Thymolphthalein is an acid-base indicator. It changes colour based on the substance it is added to and indicates whether it is an acid or base.

Usage as an Indicator

When used as an indicator, it behaves as follows:

  • Below pH 9.3 – 10.5, it is colourless.
  • Above pH 9.3-10.5, it is blue in colour.

Thus, it undergoes a transition between pH 9.3-10.5.

Thymolphthalein in action
The indicator in action (Source)

Structure of Thymolphthalein

We have the structure of chemicals as follows:

The 2-d Structure of Thymolphthalein
The 2-d Structure of the indicator (Source)

Other Uses of Thymolphthalein

Other than being used as an indicator, it can be used in:

  • Disappearing ink
  • The Kastle-Mayer test for blood
  • Making of coloured bubbles
  • Laxatives

Obtaining Thymolphthalein

The chemical can be obtained in the following ways:

Previously Synthesized

It can be obtained in powder form. Then it is dissolved in ethanol solution which is then diluted with water.

Own Synthesis

It can be synthesized by a reaction between thymol and phthalic anhydride.

Synthesis of Thymolphthalein
Synthesis of the indicator (Source)


What are the uses of thymolphthalein?

It has the following uses:
– Used as an indicator for acids and bases
– Also used as a laxative
– It is used in making coloured bubbles
– Used as disappearing ink
– Also used for a test for blood.

What is the colour of thymolphthalein in acids and in bases?

It turns colourless in acids, and blue coloured in bases.

Is thymolphthalein the same as phenolphthalein?

No, it is different from phenolphthalein. However, both are acid-base indicators and are from related chemical families.
Know more on Phenolphthalein

What is the transition pH of thymolphthalein?

The acid-base transition pH of the indicator is 9.3-10.5 pH. 

Related: Methyl Orange

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