Bromothymol blue is a color additive commonly used in various industrial, cosmetic, and scientific applications. It is valued for its vibrant color change properties and versatility as a pH indicator.

Chemical Composition and Structure

Bromothymol blue is a synthetic dye with the chemical formula C₂₇H₂₈Br₂O₅S. It is a member of the sulfonephthalein family of dyes. The structure includes two bromine atoms, which contribute to its color-changing properties, and a complex aromatic ring system that stabilizes the molecule.

Physical Properties

Bromothymol blue typically appears as a crystalline powder that can range from yellow to blue, depending on its state and pH level. It is sparingly soluble in water but soluble in ethanol and other organic solvents. The compound is known for its stability, making it suitable for various applications where precise color change is required.

Chemical Industrial Synthesis Process

• Preparation of reagents. The main raw materials include thymol, potassium bromide (KBr), and sulfuric acid (H₂SO₄).
• Synthesis of bromothymol. Thymol is dissolved in concentrated sulfuric acid. Potassium bromide is slowly added to this solution with constant stirring. The reaction between thymol and potassium bromide in the presence of sulfuric acid produces bromothymol.
• Formation of precipitate. The resulting solution is diluted with distilled water, causing the bromothymol to precipitate.
• Filtration. The suspension is filtered to separate the solid bromothymol precipitate from the aqueous solution.
• Washing. The bromothymol precipitate is washed with deionized water to remove any soluble impurities.
• Drying. The washed bromothymol is dried at controlled temperatures to remove residual moisture and obtain a dry powder.
• Grinding. The dried bromothymol is ground to obtain a fine and uniform powder. This step may involve the use of ball mills or other grinding machinery.
• Final synthesis. The bromothymol powder is treated with an alkaline solution, such as sodium hydroxide (NaOH), to form bromothymol blue.
• Filtration and washing. The suspension is filtered and washed with deionized water to remove any impurities.
• Drying. The washed bromothymol blue is dried at controlled temperatures to remove residual moisture and obtain a dry powder.
• Quality control. The bromothymol blue undergoes rigorous quality testing to ensure it meets standards for purity, color intensity, and safety. These tests include chemical analysis and spectroscopy.

What it is used for and where

Cosmetics

Restricted cosmetic ingredient as  IV/151 a Relevant Item in the Annexes of the European Cosmetics Regulation 1223/2009. Substance or ingredient reported:

• Phenol, 4,4'-(3H-2,1-benzoxathiol-3-ylidene)bis[2-bromo-3-methyl-6-(1-methylethyl)-, S,S-dioxide

Cosmetics - INCI Functions

• Colorant. This ingredient has the function of colouring the solution in which it is inserted in a temporary, semi-permanent or permanent manner, either alone or in the presence of the complementary components added for colouring.

Safety

It is an ingredient that has some important contraindications concerning the health profile: to be used only in cosmetic products that have skin contact for a short time.

Medical

Bromothymol blue is used in some medical tests and diagnostic procedures. For example, it can be used in respiratory tests to measure carbon dioxide levels, where the color change indicates the presence of CO₂.

Industrial Applications

pH Indicator: Bromothymol blue is widely used as a pH indicator in laboratory settings (1). It changes color from yellow at pH levels below 6.0 to blue at pH levels above 7.6, making it useful for monitoring pH changes in various chemical reactions and solutions.

Aquariums and Pools: It is used to monitor the pH of water in aquariums and swimming pools. The color change provides a visual indication of water quality, helping maintain optimal conditions for aquatic life and swimmers.

Molecular Formula    C₂₇H₂₈Br₂O₅S

Molecular Weight   624.4 g/mol

CAS   76-59-5

EC number 200-971-2

UNII VGU4LM0H96

DTXSID3058799

MFCD00005872

Synonyms: 

Dibromothymolsulfophthalein

References__________________________________________________________________________

(1) Marschke RJ, Kitchen BJ. Detection of bovine mastitis by bromothymol blue pH indicator test. J Dairy Sci. 1985 May;68(5):1263-9. doi: 10.3168/jds.S0022-0302(85)80955-3. 

Abstract. A simple bromothymol blue indicator test was evaluated for farm diagnosis of mastitis. The test required highly absorbent blotting paper impregnated with four spots of bromothymol blue. Indicator color scores (1 to 4) for quarter foremilks increased with somatic cell count and pH, although variability within each color score was large. Sensitivity of the bromothymol blue test ranged from 51 to 56% and specificity from 89 to 90% for most reference criteria used to classify normal and abnormal milk. Predictability of a positive test ranged from 49 to 52% (false positives 51 to 48%) and predictability of a negative test from 90 to 97% (false negatives 10 to 3%) for the same criteria. Overall the bromothymol blue test incorrectly diagnosed 11 to 20% of 3772 quarters. By classifying color score 2 as negative, predictability of a positive result was 70 to 75% and sensitivity was 26 to 30%. The test can be used by dairy producers to screen herds with a relatively high incidence of mastitis or used in combination with cow cell counts to locate abnormal quarters. The bromothymol blue test was less sensitive than the California Mastitis Test but offered several practical advantages for use on farm.

Dashtbin, R., Mahmoudi, N., Besharati, H., & Lalevic, B. (2023). Identification of sulfur-oxidizing bacteria from fishponds and their performance to remove hydrogen sulfide under aquarium conditions. Brazilian Journal of Microbiology, 54(4), 3163-3172.

Abstract. Hydrogen sulfide is a highly toxic gas that causes many economic losses in aquaculture ponds. The application of sulfur-oxidizing bacteria (SOB) to remove hydrogen sulfide is an eco-friendly approach. This study aimed to isolate and identify the most efficient SOBs from the sediment of warm-water fish farms. Enrichment and isolation were performed in three different culture media (Starkey, Postgate, and H-3) based on both mineral and organic carbon. Overall, 27 isolates (14 autotrophic and 13 heterotrophic isolates) were purified based on colony and cell morphology differences. Initial screening was performed based on pH decrease. For final screening, the isolates were assessed based on their efficacy in thiosulfate oxidation and the sulfate production on Starkey liquid medium. Among isolated strains, 3 strains of Iran 2 (FH-13), Iran 3 (FH-21), and Iran 1 (FH-14) that belonged to Thiobacillus thioparus species (identified by 16s rRNA) showed the highest ability in thiosulfate oxidation (413.21, 1362.50, and 4188.03 mg/L for 14 days) and the highest sulfate production (3350, 2075, and 1600 mg/L). In the final phase, the performance of these strains under aquarium conditions showed that Iran 1 and Iran 2 had the highest ability in sulfur oxidation. In conclusion, Iran 1 and 2 strains can be used as effective SOB to remove hydrogen sulfide in fish farms. It is very important to evaluate strains in an appropriate strategy using a combination of different criteria to ensure optimal performance of SOB in farm conditions.