Disappearing ink is a simple acid-base indicator, namely thymolphthalein, that is colored around pH 10.5 and is colorless around pH 9.3. Add about 50 mL of ethanol into a beaker and added 5 drops of thymolpthalein. Next add just enough drops of NaOH to turn the solution deep blue. If this is squirted onto white cloth, such as a dish towel, the stain will be apparent, but will lose its color as CO₂ reacts with NaOH to produce to Na₂CO₃. The ethanol evaporates leaving white Na₂CO₃. {Chemical Demonstrations: A Sourcebook for Teachers by Lee R. Summerlin, Christy L Borgford, and Julie B. Ealy.}

Flinn Scientific Inc. produced a CHEM FAX that describes disappearing ink as being prepared by 0.10 grams of thymolphthalein added to 10 mL ethanol and mixed. The mixture is next added to 90 mL of distilled or deionized water. To make the solution deep blue add 20 drops of 1 M NaOH. If this ink is applied to a cloth the stain will appear blue and disappear within a few minutes. Written on paper this disappearing ink can be made to reappear by adding a NaOH mist.