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Robert Green
Robert Green

Bromothymol Blue



Bromothymol blue (also known as bromothymol sulfone phthalein and BTB) is a pH indicator. It is mostly used in applications that require measuring substances that would have a relatively neutral pH (near 7). A common use is for measuring the presence of carbonic acid in a liquid. It is typically sold in solid form as the sodium salt of the acid indicator.




bromothymol blue



Bromothymol blue acts as a weak acid in a solution. It can thus be in protonated or deprotonated form, appearing yellow or blue, respectively. It is bright aquamarine by itself, and greenish-blue in a neutral solution. The deprotonation of the neutral form results in a highly conjugated structure, accounting for the difference in color. An intermediate of the deprotonation mechanism is responsible for the greenish color in neutral solution.[2]


The protonated form of bromothymol blue has its peak absorption at 427 nm thus transmitting yellow light in acidic solutions, and the deprotonated form has its peak absorption at 602 nm thus transmitting blue light in more basic solutions.[3] Highly acidic Bromothymol blue is magenta in color.


The presence of one moderate electron-withdrawing group (bromine atom) and two moderate donating groups (alkyl substituents) are responsible for bromothymol blue's active indication range from a pH of 6.0 to 7.6. While the conjugation is responsible for the length and nature of the color change range, these substituent groups are ultimately responsible for the indicator's active range.[2]


Bromothymol blue is sparingly soluble in oil, but soluble in water, ether, and aqueous solutions of alkalis. It is less soluble in nonpolar solvents such as benzene, toluene, and xylene, and practically insoluble in petroleum ether.[5]


Bromothymol blue may be used for observing photosynthetic activities, or as a respiratory indicator (turns yellow as CO2 is added).[7][8] A common demonstration of BTB's pH indicator properties involves exhaling through a tube into a neutral solution of BTB. As CO2 is absorbed from the breath into the solution, forming carbonic acid, the solution changes color from green to yellow. Thus, BTB is commonly used in science classes to demonstrate that the more that muscles are used, the greater the CO2 output.


Bromothymol blue has been used in conjunction with phenol red to monitor the fungal asparaginase enzyme activity with phenol red turning pink and bromothymol blue turning blue indicating an increase in pH and therefore enzyme activity.[9] However, a recent study suggests that methyl red is more useful in determining activity due to the bright yellow ring formed in the zone of enzyme activity.[10]


It may also be used in the laboratory as a biological slide stain. At this point, the bromothymol is already blue, and a few drops of BTB are used on a water slide. The specimen is mixed with blue BTB solution and fixed to a slide by a cover slip. It is sometimes used to define cell walls or nuclei under the microscope.


Bromothymol is used in obstetrics for detecting premature rupture of membranes.[11] Amniotic fluid typically has a pH > 7.2, bromothymol will therefore turn blue when brought in contact with fluid leaking from the amnion. As vaginal pH normally is acidic, the blue color indicates the presence of amniotic fluid. The test may be false-positive in the presence of other alkaline substances such as blood or semen, or in the presence of bacterial vaginosis.


SCCS (Scientific Committee on Consumer Safety), Opinion on sodium bromothymol blue (C186) (CAS No. 34722-90-2, EC No. 252-169-7), preliminary version of 24-25 October 2022, final version of 21-22 March 2023, SCCS/1645/22


The pH dependent chemical structures of bromothymol blue (BTB), which have long been under controversy, are determined by employing a combined technique of multivariate analysis of electronic absorption spectra and quantum chemistry. Principle component analysis (PCA) of the pH dependent spectra apparently reveals that only two chemical species are adequate to fully account for the color changes, with which the spectral decomposition is readily performed by using augmented alternative least-squares (ALS) regression analysis. The quantity variation by the ALS analysis also reveals the practical acid dissociation constant, pKa'. The determination of pKa' is performed for various ionic strengths, which reveals the thermodynamic acid constant (pKa=7.5) and the number of charge on each chemical species; the yellow form is negatively charged species of -1 and the blue form that of -2. On this chemical information, the quantum chemical calculation is carried out to find that BTB molecules take the pure quinoid form in an acid solution and the quinoid-phenolate form in an alkaline solution. The time-dependent density functional theory (TD-DFT) calculations for the theoretically determined chemical structures account for the peak shift of the electronic spectra. In this manner, the structures of all the chemical species appeared in equilibrium have finally been confirmed.


Bromothymol blue is a pH indicator that changes color from blue to green to yellow. When working with bromothymol blue, be careful not to leave the bottle open for extended periods of time, as exposure to carbon dioxide in the atmosphere can cause a color change to occur. If your bromothymol blue has turned green, please contact Science Interactive for a replacement bottle.


The liquid is placed in a small glass U-shaped open ended cylinder and placed inverted into the aquarium water. After about an hour or two the CO2 in the aquarium water will dissipate into the test liquid and alter its pH level therefore making the Bromothymol blue turn colour.


Color changes in the titration are done by the indicator used for the reaction. However, not all indicators will give the same color at a given pH value. It means that one indicator will give a different color compared to another indicator in acidic or basic conditions. Bromthymol blue is one example of an indicator like methyl orange, methyl red, and phenolphthalein.


Bromothymol blue is an acid base indicator. When the solution is basic it is a blue color. As the pH descends to neutral it turns sea green as it passed through a pH of 7.0, and when it crosses over into the acidic range it becomes a light yellow color. The color sequence can be reversed by slowly adding bases to the solution.


Bromothymol blue is an indicator used to detect weak acids and bases. In acidic solutions, it appears yellow, in basic solutions it is blue, and in mid pH ranges, it is green. By determining the absorbance values of solutions at all of these extremes, the equilibrium constant (Kc) of this indicator can be calculated. After taking the spectra of each the yellow, the blue and the green solutions, a Kc value of (1.2 0.19) M and a pKc value of 7.0 0.2 were calculated. When comparing the calculated value to a literature pKc value of 7.1, there is a percent difference of 1.4%.


Bromothymol blue is a chemical indicator that appears yellow in very acidic conditions, blue in basic conditions, and green in mid pH ranges. This experiment will determine the equilibrium constant of this indicator by comparing the absorbance of a buffer solution containing bromothymol blue at its yellow, blue and green states. After the spectrum of each solution is taken, the following equation can be used to determine the equilibrium constant (Kc).


However, to account for the fact that multiple species (different colors) of the bromothymol blue that may exist in the solution at one time, the revised equation below subtracts the absorbance of the invading species from the one of interest.


According to the data, the calculated equilibrium constant of the bromothymol blue indicator was (1.2 0.19) M, which translates to a pKc of 7.0 0.2. When compared to the literature pKc value of 7.1, the percent difference is 1.4%. The calculated value may have disagreed slightly because the calibration curve of the pH meter used was only 90.0. This means that the hydronium concentration calculated may be slightly off, thus throwing off the equilibrium value. Error may have also occurred during the spectrum readings since a SpectroVis Plus reader that correlated with Logger Pro 3 was used instead of a more sophisticated UV-Vis spectrometer. To improve the results, a more sensitive spectrometer and a single cuvette could be used to gain all solution spectra.


In order to demonstrate the safety of sodium bromothymol blue, this dossier submission is established on a battery of non-animal methods and approaches, including in silico and in vitro methods, assessing inter alia irritation, skin sensitisation, genetic toxicity, and percutaneous absorption, while following Good Laboratory Practice and OECD guidelines. In particular, notwithstanding the absence of data on repeated doses or reproductive/ developmental toxicity, the submitters have used the Toxicological Threshold of Concern (TTC) approach to support the safe use of this ingredient based on the very low consumer exposure estimate.


-The Applicant has provided data on two batches of sodium bromothymol blue. The SCCS has concerns about the impurities identified in one of the batches (Guanghua batch), as the levels of these impurities exceed the TTC threshold both individually and collectively. Therefore, the SCCS considers that the presence of these impurities is not safe, and a batch of sodium bromothymol blue matching the impurity profile of Guanghua batch cannot be recommended for use in marketed products.


Bromothymol Blue is another common indicator. As the name indicates, it is a blue colored solution. Bromothymol Blue remains blue in a basic solution. When it is added to a neutral solution, it turns green, and it turns yellow in an acidic solution.


A controlled experiment is an experiment in which a control group has been used. In this experiment, the control was used to see the function of the bromothymol blue by comparison. It remains blue even when a base is mixed with it, which means it must only react with acids. 041b061a72


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