Phenolphthalein Colour Change Explained: From Acid to Alkali

What Is Phenolphthalein?

Phenolphthalein is an organic compound (C₂₀H₁₄O₄), that appears as a white to pale yellow crystal, with no smell. Its melting point is approximately 261–265 °C, which means it’s stable and easy to store in powder form. Phenolphthalein is most famous as a pH indicator which is a substance you add to liquids to visually signal when they switch from an acid to an alkali.

What causes Phenolphthalein to change colour?

Phenolphthalein changes colour due to a shift in its molecular structure that depends on the pH of the solution. In acidic environments, it remains colourless because its molecular form does not absorb visible light. However, as the solution becomes more alkaline (typically above pH 8.3), phenolphthalein loses hydrogen ions, triggering a structural change. This new form absorbs visible light, giving the solution a pink to fuchsia colour. At very high pH levels (above 13), it may become colourless again due to further changes in its structure. 

Phenolphthalein’s pH Range Breakdown

Phenolphthalein’s colour change is closely tied to the pH of the solution it's in. Here is a breakdown of how it behaves across different levels:

Acidic (pH < 7) – At this point, the solution remains completely colourless.

Neutral (pH = 7) – Still clear, no hint of colour, so it won’t show in standard water.

Slightly alkaline  (pH 8.3 – pH10.0) – Always turns from pale pink to bright fuchsia as the pH increases.

Very alkaline (pH above ~13) – It will then fade back to colourless.

This makes phenolphthalein ideal for identifying the tipping point between neutral and alkaline solutions, especially in simple titration setups.

Phenolphthalein’s Discovery & Usage

Phenolphthalein was first synthesised in 1871 by the German chemist Adolf von Baeyer, who would later go on to win the Nobel Prize in Chemistry. While investigating coal tar derivatives and the development of synthetic dyes, von Baeyer created phenolphthalein as part of his work on triphenylmethane compounds. Its vivid colour transformation in different pH environments made it an instant candidate for use in analytical chemistry.

Since then, phenolphthalein has become one of the most widely used acid–base indicators in the world. It plays a central role in laboratory titrations, where its sharp transition from colourless to pink makes it easy to detect the exact moment a chemical reaction reaches its endpoint. It's occasionally used in other applications too, such as testing the alkalinity of water, demonstrating pH changes in educational displays, and even in some forensic chemistry methods.

Conclusion

From its origins in 19th-century dye research to its modern-day role in pH testing and titration, Phenolphthalein continues to be valued for its simplicity, reliability, and striking visual clarity. 

As one of the most recognisable pH indicators available, it remains a staple in both professional and educational settings. 

Where Can I Buy Phenolphthalein in the UK?

You can buy high-quality phenolphthalein solutions and powders directly from APC Pure, available in a range of concentrations for lab, educational, and home use.

Shop Phenolphthalein Now