Bromothymol blue is a commonly used indicator in chemistry and biology. It can be used to determine the pH of a solution based on the colour it turns in that solution. The colour change occurs because bromothymol blue is a weak acid that can exist in different protonation states depending on the pH. Understanding the pH range over which bromothymol blue changes colour is important for selecting the right indicator for a given application and interpreting experimental results.
Acid-Base Properties of Bromothymol Blue
Bromothymol blue (also known as BTB) has the molecular formula C27H28Br2O5S. It belongs to a class of compounds known as sulfonephthaleins. Here is the chemical structure of bromothymol blue:
Bromothymol blue contains an aromatic ring system with several bonded oxygen atoms. Two of these oxygen atoms can be protonated or deprotonated, making the compound an acid.
Specifically, bromothymol blue acts as a weak polyprotic acid, meaning it can lose multiple protons. The two ionization reactions are:
| HB+ | H2B | + | H+ | |
| B2- | HB+ | + | H+ |
Where H2B is the diprotic form, HB+ is the monoprotic form, and B2- is the deprotonated form.
The associated acid dissociation constants are:
| pKa1 = 7.0 |
| pKa2 = 8.9 |
These pKa values indicate that bromothymol blue will begin to lose its first proton at around pH 7 and its second proton at around pH 9. This is the basis for the colour change of the indicator.
Bromothymol Blue Colour Change
The different forms of bromothymol blue have different colours:
| Form | Colour |
| H2B (Diprotic) | Yellow |
| HB+ (Monoprotic) | Blue |
| B2- (Deprotonated) | Blue-green |
At very low pH values, the diprotic yellow form dominates. As the pH increases, it loses a proton to form the monoprotic blue form. At very high pH values, it loses its second proton to form the deprotonated blue-green form.
The colour transition occurs over the pH range spanning the two pKa values of bromothymol blue. Between pH 6-7.6, the colour shifts from yellow to blue. Between pH 7.6-9.4, the colour shifts from blue to blue-green.
Therefore, the main colour change pH range for bromothymol blue is approximately pH 6 to pH 9.4. Outside of this range, the indicator appears yellow (below pH 6) or blue-green (above pH 9.4).
Here is a summary of the bromothymol blue colour change:
| pH | Bromothymol Blue Colour |
| Below 6 | Yellow |
| 6 – 7.6 | Yellow to blue |
| 7.6 – 9.4 | Blue to blue-green |
| Above 9.4 | Blue-green |
Using Bromothymol Blue as a pH Indicator
The colour transition range makes bromothymol blue well-suited for indicating whether a solution is acidic, neutral or basic. Here are some examples:
- If bromothymol blue is yellow in a solution, the pH is acidic (below 6).
- If bromothymol blue is greenish-blue in a solution, the pH is basic (above 9.4).
- If bromothymol blue is blue in a solution, the pH is neutral (between 6 and 9.4).
More precise pH values can be determined by comparing the colour of bromothymol blue in the solution to a standardized colour chart.
Bromothymol blue is commonly used as an indicator in titrations involving weak acids. It allows the neutralization point to be detected, signaling the end of the titration.
It can also indicate pH changes in a solution during a chemical reaction or during biological processes like enzyme activity or cell metabolism.
Factors Affecting Bromothymol Blue Colour Change
Certain factors can affect the accuracy of bromothymol blue as a pH indicator:
1. Temperature
The pKa values of indicators are temperature-dependent. For bromothymol blue, the pKa values decrease slightly as the temperature increases. This means the transition range shifts to lower pH values at higher temperatures.
This effect is usually minor over normal lab temperatures but could become significant with large temperature variations. It’s best to use bromothymol blue near room temperature.
2. Ionic Strength
The ionic strength of a solution can also affect indicator pKa values. Increased ionic strength tends to lower pKa values. Highly concentrated salt solutions could therefore cause a downward shift in the transition range of bromothymol blue.
This effect can be minimized by using diluted solutions or by calibrating the colour chart in solutions of matching ionic strength.
3. Solvent Effects
Using bromothymol blue in non-aqueous solvents can substantially change its transition range, limiting its use. Bromothymol blue is designed to work in aqueous solutions near neutral pH. Other solvents may drastically change its acid-base behavior.
Applications of Bromothymol Blue
Some major uses of bromothymol blue as a pH indicator are:
Monitoring Water Quality
Bromothymol blue can quickly indicate if a water source is acidic, neutral or basic. Yellow means acidic conditions, while blue-green indicates basic conditions. It is commonly used for testing pools, aquariums, and drinking water.
Education
The clear, vivid colour changes make bromothymol blue ideal for chemistry and biology experiments. Students can use it to follow acid-base titrations and investigate pH changes.
Testing Carbon dioxide
Bromothymol blue in an aqueous solution will turn yellow when carbon dioxide is bubbled through it due to the formation of carbonic acid. This allows bromothymol blue to be used for monitoring CO2 levels.
Algae Culturing
Many algae species can only grow within specific pH ranges. Bromothymol blue is added to algae cultures as a pH indicator to ensure optimal acidity is maintained. A colour change alerts when the medium needs to be adjusted.
Fishkeeping
Fish tanks and ponds must be kept within the proper pH range for healthy fish. Aquarists often use bromothymol blue to monitor pH changes and detect issues.
Cell Biology Research
Tracking the pH inside cells and in cell culture media is important in biology. Bromothymol blue is used as an intracellular pH indicator due its membrane permeability.
Clinical and Microbiological Studies
Bromothymol blue is sometimes used in growth media for studying bacteria and fungi. An acidic colour change indicates the metabolism of organisms in the medium. It can also indicate infections in clinical samples like urine.
Alternatives to Bromothymol Blue
While bromothymol blue is one of the most popular pH indicators, there are a number of alternatives that can be used:
Phenol Red
Phenol red is another sulfonephthalein indicator with a colour range from yellow (pH 6.8) to red (pH 8.2). It is suitable for more acidic conditions compared to bromothymol blue.
Congo Red
Congo red is a diazo indicator that changes from blue to red over a wide pH range between pH 3-5. It is useful for very acidic conditions where bromothymol blue would appear yellow.
Thymol Blue
Thymol blue is a sulfonephthalein with two transitions – red to yellow (pH 1.2 – 2.8) and yellow to blue (pH 8-9.6). It is ideal for detecting very high or very low pH values.
Universal Indicator
Universal indicators contain a mixture of several different indicators to cover a broad pH range. They change through multiple colours and can quantify pH more precisely than single indicators.
Litmus
Litmus comes from lichens and provides a simple colour distinction between acidic (red) and basic (blue) conditions. It is less precise than purified indicators but is safe, easy to use, and inexpensive.
pH Probes
Electronic pH probes can give very accurate pH measurements numerically. They may be preferable to colour indicators when quantifying pH precisely or automation is needed. However, they are more expensive and complex.
Conclusion
Bromothymol blue is an invaluable indicator for monitoring pH changes. Its colour transition range from yellow (pH below 6) to blue (pH 6-7.6) to blue-green (pH above 7.6) allows it to easily identify acidic, neutral and basic solutions. By quantifying the acid-base behavior of bromothymol blue, we can understand exactly what pH range causes the colour change. Awareness of factors that influence indicators also helps improve the accuracy of pH measurements. While bromothymol blue has some limitations, it provides a simple, inexpensive way to determine approximate pH both visually and qualitatively.