform-xylene mixture to the generator electrode compartment and place it in a glass beaker overnight. This allows the solvent to penetrate the pores of the diaphragm and dissolve any traces of previous samples, such as crude oil. Rinse the diaphragm with chloroform and dry the electrode in a drying oven. With a clean diaphragm it takes less than 20 minutes for a titration cell with fresh reagent to become ready for use.
What is a Quveon reagent for coulometric titration with an oven?
Karl Fischer titration is a highly accurate technique for measuring water content. However, some samples represent more difficulty than others. The method is based on analysis of water that is present in the solution and if the sample tends to retain water or doesn’t dissolve fully, the result will not account for the entire amount of water and will be too low.
In many cases, specialized reagents can be used to mitigate low solubility (WaveTrace Coulometric AGR-C or AGR-Oil). However, plastics, resins, inorganic salts or other similar materials cannot be dissolved at all in any methanol-based reagent or release water only at elevated temperatures. In other cases, the sample itself interferes with the Karl Fischer reaction by producing or consuming water. The most well-known example is aldehydes and ketones, but there are others, such as vinyl ethers, ascorbic acid, etc.
To address such problematic samples, the most reliable solution is using an oven designed for Karl Fischer titration. The solid sample is introduced in a special vial and heated to a preset temperature. The temperature is usually selected between 60° C and 300° C, depending on the sample. Once the water is released, the vapor is transported to the titration cell by carrier gas (usually, dry nitrogen). The moisture is then dissolved in the reagent as it passes through it and titrated like any other sample. Since the carrier gas is usually hot (over 200° C), using regular KF reagents is not practical. The methanol in the solution would start boiling, which interferes with the electrochemical process. This can lead to incorrect and poorly reproducible results.
Quveon has developed a reagent specifically for this application: WaveTrace Coulometric AGR-Oven. It’s designed to have a higher boiling point and resistance to evaporation at working temperatures of the carrier gas. At the same time, it maintains accuracy and has high capacity. In practice, this means that over a gram of water can be titrated before the reagent in the cell needs to be replaced.