- Dissolve the thymolphthalein in ethyl alcohol and water solution.
- Add sodium hydroxide solution dropwise until the solution turns a dark blue.
CO2 + H2O ----> H2CO3
The carbonic acid then reacts with the sodium hydroxide in a neutralization reaction. This reaction form sodium carbonate which is a washing soda and the ink disappears.
2Na(OH) + H2CO3 -------> Na2CO3 + 2H2O
The carbon dioxide is the acid that neutralizes the base sodium hydroxide. The color of the thymolphthalein is colorless in the presence of acid.
We used paper. Paper allows less interaction with air, so the color change reaction takes more time. In a few second the stain will disappear. If you spray over the spot with ammonia the color will return.
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