Nonvolatile residue—
Evaporate 100 mL to dryness in a tared dish on a hot plate at 60
: the weight of the residue does not exceed 25 mg (0.025%).
Limit of tannins—
Mobile phase A—
Adjust 1000 mL of water with phosphoric acid to a pH of 2.5 ± 0.1. Filter and degas.
Mobile phase B—
Use filtered and degassed acetonitrile.
Standard solution—
Dissolve an accurately weighed quantity of tannic acid in water to obtain a solution having a known concentration of about 0.03 mg of tannic acid per mL.
Test solution—
Use Witch Hazel.
Chromatographic system
(see
Chromatography 
621
)—The liquid chromatograph is equipped with a 280-nm detector and a 5.0-mm × 15-cm column that contains packing L1 and is programmed to provide variable mixtures of
Mobile phase A and
Mobile phase B. Initially, the system is held at a mixture consisting of 90%
Mobile phase A and 10%
Mobile phase B for 5.0 minutes. The proportion of
Solvent B to
Solvent A is increased linearly to 100% over the next 15 minutes. The flow rate is about 1 mL per minute.
Procedure—
Separately inject equal volumes (about 25 µL) of the
Standard solution and the
Test solution into the chromatograph, record the chromatograms, and measure the responses for all peaks. If any peak corresponding to the retention time of tannic acid in the
Standard solution is present in the chromatogram of the
Test solution, its peak area is no more than that of the tannic acid in the
Standard solution.
