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 colourless in acidic solutions and magenta in basic solutions. It belongs to class of dyes known as phthalein dyes.
|Preferred IUPAC name
3D model (JSmol)
|Molar mass||318.33 g·mol−1|
|Density||1.277 g/cm3 (32 °C (90 °F))|
|Melting point||258–263 °C (496–505 °F; 531–536 K) |
|400 mg/L in water|
|Solubility in other solvents||Insoluble in benzene or hexane, very soluble in ethanol and ether, slightly soluble in DMSO|
|UV-vis (λmax)||552 nm (1st)|
374 nm (2nd)
|GHS signal word||Danger|
|H341, H350, H361|
|P201, P281, P308+313|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
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 colourless, and the phenolphthalein ion is pink.When a base is added to the phenolphthalein, the molecule ⇌ ions equilibrium shifts to the right, leading to more ionization as H+ ions are removed. This is predicted by Le Chatelier's principle.
Phenolphthalein adopts four different states in aqueous solution: Under very strongly acidic conditions, it exists in protonated form (HIn+), providing an orange colouration. Between strongly acidic and slightly basic conditions, the lactone form (HIn) is colourless. The doubly deprotonated (In2-) phenolate form (the anion form of phenol) gives the familiar pink colour. In strongly basic solutions, phenolphthalein is converted to its In(OH)3− form, and its pink colour undergoes a rather slow fading reaction and becomes completely colourless above 13.0 pH.
|Conditions||strongly acidic||acidic or near-neutral||basic||strongly basic|
|Color||orange||colourless||pink to fuchsia||colourless|
|An animation of the pH dependent reaction mechanism: H3In+ → H2In → In2− → In(OH)3−|
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 colourless, it shows that the concrete has undergone carbonation. In a similar application, spackling used to repair holes in drywall contains phenolphthalein. When applied, the basic spackling material retains a pink colouration; when the spackling has cured by reaction with atmospheric carbon dioxide, the pink colour fades.
In a highly basic solution, phenolphthalein's slow change from pink to colourless as it is converted to its In(OH)3− form is used in chemistry classes for the study of reaction kinetics.
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 colour 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 colour 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 colour is desired.
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. 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 Calcium release activated channel § Structure). This is effected by its inhibiting thrombin and thapsigargin, two activators of SOCE that increase intracellular free calcium.
A reduced form of phenolphthalein, phenolphthalin, which is colourless, 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.
Phenolphthalein can be synthesized by condensation of phthalic anhydride with two equivalents of phenol under acidic conditions (hence the name). It was discovered in 1871 by Adolf von Baeyer.
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- Max Hubacher, U.S. Patent 2,192,485 (1940 to Ex Lax Inc)
- U.S. Patent 2,522,939
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