Moisture determination: How is moisture measured?In alcoholic solvents, water and iodine are consumed in a 1:1 ratio, and in a 2:1 ratio in non-alcoholic solvents. Once the water is completely consumed, the excess iodine is calculated by an indicator electrode in the titrator, indicating the end-point of the titration. The amount of water that was present in the sample is then calculated from the concentration of iodine in the titrating reagent and the amount of reagent consumed in the titration. Typically, the recommended pH range for Karl Fischer titration is 5.5 to 8 for the maximum rate of reaction. For pH levels outside of this range, the titration speed is either significantly slower (values less than 4) or results in a vanishing endpoint (values higher than 8). As such, samples outside of the optimal range should be buffered to reach an overall pH in the recommended range.
Type of Karl Fischer Analyses: Coulometric Titration vs Volumetric TitrationThere are two different techniques to determine water content using the Karl Fischer method:
- Volumetric Karl Fischer Titration
- Water Concentration: 100ppm to 100%
- Iodine is added using a burette during titration.
- Coulometric Karl Fischer Analysis
- Water Concentration: 1ppm to 5%
- Iodine is generated electrochemically
Volumetric Karl Fischer Titration
Iodine is added to the sample with solvent, where water is measured by the consumption of Karl Fischer reagent. There are two types of volumetric KF systems:
One Component ReagentLess expensive and easier to handle, but has a less stable titer, and slower titration speed.
Two Component ReagentHas a longer term stability and faster titration time, but have solvent capacity restrictions. Available through PROAnalytics: Karl Fischer Volumetric Titrator
Coulometric Karl Fischer Titration
Iodine is generated electrochemically during the titration, and water is measured through the total charge passed (1 mg H2O = 10.72 Coulombs). Advantages to a Coulometric KFT include:
- Lower reagent cost
- Reduced downtime and maintenance cost
- More stable titer
- More suitable for automation
Application: Determining Water Content in Magnesium AspartateWater content is a critical parameter for quality control in raw materials and finished pharmaceutical products. In many situations, the potency and effectiveness of the active ingredients in such products may be strongly affected by the water content. Magnesium is crucial in basic cellular function and muscle cell metabolic processes. Both compounds in the application note are used as dietary supplements for magnesium utilization and are found in the form of salts. Water can be present in solid samples such as magnesium aspartate and magnesium gluconate as either surface water, or within cells and capillaries. In order to appropriately determine the water content, suitable sample preparation must be exercised: crushing the sample, dissolving it in high temperatures, or evaporating water in a KF-oven. Proper preparation is needed to ensure water is released completely and dissolved in the alcohol solution. Both volumetric and coulometric Karl Fischer Titration methods can be used, based on the sample water content. The following KF methods have been developed and optimized by Mettler Toledo:
- KF Volumetry: Direct titration in methanol: formamide 2:1 v/v, 50 °C Equipment: V30 Compact KF Volumeter
- KF Coulometry: Gas phase extraction T = 180 °C, t = 1800 s, Nitrogen gas Equipment: C30 Compact KF Coulometer, Stromboli KF Oven Sample Changer
- KF Coulometry: External dissolution Methanol: Formamide1:1 v/v Equipment: C30 Compact KF Coulometer