Phenolphthalein

Phenolphthalein (pH indicator)
below pH 8.2		above pH 10.0
colorless	↔	pink

Phenolphthalein is a sensitive pH indicator with the formula C₂₀H₁₄O₄. Often used in titrations, it turns from colorless in acidic solutions to 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 three forms:

H₂Phenolphthalein
acidic
no color

↔

Phenolphthalein^2-
basic
pink

↔

Phenolphthalein(OH)^3-
strong alkaline
no color

The fading reaction is sometimes used in undergraduate 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.

Uses

Phenolphthalein has been used for over a century as a laxative, but is now being removed from the market because of concerns over carcinogenity. However, the small amounts usually used in experiments are harmless. Phenolphthalein is also commonly used in a mixture, primarily by forensic scientists, to test for the presence of blood.

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 $OH^{-}_{(aq)} + CO_{2 (g)} \longrightarrow CO^{2-}_{3 (aq)} + H^{+}_{(aq)}$ .

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 colour 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. ^*

External links

Page on different titration indicators, including phenolphthalein

pH indicators | Triarylmethane dyes

Gourt Home::Gourt :: Phenolphthalein

Uses

See also

External links