In tests for readily carbonizable substances, unless otherwise directed, add the specified quantity of the substance, finely powdered if in solid form, in small portions to the comparison container, which is made of colorless glass resistant to the action of sulfuric acid and contains the specified volume of sulfuric acid TS (see under Test Solutions).
Stir the mixture with a glass rod until solution is complete, allow the solution to stand for 15 minutes, unless otherwise directed, and compare the color of the solution with that of the specified matching fluid in a comparison container, which also is of colorless glass and has the same internal and cross-section dimensions, viewing the fluids transversely against a background of white porcelain or white glass.
When heat is directed in order to effect solution of the substance in the sulfuric acid TS, mix the sample and the acid in a test tube, heat as directed, and transfer the solution to the comparison container for matching with the designated Matching Fluid (see Color and Achromicity 631).
Special attention is directed to the importance of the concentration of sulfuric acid used in this test. The reagent of the required strength, i.e., 95.0 ± 0.5 percent of H2SO4, is designated as a “Test Solution.”

Auxiliary Information—
Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
General Chapter Antonio Hernandez-Cardoso, B.S.
Scientist, Latin American Specialist
(GC05) General Chapters 05
USP32–NF27 Page 138
Pharmacopeial Forum: Volume No. 33(6) Page 1258