Karl Fischer (KF) titration is a redox reaction which uses the consumption of water during the reaction to measure the amount of water in a sample. It is the reference method for water determination because of its specificity, accuracy and speed of measurement. It takes place in an organic solvent.
Different solvents are necessary because of the difficulty in dissolving various samples. The presence of incompletely dissolved sample leads to a slow release of water with a drifting end point. Physical techniques such as grinding of the sample and warming of the titration cell are also sometimes used to ensure complete dissolution.
Types of Samples
The most common sample types include:
Chemicals
Inorganic compounds including alkali and alkali earth salts do not dissolve well in methanol, and so other mixtures such as 50% formamide and 50% methanol may be used. If no organic solvent is suitable, a KF oven is used to evaporate the water which is then directly titrated using the KF reaction. Strong reducing and oxidizing agents are not suitable for this titration because they consume the reagent.
The pH must be kept fairly neutral because at acidic pH the reaction is too slow to achieve an end point, while at an alkaline environment the iodine is consumed by a side reaction which gives a falsely high result or a vanishing end point. A buffer is therefore used in such a situation.
Organic compounds with reactive functional groups that could take part in a redox reaction are not suitable for this method. This includes strong acids. However, hydrocarbons, esters and ethers, and alcohols can be titrated. Long-chain hydrocarbons may dissolve completely only if propanol or chloroform are added. Proteins and sugars, being polar organic compounds including require volumetric titration, also need a modified solvent, and warming the cell to speed up dissolution.
Ketones react with methanol to form water, resulting in a vanishing end point, so special methanol-free KF reagents are used with such chemicals. Sulfur dioxide is also used up by aldehyde groups resulting in a falsely low water result.
Mercaptans and peroxides are redox agents and react with the iodine of the reagent, leading to false results. Test titrations with validation are recommended to achieve accurate titration in such cases.
Pharmaceutical Products
Whether raw or finished, pharmaceutical products need to have their moisture content determined. These include the basic material, inorganic salts, solvents and excipients. The raw material is typically pure but in a few cases other components may be present, which will require validation of the testing.
KF titration is preferred if the water content is within the specified limit. Intermediates also must be tested for water content as this affects the quality of the products, including their adhesion characteristics or flux. When the drug is formulated as a tablet powder, solubility issues must be solved.
In the finished product, water determination will show whether the product has the desired physical characteristics and shelf life. KF titration is difficult because of poor solubility and effervescence of many formulations. Liquids, creams and suppositories are suitable as they can be directly added and dissolved, if required, in chloroform or propanol in a warmed cell.
Petroleum Products
KF titration is not recommended for petroleum-derived oils and lubricants, both because they do not readily dissolve in the usual reagent and because they contain only minute amounts of water in most cases.
However, coulometric KF titration may be used with a special reagent designed for this application, containing chloroform or long-chain alcohols. Complete homogenization with a mixer or ultrasonic bath is required for uneven mixtures. With crude oil, xylene or toluene should be added to the medium to keep tar in solution and so prevent it from sticking to the electrodes.
Problems may occur with lubricants and other products which have additives like antioxidants, mercaptans, ketones or metal oxides which react with iodine or generate water by side reactions leading to falsely high results. This is prevented by using a KF oven and coulometry as per ASTM or ISO standards.
Plastics
Plastics are available as fibers, granules or solutions, often with minute water concentrations. Coulometry is therefore the preferred method, with the addition of chloroform or methyl pyrrolidone to dissolve them completely. Volumetric titration may be performed for fine plastic polymer powders.
Their addition to the methanol leads to the extraction of water which is then determined. Otherwise, with coarser samples, a KF oven is used with a preset 10 minute period to prevent premature termination of the process due to the extremely slow rate at which water is released.
The temperature must be carefully controlled as further polymerization may occur on heating, which can release more water. Decomposition is another possibility. The settings are therefore customized for each product.
Foodstuffs
Water content affects storage stability, appearance, texture and taste, as well as other physicochemical characteristics of food, and is legally stipulated as well. KF titration of foods is complicated by the multitude of substances present in a single food.
The sample preparation is therefore complex as well, to include polar as well as nonpolar materials, as well as substances that can cause side reactions. Heating must be controlled to prevent the breakdown of the food, which releases additional water.
Suspended and intracellular material must be considered. For this category a titration curve is therefore necessary to understand how the titration is proceeding and whether side reactions occur. Volumetric KF titration is typically used in this case, unless for vegetable oils and such foodstuffs with a very low expected water content.
Warming of the cell and modified working media are used for different polar and nonpolar materials in foodstuffs, especially when they are combined in the same sample. Physical preparation by fine chopping, homogenizers or high-speed mixing is very often required. As above, water in starch is extracted by adding the fine powder to methanol. Such preparation techniques can lead to short titration times with clear-cut end points.
Further Reading