Chemical Reactions and Equations

Phenolphthalein – Structure and Why it Changes Color?


People know the terms acid or base (acidic or alkaline) from common household things. But, the function of the pH indicator is far advanced. Furthermore one of the pH testers is Phenolphthalein, which is usually colourless but on coming in contact with aid or base changes its colour from pink (acidic) to purple (base).

Phenolphthalein Test

It is an organic compound that we use as an acid or base indicator. Furthermore, the compound is pinkish in basic solution and colourless in acidic solution.

Besides, it’s chemical formula is C20H14O4. It means that it contains 20 carbon atoms, 14 hydrogen atom, and 4 oxygen atoms.

Most noteworthy, it does not dissolve very well in water so usually, alcohol titration is used to prepare this solution. Moreover, when you add a drop of acidic indicator in acid you sometimes see a white cloud generate in it.


This is a precipitate of solid Phenolphthalein that forms as a result of high local concentrates, which exceeding the solubility product. Besides, it disappears when you shake the solution.

Furthermore, it turns pink when it comes in contact with a substance of a pH of 8.2 and becomes purple at an even higher pH. Moreover, this change in colour is a result of ionization; that alter the charge and shape of Phenolphthalein molecule.

Besides, on exposure to the alkaline substance, they produce a pink to purple shade or hue.

Structure of Phenolphthalein

The structure of Phenolphthalein has 3 hexagonal structures, one pentagonal structure, 2 alcoholic groups, and one ketone group. Besides, Phenolphthalein structure form of the carbon, hydrogen, and oxygen chains.

How does Phenolphthalein Indicator Work?

The pH scale has a marking from 0 to 14 fro detecting acid and base. Besides, from 0 to 6.9 resides all the acids and the less the number of acid the more acidic they are. While the pH 7 is neutral which is the pH of water.

Furthermore, from 7.1 to 14 reside the entire base and the higher the number of base on a pH scale the more basic nature they possess. Besides, in a common practice chemist use the litmus paper to test the pH of a compound.

Most noteworthy, the litmus paper turns red on contact with acid and turns blue on coming in contact with the base.

Phenolphthalein is naturally colourless and works differently then litmus paper. Also, it turns in pink colour in an alkaline solution or base.

Moreover, the compound remains colourless in acids but starts to turn pink on pH 8.2 and continues to turn bright purple in strong bases.

Colour Change of Phenolphthalein

Ionization is the process that is responsible for the colour change of the compound (Phenolphthalein). Besides, ionization occurs when a molecule loses or gain an electron, which gives the molecule a positive or negative charge.

Furthermore, the ionized molecule attracts the opposite change and repels the same charge. Moreover, it also affects the molecule shape. Also, the charge determines how it will react to light.

Normally, all colours of the light pass through the Phenolphthalein solution and it appears colourless. But, when it exposed to alkaline then it starts to block the blue colour of the light spectrum, which turns the light pink.

And the more strong the alkaline solution the more Phenolphthalein molecules will change and the darker the colour will get.

Solved Question for You

Question. What is the pH of pure water?

A. 6.0
B. 7.0
C. 8.0
D. 9.0

Answer. The correct answer is option B. As option A is an acid, and option C and D are bases.

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