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Thymolphthalein is an acid–base (pH) indicator. Its transition range is around pH 9.3–10.5. Below this pH, it is colorless; above, it is blue. The molar extinction coefficient for the blue thymolphthalein dianion is 38,000 M−1 cm−1 at 595 nm
T 1757 (OTTO) Thymolphthalein Cas 125-20-2 - used as a pH indicator and as a reagent for blood after decolorizing the alkaline solution by boiling with zinc dust.