Bromothymol Blue is a chemical compound used as a pH indicator dye in scientific and educational settings. Its primary function is to measure the acidity or alkalinity of a liquid sample. The dye provides a simple, visible means of determining the concentration of hydrogen ions in a solution. This makes it a valuable tool for monitoring chemical reactions and biological processes that involve shifts in pH, especially for substances that maintain a relatively neutral balance.
Chemical Structure and Classification
Bromothymol Blue (BTB) has the full chemical name Bromothymol Sulfone Phthalein. It belongs to the sulfone-phthalein class of dyes, a subgroup of the broader triphenylmethane dye family. The compound has the molecular formula $\text{C}_{27}\text{H}_{28}\text{Br}_{2}\text{O}_{5}\text{S}$, featuring three aromatic rings.
The structure includes a sulfur atom linked to a central carbon, which allows the molecule to function as a weak acid in a solution. The presence of bromine atoms and alkyl substituents on the rings dictates the specific range in which the dye changes color. This weak acid characteristic means the molecule can either hold onto or release a hydrogen ion (protonated or deprotonated form), which is the basis for its color-changing ability.
How Bromothymol Blue Changes Color
The color change of Bromothymol Blue is directly linked to the concentration of hydrogen ions, or the pH, in the solution. The dye exhibits a distinct transition interval between pH 6.0 and 7.6, which is close to the neutral point. Outside of this range, the dye settles into one of two stable forms, each absorbing light differently.
When the solution is acidic (pH below 6.0), the dye exists in its protonated form, appearing yellow. Conversely, in a basic solution (pH above 7.6), the dye loses a proton and shifts to its deprotonated form, producing a blue color. This shift is known as ionization, where the loss or gain of a hydrogen ion causes a rearrangement in the molecule’s internal structure.
In the intermediate range between pH 6.0 and 7.6, the solution takes on a green color, which is a mix of the yellow and blue forms. This greenish color results from the simultaneous presence of both the protonated and deprotonated structures. The color transition occurs gradually across the range, with each slight change in hydrogen ion concentration causing a subtle shift in the shade of green.
Common Uses in Laboratory Settings
Bromothymol Blue’s ability to indicate pH changes near neutrality makes it valuable in biological and chemical laboratories. One of its most frequent applications is monitoring respiration and photosynthesis experiments. When an organism respires, it releases carbon dioxide ($\text{CO}_{2}$), which dissolves in water to form carbonic acid, lowering the solution’s pH and causing the indicator to turn yellow.
The dye is also used in acid-base titrations to precisely determine the point where an acid is neutralized by a base. Its narrow transition range around pH 7.1 makes it a reliable indicator for reactions that end near neutral conditions.
BTB assists microbiologists in tracing metabolic activity in cultured organisms. If microbes produce acidic byproducts as they grow, the indicator in the culture medium will shift toward a yellow color, providing a visible signal of their activity. The dye is additionally employed in aquarium testing and in obstetrics for detecting premature rupture of membranes, as amniotic fluid is slightly basic and causes the indicator to turn blue.