pH indicator Phenolphthalein's common use is as an indicator in acid-base titrations. It also serves as a component of
universal indicator, together with
methyl red,
bromothymol blue, and
thymol blue. Phenolphthalein adopts different forms in aqueous solution depending on the pH of the solution. Inconsistency exists in the literature about hydrated forms of the compounds and the color of sulfuric acid. Wittke reported in 1983 that it exists in protonated form (H3In+) under strongly acidic conditions, providing an orange coloration. The lactone form (H2In) is colorless and dominates between strongly acidic and slightly basic conditions. The doubly deprotonated (In2-) phenolate form (the
anion form of phenol) gives the familiar pink color. In strongly basic solutions, phenolphthalein is converted to its In(OH)3− form, and its pink color undergoes a rather slow fading reaction
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) → carbonate|(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.
Detection of blood 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 test indicates the sample contains hemoglobin and, therefore, is likely blood. A false positive can result from the presence of substances with catalytic activity similar to hemoglobin. This test is not destructive to the sample; it can be kept and used in further tests. This test has the same reaction with blood from any animal whose blood contains hemoglobin, including almost all vertebrates; further testing would be required to determine whether it originated from a human.
Laxative Phenolphthalein has been used for over a century as a
laxative, but is now being removed from
over-the-counter laxatives over concerns of
carcinogenicity. Laxative products formerly containing phenolphthalein have often been reformulated with alternative 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 based on rodent studies, the use of phenolphthalein as a laxative is unlikely to cause
ovarian cancer. Some studies suggest a weak association with
colon cancer, while others show none at all. Phenolphthalein is described as a stimulant laxative. Phenolphthalein has been added to the
European Chemicals Agency's candidate list for
substance of very high concern (SVHC). It is on the
IARC group 2B list for substances "possibly carcinogenic to humans". ==Synthesis==