Phenolphthalein / ̩ f i ː n ɒ l f ̍ θ eɪ l i ː n / [1] is a chemical compound with the formulaC20H14O4 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. The molecule has four forms:

Species H3In+ 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 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, 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.

Contents

1 Synthesis

2 Uses

3 Physiology

4 See also

5 References

6 External links

Synthesis[edit]

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.[2][3][4]

Uses[edit]

Phenolphthalein has been used for over a century

as a laxative, but is now being removed from over-

the-counter laxatives[5] because of concerns

over carcinogenicity.[6][7]

Phenolphthalein is used in a test to identify

substances that are thought to be, or to contain,

blood. This test 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 contains hemoglobin, it will turn pink.

This is considered a positive test and indicates that the sample contains hemoglobin and, therefore, is

likely blood. 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 HairBarbie hair. In the ink it is mixed with sodium hydroxide, which reacts with carbon

Phenolphthalein (pH indicator)

below pH 8.2 between pH 10.0 and 13.0

colorless ⇌ fuchsia

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.[8]

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).[9]

The acid-base indication abilities of phenolphthalein 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. The pH of the ionic water solution present

in the pores of fresh concrete may be over 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

8½ to 9, although that reaction usually is restricted to a thin layer at the surface. When a 1%

phenolphthalein solution is applied to normal concrete it will turn bright pink. If the concrete has

undergone carbonation, no color change will be observed