Determination of the water content of a substance is essential for quality control in many applications, including pharmaceutical agents, foodstuffs and chemicals. One of the most widely used and accurate methods of quality control is Karl Fischer (KF) titration.
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Karl Fischer titration is based on a reaction that consumes the water present in the sample tested. The end-point is determined by the absence of any waterand when the iodine titrant reaches excess in the solution, as shown by a change in color and electrical conductivity.
Advantages of Karl Fischer titration
- Extreme accuracy because KF titration is based upon a chemical reaction which depends upon the presence of water.
- Specificity for water determination because it does not detect the loss of any other volatile substances unlike other methods such as loss on drying which are based upon the heat-induced loss of moisture and the resulting reduction in weight of the sample.
- Wide range of determination from 0.001 to 100%, or from 100 percent to 1 in 1 million parts per million (ppm) with smaller percentages up to 1% being suited for coulometric KF titration while samples with more than 2% water are tested by the volumetric method (applied for samples with 0.1 to 500 mg of water).
- The coulometric method avoids the need for titer determination and is thus an absolute method, with the amount of current consumed being exactly related to the amount of iodine generated.
- The presence of two methods increases the flexibility of the application range, as the volumetric KF titration handles samples dissolved in several common solvents, while the coulometric method depends on current consumption till the end point to determine the water content.
- It works with small samples unlike several other methods, and this makes it more accurate and repeatable.
- It is a rapid method and requires minimal sample preparation apart from dissolving them in a suitable solvent and pre-titrating the solution and the cell to get rid of all traces of water before starting the titration.
- It is suitable for water determination in solids, liquids and gases.
- The process lends itself to automation, making water determination a matter of minutes. The coulometric method also uses the same cell to determine water content in many different samples without the need to reload or clean the cell.
- KF reagents have water equivalents which change with time, and therefore they should be regularly calibrated when the volumetric method is used to ensure accuracy of determination. This is done using certified water standards which have tested water equivalents (i.e. the amount of water in milligrams which can be taken up using 1 milliliter of the reagent). This helps avoid the need to calibrate with pure water which is a difficult process. The water equivalents for commonly used KF reagents are 2-6 mg water/mL reagent. The KF reaction is a linear one, and so single-point calibration is sufficient, and no calibration curves are needed.
- The coulometric titrator detects free water, emulsified water and dissolved water unlike other methods even at low quantities.
- A spectrophotometric detector can be incorporated to allow visual detection of the end point or by the use of conductivity assessment by a double platinum electrode.
Limitations of Karl Fischer titration
- Manual volumetric KF titration requires reloading for each determination and hence has a high solvent consumption.
- The margin of error is relatively large when manual volumetric KF titration is applied to materials that contain starch.
- The manual titration requires considerable operator input.
- KF titration is a destructive technique.
- Coulometric KF titration is suitable only for samples containing small amounts of water and larger amounts may overwhelm the reagent capacity and yield false results, besides taking excessively long periods for the determination.
- KF titration depends upon a redox reaction and thus any component of the sample which is an active redox chemical such as dimethyl sulfoxide will react with the iodine in the reagent and generate false results.
- Ketones and aldehydes, boric acid and metal peroxides, as well as silanols and strong acids, are not suitable for this titration without modification, as their reaction with the methanol solvent produces water, resulting in a vanishing end point and falsely high water content, requiring the use of methanol-free reagents with such substances.
- Some substances, such as chocolate, have tightly bound water which is released very reluctantly, often after breaking down the sample with a high-shear mixer. Others are insoluble in commercially available KF reagents and this method cannot be used in the solid state without dissolution of the sample.
- Compounds like lithium chloride have tightly bound water of hydration, so when they are part of the solvent, it is difficult to use with KF titration.
- Carbonates, oxides and hydroxides also undergo side reactions and are not suitable for KF titration.
- The drift of the cell must be calculated before the actual testing, and this takes up to 30 minutes before the actual run. Drift is due to apparent water in the sample and must be calculated in a dry run and subtracted from the final result.