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1.4 pH indicators

Methyl Red C15H16ClN3O2 = 305.76

Dark red powder or violet crystals. Sparingly soluble in water; soluble in ethanol. Transition interval: from pH 4.2 to 6.3. Colour change: from red to yellow.

Methyl Red TS  Dissolve 10 mg of methyl red in 100 mL of ethanol, and filter if necessary.

Phenolphthalein C20H14O4 = 318.33

White or faintly yellowish white, crystalline powder. Insoluble in water; soluble in ethanol.

Transition interval: from pH 8.3 to 10.0. Colour change: from colourless to red.

SENSITIVITY Dissolve 100 mg in 10 mL of neutralized ethanol, add 0.1 mL of this solution to 200 mL of carbon dioxide-free water, and then add 0.05 mL of 0.10 M sodium hydroxide: the solution is pink in colour when compared with an equal volume of water.

Phenolphthalein TS  Dissolve 1 g of phenolphthalein in 100 mL of ethanol (50 per cent).

APPENDICES • 1.4 pH indicators
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หมายเหตุ / Note : THP 2021 Page 848