Karl Fischer Titration to Measure the Water Content of Samples that Do not Readily Release Water
Karl Fischer titration for the determination of water is probably the best-known and most widely used titration method. The reaction mechanism has been well explored, and a large number of suitable reagents and instruments are available. The method is described in many regulations and guidelines and has thousands of applications. As part of method development, the type of sample should be considered. Specifically, how best to release the water in it so that it is fully available for titration.
the selective and stoichiometric reaction between water in the sample and iodine in the reagent, the Karl Fischer method only determines water and not other compounds. There are a number of different methods to release moisture from a sample and determine the correct moisture content. Before starting, the following prerequisites and questions must be considered:
lThe water in the sample needs to be completely released. What is the best solvent or method to do this?
lAny contamination with moisture in the environment must be avoided. Is the sample hygroscopic? Is the instrument sealed? Fast working procedures are required for hygroscopic samples and must be kept in closed containers.
lThere is a need to suppress or avoid side reactions of the formation and consumption of water or iodine. Is the sample reacting with components in the Karl Fischer reagent and how can this be prevented?
How is the Water Present in the Sample and How Can It Be Completely Released?
The first point to consider is whether the sample can be quickly and completely dissolved in a Karl Fischer solvent without any additional treatment, or whether the addition of co-solvents (solubilizers) or any sample preparation is required to extract the moisture.
Solid samples can bind water in different forms:
lbound water
lcrystal water
lsurface water
To measure the total water content of a solid sample, either the solid sample is completely dissolved or the water is completely extracted.
If only surface water should be measured, it must be ensured that the sample is not dissolved, which is usually achieved by adding chloroform and a quick procedure.
If the sample is soluble in a Karl Fischer solvent, it can be added directly to the titration cell without any additional sample preparation. Some samples may require additional preparation in order to dissolve quickly and completely in the titration cell, e.g. heating, homogenization, prolonged stirring, or addition of auxiliary solvents. Sample preparation is especially important for those samples that are not soluble in Karl Fischer solvents or cause side reactions. In coulometric methods, it is not recommended to add solid samples directly to the titration cell. This will require opening the coulometric titration cell, and depending on ambient humidity and operation, up to 10 µg of water may enter the sample. In coulometric assays, samples with low water content (for example in the range of 100 ppm) are measured, which can lead to errors, e.g. 1 g of sample with water content as high as 10%. For liquid samples, a diaphragm is used to introduce the sample into the titration cell to prevent moisture from the environment from entering at the same time. For volumetric titration, more samples are usually added, so when directly adding solid samples, the relative error due to the addition of ambient moisture is very small and can be ignored. Many liquid and solid samples do not release their water quite quickly in Karl Fischer solvents. They either do not dissolve or dissolve very slowly and often homogeneously. This can lead to a sustained release of water during the titration of such samples, leading to incorrect and irreproducible results. Other samples will have side reactions with the Karl Fischer reagent, which can also lead to erroneous water content values.
Methods of Dissolving or Extracting Water from a Sample
After homogenizing the sample by grinding, mixing, or dispersing, there are different options to completely release the water for titration:
lExternal extraction/dissolution. Add the sample in a suitable solvent outside the titration cell to release moisture, then add an accurately weighed aliquot of the sample to the titration cell;
lGas phase extraction (Karl Fischer furnace method). The sample is heated in a Karl Fischer furnace to extract water and the evaporated water is transferred to a Karl Fischer titration cell.
Solubilizer for Dissolving Samples in Karl Fischer Solvents
Many samples are insoluble in Karl Fischer solvents, usually methanol. They require the addition of solubilizers to methanol-containing Karl Fischer solvents, or some specific Karl Fischer solvents already contain co-solvents. Ethanol-based solvents can only be used for samples dissolved in ethanol, in which case it is not recommended to add auxiliary solvents.
In order to find a suitable solubilizer, the different possible solubilizers were first added separately to the Erlenmeyer flask and then the sample. After mixing, check in which solubilizer the sample is completely dissolved and then use this solvent as an auxiliary solvent in the Karl Fischer titration. An overview of co-solvents is listed in Table 1.
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Table 1. Examples of recommended co-solvents/solubilizers by sample type
It should be noted that the addition of the co-solvent mentioned above will affect the conductivity and other conditions of the system.
Add no more than 40-50% solubilizer in volumetric solvents and 20-30% in coulometric reagents to avoid indication problems. If endpoint indication or over-titration is a problem, reduce the amount of co-solvent used.
Internal Extraction/Dissolution
In a direct titration, the sample is dissolved in the titration vessel before the titration starts. For some samples that dissolve slowly in the solvent, it is necessary to prolong the stirring time.
The internal extraction is carried out in a specific heated bath at 50°C, suitable for samples that are insoluble in water and release slowly. Examples include the application of coffee and starchy products such as wheat, flour, or rice. Solubility can also be increased by adding co-solvents such as formamide. A combination of the sample preparation methods described above will be required for some samples.
External Extraction/Dissolution
Some samples require external extraction or dissolution because side reactions may occur, or they have low water content or are insoluble in Karl Fischer solvents. For such samples, first, determine the water content of the solvent used, then weigh the exact amount of solvent in a closed flask, add the weighed sample, and mix well. Some samples will have a slow water release, requiring extended extraction or mixing times. The slow water release can also be accelerated by performing the titration at elevated temperatures up to 50°C. These methods can be used in combination and should be evaluated during method development to obtain reproducible results. After the water has been completely released, add an accurately weighed aliquot to the Karl Fischer titration cell and start the titration. Once complete, subtract the water content of the solvent from the result.
Gas Phase Extraction/Karl Fischer Furnace Method
For critical samples that are water-insoluble, cause side reactions, or have solids with low water content, it is more convenient to combine a Karl Fischer furnace with a volumetric or coulometric titrator. However, samples need to be thermally stable and should not decompose during heating. A temperature ramp test should first be performed on each sample to determine the exact heating temperature required. The sample is then heated to this specific temperature to evaporate the water in it and conveyed to the titration vessel by a continuous flow of dry air or nitrogen.