Phenolphthalein


Phenolphthalein is a chemical compound with the formula C20H14O4 and is often written as "HIn" or "phph" in shorthand notation. Phenolphthalein is often used as an indicator in acid–base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions. It belongs to the class of dyes known as phthalein dyes.
Phenolphthalein is slightly soluble in water and usually is dissolved in alcohols for use in experiments. It is a weak acid, which can lose H+ ions in solution. The phenolphthalein molecule is colorless, and the phenolphthalein ion is pink. When a base is added to the phenolphthalein, the equilibrium shifts, leading to more ionization as H+ ions are removed. This is predicted by Le Chatelier's principle.

Uses

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 at least four different states in aqueous solution as a result of pH changes. Under strongly acidic conditions, it exists in protonated form, providing an orange coloration. Between strongly acidic and slightly basic conditions, the lactone form is colorless. The doubly deprotonated phenolate form gives the familiar pink color. In strongly basic solutions, phenolphthalein is converted to its In3− form, and its pink color undergoes a rather slow fading reaction and becomes completely colorless above 13.0 pH.
SpeciesH3In+H2InIn2−In3−
Structure
Model
pH<-10−8.28.2−10.0>10.0
Conditionsstrongly acidicacidic or near-neutralbasicstrongly basic
Colororangecolorlesspink to fuchsiacolorless
Image

An animation of the pH dependent reaction mechanism: H3In+ → H2In → In2− → In3−

Concrete carbonation

Phenolphthalein's pH sensitivity is exploited in other applications: concrete has naturally high pH due to the calcium hydroxide formed when Portland cement reacts with water. As the concrete reacts with carbon dioxide in the atmosphere, pH decreases to 8.5-9. When a 1% phenolphthalein solution is applied to normal concrete, it turns bright pink. However, if it remains colorless, it shows that the concrete has undergone carbonation. In a similar application, some spackling used to repair holes in drywall contains phenolphthalein. When applied, the basic spackling material retains a pink 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 In3− 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:
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 concerns over 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. 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.
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.

Synthesis

Phenolphthalein can be synthesized by condensation of phthalic anhydride with two equivalents of phenol under acidic conditions. It was discovered in 1871 by Adolf von Baeyer.
The reaction can also be catalyzed by a mixture of zinc chloride and thionyl chloride.