Thymol Blue | CAS 76-61-9 | Indicator Grade

Technical Specifications

Product Description & Scientific Applications

Thymol Blue (thymolsulfonephthalein) is a sulfonephthalein dye of the Clark–Lubs series and a two-range acid–base pH indicator. The molecule has a triarylmethyl core in which two thymol-derived phenolic rings are attached at a quaternary carbon to a benzoxathiole-1,1-dioxide ring. The two phenols and the sulfonate group give the dye three protonation states accessible across the experimental pH window. It is supplied as a brownish-green to reddish-brown crystalline powder.

Thymol Blue is poorly soluble to practically insoluble in water, and soluble in ethanol, dilute alkali, and sodium hydroxide solution. The dye has two acid dissociation constants and two corresponding colour transitions. The lower transition (pKa1 ≈ 1.65) spans pH 1.2–2.8 and shifts from red to yellow as the protonated form deprotonates. The upper transition (pKa2 ≈ 8.9–9.0) spans pH 8.0–9.6 and shifts from yellow to blue as the para-phenol deprotonates to the phenolate. The yellow intermediate form has λmax 435 nm and the blue alkaline form has λmax 596 nm. Both transitions are sharp enough for visual endpoint detection and for spectrophotometric titration based on the absorbance ratio A596/A435. The apparent pKa2 is sensitive to temperature, salinity, and the presence of lot-specific light-absorbing impurities; for high-precision environmental and oceanographic monitoring, commercial dye batches typically require preparative chromatography to remove structurally related impurities, which can otherwise introduce systematic pH errors of approximately 0.01–0.015 pH units.

Applications:

• Two-range pH indicator for acid–base titrations. Thymol Blue is one of the canonical Clark–Lubs sulfonephthalein indicators, with two transition ranges in a single reagent. The lower transition (pH 1.2–2.8) is used for strongly acidic endpoints; the upper transition (pH 8.0–9.6) is used for weak-acid/strong-base titrations and for endpoints in the alkaline region.
• Visual indicator in non-aqueous titrations. Used in selected non-aqueous alkalimetric titrations of weakly acidic compounds where aqueous endpoints are poorly defined. The upper transition of Thymol Blue gives a yellow-to-blue endpoint in alcohol, DMF, and related non-aqueous media, including pharmaceutical-analysis workflows for weak acids such as sulphonamides and barbiturates.
• Ion-pair extractive spectrophotometry for basic nitrogenous compounds. In acidic aqueous buffers (pH 3–4), the anionic sulfonate group of Thymol Blue forms 1:1 ion-pair complexes with basic nitrogenous compounds, including tertiary amines, alkaloids, phenothiazines, and analogous basic active ingredients. The yellow-coloured complexes are quantitatively extractable into chloroform and quantified by visible spectrophotometry near 400–415 nm, providing a sensitive determination route for basic-nitrogen compounds in complex matrices.
• Component of universal indicator and wide-range pH papers. Thymol Blue is a standard component of Yamada-type universal indicator formulations (1933 patent), where it pairs with methyl red, bromothymol blue, and phenolphthalein to extend the colour coverage of the mixture across pH 1–12. The two transition ranges of Thymol Blue cover both the strongly acidic and weakly alkaline regions of the wide-range scale.
• Spectrophotometric pH measurement in seawater, estuaries, and rivers. Purified Thymol Blue is used as an indicator dye in spectrophotometric determination of seawater pH on the total hydrogen ion scale, with the absorbance ratio A596/A435 as the analytical signal. The method is established in oceanographic carbon-cycle research and in monitoring of CO2-driven ocean acidification; characterization of purified Thymol Blue extends the upper pH range accessible by spectrophotometric methods by approximately 0.5 pH units beyond m-cresol purple, making it particularly suited to surface waters and photosynthesis-driven high-pH regimes.
• Solid-state pH sensing in sol-gel silica matrices. Thymol Blue can be entrapped in transparent monolithic silica matrices prepared by acid-catalysed sol-gel reaction of tetraethylorthosilicate. The immobilised dye retains its pH-dependent absorption behaviour and is used as the chromophore in solid-state and optical-fiber pH sensors operating in the alkaline range (pH 8–12).
• Synthetic precursor for bromothymol blue. Thymol Blue is the parent sulfonephthalein from which bromothymol blue (CAS 76-59-5) is prepared by aromatic bromination of the thymol-derived rings. Reported preparative methods include classical elemental-bromine routes, electrochemical in-situ bromine generation as a green-chemistry variant, and enzymatic bromination using chloroperoxidase from Caldariomyces fumago.

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