Thymolphthalein
[CAS# 125-20-2]

Identification

• Classification: Biochemical >> Biochemical reagent >> Acid-base indicator
• Name: Thymolphthalein
• Synonyms: 3,3-Bis(4-hydroxy-5-isopropyl-o-tolyl)phthalide
• Molecular Formula: C₂₈H₃₀O₄
• Molecular Weight: 430.54
• CAS Registry Number: 125-20-2
• EC Number: 204-729-7
• SMILES: CC1=CC(=C(C=C1C2(C3=CC=CC=C3C(=O)O2)C4=CC(=C(C=C4C)O)C(C)C)C(C)C)O

Properties

• Solubility: insoluble (water), soluble (alcohol, acetone, dilute alkalies, H2SO4) (Expl.)
• Density: 1.2±0.1 g/cm³, Calc.^*, 0.92 g/mL (Expl.)
• Melting point: 251-253 °C (Expl.)
• Index of Refraction: 1.613, Calc.^*
• Boiling Point: 571.6±50.0 °C (760 mmHg), Calc.^*
• Flash Point: 187.5±23.6 °C, Calc.^*

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS08 Danger
• Risk Statements: H341-H350-H361
• Safety Statements: P203-P280-P318-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Reproductive toxicity	Repr.	2	H361
Carcinogenicity	Carc.	1B	H350
Germ cell mutagenicity	Muta.	2	H341
Acute hazardous to the aquatic environment	Aquatic Acute	1	H400
Acute toxicity	Acute Tox.	4	H302
Chronic hazardous to the aquatic environment	Aquatic Chronic	4	H413

Discovery and Applications

Thymolphthalein is a chemical compound commonly used as a pH indicator. It was first synthesized in the early 20th century through the condensation reaction of thymol with phthalic anhydride in the presence of a strong acid catalyst. This synthesis method is similar to the process used for creating other phthalein-based indicators such as phenolphthalein. Thymolphthalein’s structure consists of two thymol groups linked by a central phthalic structure, giving it distinct color-changing properties.

The primary method for synthesizing thymolphthalein involves the reaction of thymol with phthalic anhydride under acidic conditions. The resulting product is then purified and recrystallized to obtain the final indicator compound. This synthesis pathway is efficient and widely used in laboratories and industrial settings.

Thymolphthalein is best known for its application as a pH indicator. It is colorless in acidic to neutral solutions and turns a deep blue in basic conditions, specifically within a pH range of 9.3 to 10.5. This makes it useful for titrations in analytical chemistry where the detection of alkaline conditions is required. It is also employed in educational demonstrations and laboratory experiments to illustrate the concept of acid-base reactions.

Apart from its use as a pH indicator, thymolphthalein is used in invisible ink formulations. When dissolved in alcohol and applied to paper, the writing remains invisible until exposed to a basic solution, which reveals the blue color. This property has been utilized for security and novelty applications.

In industrial applications, thymolphthalein is sometimes used in chemical sensors and diagnostic tools where accurate pH detection is critical. Its stability and clear color transition make it a reliable choice for monitoring pH changes in various chemical processes. Ongoing research into new applications continues to expand its utility in analytical and applied chemistry.

References

1921. Der Farbenumschlag der Indicatoren. Der Gebrauch von Farbenindicatoren.
DOI: 10.1007/978-3-662-26440-9_3

2023. Design, synthesis and application of tetraphenylethene substituted chalcones as pH and Et3N indicators. Chemical Papers.
DOI: 10.1007/s11696-023-02799-z

2024. Rapid method for screening of both calcium and magnesium chelation with comparison of 21 known metal chelators. Journal of Biological Inorganic Chemistry : JBIC : a Publication of the Society of Biological Inorganic Chemistry.
DOI: 10.1007/s00775-024-02078-6