What is the Difference Between Thymolphthalein and Phenolphthalein

The key difference between thymolphthalein and phenolphthalein is that the color change of thymolphthalein occurs from colorless to blue, whereas the color change of phenolphthalein occurs from colorless to pink color upon changing the reaction conditions from acidic to basic.

Thymolphthalein and phenolphthalein are two different pH indicators that are very useful in titrimetric analytical processes.

1. Overview and Key Difference
2. What is Thymolphthalein
3. What is Phenolphthalein
4. Thymolphthalein vs Phenolphthalein in Tabular Form
5. Summary – Thymolphthalein vs Phenolphthalein

What is Thymolphthalein?

Thymolphthalein is a type of phthalein dye that is useful as an acid-base indicator. The chemical formula of thymolphthalein is C28H30O4. It is a pH indicator that gives its color change upon the changes in pH of the reaction mixture. The transition pH range of this indicator is around 9.3 – 10.5. Thymolphthalein is colorless below pH 9.3, whereas it appears in blue color at pH values above 10.5. Moreover, the molar extinction coefficient of thymolphthalein is 38 000 M^-1cm^-1 at 595 nm for the blue color indicator anion.

Thymolphthalein and Phenolphthalein - Side by Side Comparison

Figure 01: The Chemical Structure of Thymolphthalein Indicator

The synthesis of thymolphthalein can be performed using thymol and phthalic anhydride. The product of this synthesis reaction is a white powder which is also the commercially available form of thymolphthalein. At high temperatures, this substance tends to decompose. Furthermore, this substance is used as a laxative and for disappearing ink as well.

What is Phenolphthalein?

Phenolphthalein is a pH indicator that is useful as an acid-base titration indicator. This is a very common indicator that we often use in our laboratory titration processes. The chemical formula of this substance is C20H14O4. We can write this term simply as “Hin” or as “phph”. The acidic color of phenolphthalein is colorless, while the basic color of phenolphthalein is pink. The pH range for this color change to occur is around 8.3 – 10.0 pH.

Moreover, the phenolphthalein indicator is slightly water-soluble, and often, it dissolves in alcohols. This way, we can easily use them in titrations. Phenolphthalein is a weak acid that can release protons to the solution. The acidic form of phenolphthalein is nonionic, and it is colorless. The deprotonated form of phenolphthalein is pink colored, and it is an ionic form. If we add a base to the reaction mixture consisting of phenolphthalein indicator, the equilibrium between the ionic and nonionic forms tends to shift towards the deprotonated state because the protons are removed from the solution.

Thymolphthalein vs Phenolphthalein in Tabular Form

Figure 02: Basic Color of Phenolphthalein

Concerning the synthesis of phenolphthalein indicator, we can produce it from the condensation of phthalic anhydride in the presence of two equivalents of phenol under acidic conditions. Moreover, this reaction can be catalyzed using a mixture of zinc chloride and thionyl chloride.

What is the Difference Between Thymolphthalein and Phenolphthalein?

Thymolphthalein and phenolphthalein are two different pH indicators that are very useful in titrimetric analytical processes. The key difference between thymolphthalein and phenolphthalein is that the color change of thymolphthalein occurs from colorless to blue, whereas the color change of phenolphthalein occurs from colorless to pink color upon changing the reaction conditions from acidic to basic. Moreover, the active pH range of thymolphthalein is 9.3 to 10.5 whereas the active pH range of phenolphthalein is 8.3 to 10.0.

The below infographic presents the differences between thymolphthalein and phenolphthalein in tabular form for side by side comparison.

Summary – Thymolphthalein vs Phenolphthalein

Reference:

1. “Phenolphthalein.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine.

Image Courtesy:

1. “Phenolphthalein-at-pH-9” By Ben Mills – Own work (Public Domain) via Commons Wikimedia
2. “Thymolphthalein” By NEUROtiker – Own work (Public Domain) via Commons Wikimedia