Presence of moisture plays a vital role in deciding characteristic properties and stability of pharmaceutical and food products. Merits and demerits of water determination methods have been covered earlier. Karl Fischer is the preferred method adopted by most laboratories due to its high specificity and selectivity towards water present even in traces levels in a product.
The results of Karl Fischer titration are highly reliable and at same time the analysis does not require large investments. However, required precautions should be taken in handling of samples as well as the Karl Fischer reagent.
The reaction cell and electrodes should be cleaned properly to remove traces of earlier samples and residual moisture. After rinsing with high purity grade solvent the cell should be dried in hot air oven to ensure removal of any residual moisture. The sample port should be opened only briefly for sample introduction and the jacket on lid should be packed with silica gel to prevent moisture from atmosphere interfering with the estimations.
The sample must be stored in water resistant packs or sealed vials to minimize contamination during storage and transportation. It is essential to homogenize it to a fine powder if it is in granular or crystalline state. The measured samples should be introduced into the cell with minimum exposure.
It is necessary to determine the water factor in the blank methanol solvent every time. This factor depends on the amount of water present in the pure methanol solvent taken in the reaction vessel and depends on volume of Karl Fischer reagent required to neutralize it.
Use of Buffers
Karl Fischer titrations are best carried out in neutral solutions, commonly between pH 5-5 to 8.0. Moisture content in acidic and basic samples should be determined after buffering the solvent in reaction vessel before sample introduction. Commonly imidazole is used for acidic samples and salicylic acid is used for bases.
Calibration is commonly carried out using disodium tartrate dihydrate as a primary standard. The standard contains 15.66% water. It dissolves in methanol slowly (2-3 minutes) so it should be allowed to dissolve completely after introduction and starting the titration.
Safe use of Karl Fischer Reagent
Karl Fischer reagent contains potentially hazardous and corrosive constituents. Care should be exercised to prevent contact with eyes or skin. It can prove fatal if accidentally swallowed. It can also catch fire easily as it is flammable. It is advised that you should familiarize yourself with its MSDS before handling Karl Fischer reagent so that you are aware of its hazardous properties.