Phenolphthalein Explained with Structure, Formula, Application and Uses

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.

Chemistry Notes Free PDFs

Topic	PDF Link
Class 11 Chemistry Short Notes PDF	Download PDF
Class 12 Chemistry Complete Notes	Download PDF
Study Notes for Physics	Download PDF
Class 11 Biology Short Notes PDF	Download PDF
Class 12 Biology Short Notes PDF	Download PDF

• 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.

pH Range	Dominant Species	Condition	Observed Colour
< 0	H₃In⁺	Strongly acidic	Orange
0 – 8.2	H₂In	Acidic or near neutral	Colourless
8.2 – 10.0	In²⁻	Basic	Pink to Fuchsia
> 10.0	In²⁻ + OH⁻₃	Strongly basic	Colourless

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

Hope this article was helpful for your exam preparation. Find more of such Chemistry topics and score well in your exam with the help of carefully curated study material by the professionals available for you on Testbook. Get free assistance for your exam preparation, practice sets for exams, and personalised career guidance on the free Testbook App. Boost your exam preparation with some special offers waiting for you!

Important Links
NEET Exam
NEET Previous Year Question Papers	NEET Mock Test	NEET Syllabus
CUET Exam
CUET Previous Year Question Papers	CUET Mock Test	CUET Syllabus
JEE Main Exam
JEE Main Previous Year Question Papers	JEE Main Mock Test	JEE Main Syllabus
JEE Advanced Exam
JEE Advanced Previous Year Question Papers	JEE Advanced Mock Test	JEE Advanced Syllabus