Thymolphthalein is a phthalein dye used as an acid–base (pH) indicator. It is colorless in acidic form and deep blue in basic form. Its transition range is around pH 9.3–10.5. Below this pH, it is colorless; and above this, it is blue. Acidic form retains hydrogen on each hydroxyl group. The molar extinction coefficient for the blue thymolphthalein dianion is 38000 M-1cm-1 at 595 nm.
Thymolphthalein can be synthesized from thymol and phthalic anhydride.
It is white fine crystalline powder which is insol in water, soluble in alcohol, acetone and also soluble in dilute alkalies with a blue color, in H2SO4 with a carmine-red color.
Thymolphthalein is also known to have used as a laxative and for disappearing ink. An alkaline solution of thymolphthalein in alcohol makes a very good disappearing ink. The solution starts out blue, but when it absorbs carbon dioxide from the air, carbonic acid is formed, neutralizing the alkaline solution and causing the blue color to vanish. The color returns when the ink spot is treated with an alkaline solution.
Thymolphthalein was suitable as indicator during titration of hyamine hydroxide.
|Molecular weight||430.54 g·mol−1|
|Appearance||white fine crystalline powder|
|Clarity||Clear without any particles|
|Melting Point||248 to 252 °C|
|Boiling point||571.6 °C at 760 mmHg|
|Solubility||Insoluble in water|
|Storage||Stored at room temperature. Store at 10-30°C|
|indicator (pH 8.8-10.5)|
|251-253 °C (lit.)|