color; when the spackling has cured by reaction with atmospheric carbon dioxide, the pink color fades. Education In a highly basic solution, phenolphthalein's slow change from pink to colorless as it is converted to its Ph(OH)3− form is used in chemistry classes for the study of reaction kinetics. Entertainment Phenolphthalein is used in toys, for example as a component of disappearing inks, or disappearing dye on the "Hollywood Hair" Barbie hair. In the ink, it is mixed with sodium hydroxide, which reacts with carbon dioxide in the air. This reaction leads to the pH falling below the color change threshold as hydrogen ions are released by the reaction: OH−(aq) + CO2(g) → (aq) + H+(aq). To develop the hair and "magic" graphical patterns, the ink is sprayed with a solution of hydroxide, which leads to the appearance of the hidden graphics by the same mechanism described above for color change in alkaline solution. The pattern will eventually disappear again because of the reaction with carbon dioxide. Thymolphthalein is used for the same purpose and in the same way, when a blue color is desired. Medical uses Phenolphthalein has been used for over a century as a laxative, but is now being removed from over-the-counter laxatives because of risk of carcinogenicity. Laxative products formerly containing phenolphalein have often been reformulated to have alternate active ingredients: Feen-a-Mint switched to bisacodyl and Ex-Lax was switched to a senna extract. Thymolphthalein is a related laxative made from thymol. Despite concerns regarding its carcinogenicity, the use of phenolphthalein as a laxative is unlikely to cause ovarian cancer. Phenolphthalein has been found to inhibit human cellular calcium influx via store-operated calcium entry (SOCE, see ). This is effected by its inhibiting thrombin and thapsigargin, two activators of SOCE that increase intracellular free calcium. Phenolphthalein has been added to the European Chemicals Agency's candidate list for Substances of Very High Concern (SVHC). A reduced form of phenolphthalein, phenolphthalin, which is colorless, is used in a test to identify substances thought to contain blood, commonly known as the Kastle–Meyer test. A dry sample is collected with a swab or filter paper. A few drops of alcohol, then a few drops of phenolphthalin, and finally a few drops of hydrogen peroxide are dripped onto the sample. If the sample contains hemoglobin, it will turn pink immediately upon addition of the peroxide, because of the generation of phenolphthalein. A positive