The bromothymol blue It is a dye sensitive to the pH of its environment and has been used as an indicator in many laboratory reactions. An indicator is an organic dye whose color depends on the concentration of the H ion.3OR+ or the pH of the medium. By the color it displays, the indicator can indicate the acidity or basicity of the solution.
Bromothymol blue has an active indication scale between pH 6.0 and 7.6. At a pH of the medium lower than 6.5 it shows a yellow coloration. When the pH of the medium varies between 6.5 and 7.6, it acquires a green color. At pH greater than 7.6 its color is blue.
This characteristic makes this indicator very useful, since it can be used in a pH scale close to neutrality; precisely where physiological processes occur.
The ionization of bromothymol blue in aqueous solution can be outlined as follows:
HIn (yellow color) + HtwoOR <=> In- (blue color) + H3OR+
When the indicator is protonated (HIn) it acquires a yellow color; whereas if the deprotonated indicator (In-) turns blue.
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The upper image represents the structure of bromothymol blue with a bar and sphere model at a pH lower than 7.1. The brown bars correspond to bromine atoms, two in total.
Each of the three aromatic rings composes, together with the methyl groups, CH3, and tertbutyl, the thymol group; and having linked a Br, from there arises the reason why it is called 'bromothymol'.
At the bottom is shown the sulfonate group, R-SO3-, with red spheres and a yellow one. This group can be linked through an O atom to the central carbon that joins the three aromatic rings.
Although this molecular structure does not change radically with acidic or basic pH, its electronic states are altered, reflected by changes in the color of the solutions.
Below pH 7, the indicator is yellow in color and adopts the structure of the image; but if the pH becomes basic, the -OH groups deprotonate and a C = O double bond can form.
As a result, the conjugated system of the molecule (that of all its resonant double bonds) changes enough that the electrons no longer absorb the same photons, and the solution turns from yellow to bluish..
If, on the other hand, the pH is well below 7, the color of the indicator changes from yellow to red. Here the change in the conjugated system is due to the protonation of the -OH groups to -OHtwo+. -
In both acidic and basic environments, the molecule loses stability, absorbing photons of lower energy to allow the electronic transitions responsible for the colors perceived by the spectators..
And what about the green color of the main image? As the pH approaches 7.1, a small portion of molecules lose protons, giving off a blue color that, when mixed with yellow, gives the color green..
Bromothymol blue or 3,3'-dibromothymolsulfonaphthalein.
C27H28BrtwoOR5S.
624.384 g / mol.
Solid powder with a color ranging from pink to purple.
It acts as a weak acid in solution. When it is protonated in an acidic medium it acquires a yellow color, at neutral pH it is green, and when it is deprotonated in an alkaline pH it is blue.
7.9 x 10-8.
pH 6.0 to 7.6. This is due to the presence of bromine atoms that act to extract electrons, and two moderate electron donor groups (alkyl substituents).
1.25 g / cm3.
202 ºC (396 º F).
184.9 ºC.
It is poorly soluble in water and oil. Likewise, it is poorly soluble in non-polar solvents such as benzene, toluene and xylene, and practically insoluble in petroleum ether. It is soluble in aqueous alkali solutions and in alcohol.
It is stable at room temperature and incompatible with strong oxidizing agents.
The protonated form has its maximum absorption at a wavelength (λ) of 427 nm, thus transmitting yellow light in acid solutions and the deprotonated form has a maximum absorption at a wavelength (λ) of 602 nm, transmitting a blue light. at alkaline pH.
Bromothymol blue is used in many processes in which carbon dioxide (COtwo), with the consequent generation of carbonic acid (HtwoCO3), which acidifies the medium; evidenced by a change in colouration of bromothymol blue.
If a person is doing physical exercise and is connected to a tube that, in turn, is connected to a container with a bromothymol blue solution, the following phenomenon will be observed: as a consequence of increased physical activity and metabolism, there will be an increase in the production of COtwo.
The COtwo will react with water, producing carbonic acid, HtwoCO3, which subsequently dissociates releasing the H ion+ and acidifying the medium. As a consequence, the bromothymol blue acquires a yellow color, highlighting the acidity of the medium..
Bromothymol blue is used in obstetrics to show early rupture of membranes. Amniotic fluid normally has a pH greater than 7.7, so bromothymol blue will turn blue when it comes into contact with the fluid leaking from the amnion..
Vaginal pH is usually acidic, so bromothymol blue has a yellow tint. The change in its color to blue highlights the presence of amniotic fluid in the vaginal region.
Bromothymol blue also has application in the following equipment and functions:
-Optical sensors
-Gas combustion detection systems
-In the manufacture of paints, toys, cleaning products, detergents, and textiles
-Food freshness sensors
-As a handle for fresh and stale rice
-In the detection of microorganisms.
It is also used in bacterial growth analysis, psychoactive drugs, and dental materials..
100 mg of bromothymol blue is dissolved in 100 mL of diluted alcohol, and filtered if necessary. It is also prepared as follows: 50 mg of bromothymol blue is dissolved in a mixture of 4 mL of 0.02 N sodium hydroxide and 20 mL of alcohol, making up to 100 mL with water.
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