Water Determination in Plants by Karl Fischer with Extraction

Water determination for plant materials generally uses loss on drying (LOD) methods. The azeotropic-toluene distillation (ATD) method is often used in pharmacopeial and food compendial monographs to determine the water content of plant materials containing volatile components because LOD will lose both water and volatile components. ATD is an environmentally hazardous method since it uses relatively large amounts of toluene with distillation.

To seek a greener approach for water determination in plant materials, USP <921> Water Determination, Method I, also known as the Karl Fischer (KF) test, is recommended due to its efficiency, specificity, and accuracy. However, it may be difficult to determine water content of plant materials using direct titration since water is tightly trapped in plant cells. An alternative approach is needed.

The United States Pharmacopeial Convention (USP) laboratory therefore developed and validated an alternative method-KF direct titration with extraction using formamide as the extraction solvent.

The validation results demonstrated that the newly developed method met the required criteria for accuracy, repeatability and robustness8.

Using the newly established method, the water content in different species with different plant parts including fruit peel, root, rhizome, bark, cremocarp and bulb was effectively determined.

The results generated using the KF direct titration with extraction were also compared with results of LOD applying different heating times and direct titration using different solvents (see Table 1). The differences in the results between the methods will be discussed.