Water Determination in Herbal Drugs

A revised version of the previously updated USP chapter <561> Articles of Botanical Origin (ABOs) has been published for comment in the Pharmacopeial Forum PF 52(4). The comment deadline is 30 September 2026.

Additional methods for the determination of water content

Currently, this chapter includes only one method for determining water content: USP General Chapter <921> Water Determination, Method III. This method is based on USP General Chapter <731> Loss on Drying and is suitable for plant materials that do not contain volatile components.

The azeotropic toluene distillation (ATD) method described in <921>, Method II, is frequently used in pharmacopeial monographs to determine the water content of plant materials containing volatile components. However, ATD is environmentally hazardous because it requires relatively large quantities of toluene and distillation at high temperatures.

To provide a greener approach to water determination in plant materials, <921>, Method Ia, is recommended because of its efficiency, specificity, and accuracy. Direct titration of plant material, however, is difficult because water is tightly trapped in plant cells. Therefore, an alternative approach is extracting water with formamide followed by titrating the water extract.

The following three methods are being added:

• Method I: Direct Titration with Solvent Extraction. This method includes water extraction with formamide followed by titration using USP <921>, Method Ia (also known as the Karl Fischer (KF) test).
• Method II: Loss on Drying (LOD): This is the current method with changes and suitable for plant materials that do not contain volatile components.
• Method III: Azeotropic–Toluene Distillation. This method uses USP <921>, Method II and is suitable for plant materials containing volatile components.

More information is available in the Pharmacopeial Forum.