Indicators are required in the 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.”

Solutions 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 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. When the indicator has dissolved, dilute the solution with carbon dioxide-free water to 200 mL (0.05%). Store the solutions in suitably resistant containers, protected from light.

Listed in ascending order of the lower limit of their range, useful pH indicators are: thymol blue, pH 1.2–2.8; methyl yellow, pH 2.9–4.0; bromophenol blue, pH 3.0–4.6; bromocresol green, pH 4.0–5.4; methyl red, pH 4.2–6.2; bromocresol purple, pH 5.2–6.8; bromothymol blue, pH 6.0–7.6; phenol red, pH 6.8–8.2; thymol blue, pH 8.0–9.2; and thymolphthalein, pH 8.6–10.0.

Loss on drying 731 —Dry it in vacuum at 60 for 1 hour: it loses not more than 5% of its weight.

Melting range 741: between 261 and 264.

Sensitiveness —Dissolve 100 mg in 100 mL of glacial acetic acid, and mix. Pipet 1 mL of the solution into a 100-mL volumetric flask, and dilute with glacial acetic acid to volume: the solution is violet-blue in color and does not show a reddish tint. Pipet 20 mL of the diluted solution into a beaker, and titrate with 0.1 N perchloric acid VS, adding the perchloric acid slowly from a microburet: not more than 0.10 mL of 0.1 N perchloric acid is required to produce an emerald-green color.

Sensitiveness —To 10 mL of a 1 in 200,000 solution in a mixture of equal parts of methanol and water add sodium hydroxide solution (1 in 100) until the pH is 10: the solution is pure blue in color and free from cloudiness. Add 0.01 mg of magnesium ion (Mg): the color of the solution changes to red-violet, and with the continued addition of magnesium ion it becomes wine-red.