Quick and Accurate Moisture Determination
Karl Fischer Reagents
German chemist Karl Fischer is credited with creating his namesake assay in 1935. Karl Fischer (KF) titrations can quickly and accurately determine the amount of water or moisture in a sample based on a reaction that consumes water and iodine in a 1:1 stoichiometric ratio. The amount of iodine consumed in the reaction can be measured; that same amount is therefore equal to the amount of water in the sample.
The reaction occurs quantitatively and selectively with water. It is both accurate and precise (with very small standard deviations) and can be performed in one to three minutes.
There are two primary forms of KF titrations:
- Volumetric assays are used for samples with higher water content (usually 1 to 100mg per sample).
- An iodine-containing solution serves as the titrating agent; the volume and titer of the agent is used to calculate the water content of the sample. Two types of reagents are used for the volumetric assay.
- One-component: All of the reactants (iodine, sulfur dioxide and a base) are combined in a suitable alcohol.
- Two-component: Reactants are separated to provide better stability of the titrating agent and increase the speed of the reaction.
- Coulometry is a micro-method used for samples with lower water content (10μg to 10mg). In this method, the iodine is generated electrochemically in the titration vessel by oxidation of iodide in the reagents. A change in electrical charge is used to calculate the amount of water in the sample based on the consumption of iodide.
The choice of working medium and the pH range are two important factors that ensure an interference-free KF reaction. The working medium is the solvent or solvent mixture in which the sample is dissolved and the KF reaction occurs. Methanol is the preferred solvent for the working medium, but is often mixed with other solvents to increase the solubility of the sample or prevent interfering side reactions. Buffer solutions can be used to keep the pH of the mixture between 5 and 7, the optimal range for KF reactions.