Phenolphtahlein is a common chemical that many students come across in school labs, especially during titration experiments. It’s best known as an acid-base indicator because it changes colour depending on pH of a solution. When added to a basic solution, it turns pink, but in an acidic solution, it stays colourless. This simple colour makes phenolphthalein very useful in chemistry to find out whether a solution is acidic or basic. In this article, we’ll explore what phenolphthalein is, how it works, its structure, formula, and the different ways it is used in labs.

What is Phenolphthalein?

Phenolphthalein is a chemical used to test whether a solution is acidic or basic. It is colourless in acids and turns pink in basic (alkaline) solutions. In its natural state, phenolphthalein exists as an acid. It is a colorless crystalline solid with the chemical formula C20H14O4. Sometimes it may be in a pale yellow or orange powder form. Phenolphthalein was discovered in the year 1871 by Adolf van Baeyer, who discovered it when he heated phenol with phthalic anhydride in the presence of an acid.

• Molecular formula: C20H14O4
• IUPAC Name: 3,3-bis(4-hydroxyphenyl)-2-benzofuran-1(3H)-one

Basic Properties:

• Appears as a colourless or pale yellow solid
• Turns pink in basic solutions, but remains colourless in acids
• Slightly soluble in water but dissolves well in alcohol 
• Acts as a weak acid in its natural form 

Structure of Phenolphthalein 

The images below show the structure of phenolphthalein. A simple chemical structure, as well as the ball and stick model figures, are shown here.

The presence of two phenols in the structure of phenolphthalein explains its acidic nature—the deprotonation of phenol rings by adding a base that results in the pink colour.

This change can be explained using a process called ionization. The lactone ring is intact in an acidic solution, and phenolphthalein remains colourless. The two phenolic protons are removed when a base is added, and the lactone ring opens to give a quinone-type structure. The negative charge is spread over the quinone ring and results in the delocalization of the charge. This has lower energy, and light emission will happen in the visible region, rendering the pink colour visible to our eyes.

Properties of Phenolphthalein 

The hydrogen on the OH of the phenol is very acidic. It is because of the resonance stabilization of the phenol anion.

Physical Properties of Phenolphthalein 

The physical properties of phenolphthalein are:

• Phenolphthalein usually exists as a colourless crystalline solid. It can sometimes be found as pale yellow or orange powder as well.
• It has a melting point of 262.5 degree celsius
• It has better solubility in alcohols compared to water.

Chemical Properties of Phenolphthalein 

The chemical properties of phenolphthalein are:

• Phenolphthalein decomposes on heating and produces irritating fumes.
• It has an acidic nature and reacts with bases.

Application and Uses of Phenolphthalein

Phenolphthalein is used widely as an indicator, but it has other applications.

• For more than a hundred years, phenolphthalein was used as a laxative. It has been banned recently due to its carcinogenic effect.
• Another form of phenolphthalein has found its application as part of the Kastle-Meyer test. It is a test for checking the presence of blood. The active compound turns pink if haemoglobin is present.
• Phenolphthalein has been used as a component in toys such as disappearing inks and the hair of Barbie dolls.

Limitations of Phenolphthalein

• Only works in a specific pH range (about 8.2 to 10); doesn’t show colour change in strong acids or very weak bases
• Not useful for strong acidic or very basic solutions, as it stays colourless
• Can give false results if the solution has strong colours or impurities
• Not suitable for redox titrations used only for acid base reactions

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