Indicators and Indicator Test Papers

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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INDICATORS

1 SCOPE

Indicators are required in Pharmacopeial tests and assays either to indicate the completion of a chemical reaction in volumetric analysis or to indicate the hydrogen-ion concentration (pH) of solutions. The necessary solutions of indicators are listed among the Test Solutions, abbreviated TS.

2 PREPARATION OF SOME INDICATORS

Solution of indicators of the basic type and of the phthaleins are prepared by dissolving in alcohol.

With indicators containing an acidic group, the acid must first be neutralized with sodium hydroxide (NaOH) as follows. Triturate 100 mg of the indicator in a smooth-surfaced mortar with the volume of 0.05 N sodium hydroxide specified in the directions for preparing its test solution, or with the equivalent of 0.02 N sodium hydroxide. After solubilization of the indicator, dilute the solution with carbon dioxide-free water to 200 mL (final concentration 0.05%). Store the solution in suitably resistant containers, protected from light.

3 pH RANGE AND COLOR CHANGE FOR SOME USEFUL INDICATORS