phenolphthalein

Phenolphthalein
IUPAC name	3,3-bis(4-hydroxyphenyl)isobenzofuran-1(3H)-one
Identifiers
CAS number	77-09-8 Y
SMILES	Oc1ccc(cc1)C3(OC(=O)c2ccccc23)c4ccc(O)cc4
Properties
Molecular formula	C20H14O4
Molar mass	318.32 g mol−1
Density	1.277 g cm−3, at 32 °C
Melting point	262.5 °C
Boiling point	N/A
Solubility in water	Insoluble
Solubility in other solvents	Insoluble in benzene, very soluble in ethanol and ether, slightly soluble in DMSO
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Phenolphthalein is a chemical compound commonly pronounced [ˌfi nɔl ˈθei lin]. It has the formula C₂₀H₁₄O₄ and is often written as "HIn" or "phph" in shorthand notation. Often used in titrations, it turns colorless in acidic solutions and pink in basic solutions. If the concentration of indicator is particularly strong, it can appear purple. In strongly basic solutions, phenolphthalein's pink color undergoes a rather slow fading reaction and becomes colorless again. In other words, the molecule has four forms:

Species	In	H2In	In2−	In(OH)3−
Structure
Model
pH	<0	0−8.2	8.2−12.0	>12.0
Conditions	strongly acidic	acidic or near-neutral	basic	strongly basic
Color	orange	colorless	pink to fuchsia	colorless
Image

The rather slow fading reaction that produces the colorless InOH³⁻ ion is sometimes used in classes for the study of reaction kinetics.

Phenolphthalein is insoluble in water and is usually dissolved in alcohols for use in experiments. It is itself a weak acid, which can lose H⁺ ions in solution. The phenolphthalein molecule is colorless. However, 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.

Synthesis

Phenolphthalein is 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.

Uses

Phenolphthalein has been used for over a century as a laxative, but is now being removed from over-the-counter laxatives^[1] because of concerns over carcinogenicity.^[2][3]

Phenolphthalein is used to perform a presumptive blood test, and is commonly known as the Kastle-Meyer test. A dry sample is collected with a swab or filter paper. First a few drops of alcohol, then a few drops of phenolphthalein and finally a few drops of hydrogen peroxide are dripped onto the sample. If the sample turns pink then it is a positive test. This test is nondestructive to the sample; it can be kept and used in further tests at the lab. This test has the same reaction with blood from any animal, so further testing would be required to determine whether it originates from a human.

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 via the reaction:

OH⁻ (aq) + CO₂ (g) → CO₃²⁻ (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 by the same reaction with carbon dioxide detailed above. Thymolphthalein is used for the same purpose and in the same way, when blue color is desired. [1]

Phenolphthalein in alkaline solution

Phenolphthalein is used as an acid or base indicator where in contact or presence of acid it will turn colorless and with base, it will turn into a pinkish violet color. It is also a component in universal indicator, a solution consisting of a mixture of pH indicators (usually phenolphthalein, methyl red, bromothymol blue, and thymol blue).^[4]

The acid-base indication abilities of phenolpthalien also make it useful for testing for signs of carbonation reactions in concrete. Concrete has naturally high pH due to the calcium hydroxide formed when portland cement reacts with water. Fresh concrete pore solution can be over a pH of 14. Normal carbonation of concrete occurs as the cement hydration products in concrete react with carbon dioxide in the atmosphere and can reduce the pH to 8 1/2 to 9, although that reaction usually is restricted to a thin layer at the surface. When a 1% phenolpthalien solution is applied to normal concrete it will turn bright pink. If the concrete has undergone carbonation, no color change will be observed.

See also

• Bromothymol blue
• pH indicator
• Universal indicator

References

External links

• Page on different titration indicators, including phenolphthalein