Bromothymol Blue to Yellow by Breath
Publication information:
Abstract
A solution with bromothymol blue indicator changes from blue to yellow as the instructor blows into the liquid with a straw.
Video of Professor's Acid Breath can be found here https://youtu.be/hfhNZEzBEtg
What it Shows
A person’s breath contains carbon dioxide, which reacts with water to form carbonic acid in solution. The carbonic acid then converts the acid-base indicator bromothymol blue to its acidic yellow form.
How it Works
Bromothymol blue is an acid-base indicator with a pKa of 7.2. It is blue in solutions with a pH ≥ 7.6, yellow in pH ≤ 6.0, and greenish in between. A person’s breath contains carbon dioxide which reacts with water to produce carbonic acid according to the following equilibrium reaction:
CO2 (g) + H2O (l) ⇆ H2CO3 (aq)
Initially, the solution is made basic using sodium hydroxide, and the solution is blue. As a person blows into the solution, more and more CO2 dissolves in the solution, producing carbonic acid in solution. After several breaths, the solution turns more and more yellow as the pH of the solution reaches 6.0. The color change is striking, and students will enjoy watching the instructor puff away into the solution.
Safety
Wear eye protection when preparing and performing the demonstration. Only blow into the solution - do not inhale!
Setting it Up
Equipment needed: 500 mL flask with a stopper, Acidified bromothymol blue solution (0.04%, can also use plain bromothymol blue and add some acid to it), 1 M NaOH, DI water, two straws taped together, safety glasses. A light box can be used to illuminate the flask, or it can be placed under a document camera.
Put about 250 mL of DI water in the flask and add a squirt or two of bromothymol blue. Add drops of 1 M NaOH until the soluiton is a blue color. Stopper the flask immediately - if you don't, the CO2 in the air will start acidifying the solution!
Presenting to the class: Take off the straw wrappers and place the long straw into the solution. Start blowing into the solution until it turns yellow. You will probably need at blow in at least 2 or 3 breaths before the color changes.
Disposal: The solution can be poured down the drain.
Comments
This demo can also be done with lime water (saturated solution of calcium hydroxide). Other indicators would work also, such as phenolphthalein, red cabbage juice, universal indicator, though the SDS on these indicators should be consulted, except for the cabbage juice. Best done at the start of a lecture as a puzzler or for a lecture on acids and bases, ocean acidification, climate change, etc. Do not give away the color change - just tell students to carefully observe what happens as you blow into the solution. Ask students for observations of the solution before and after you blow into it. Revisit the demo after students have learned more about acids, bases, and indicators.
References
Rutherford, S., & Coffman, M. (2005). Bromothymol blue: The demo we all do! The Science Teacher, 72(8), 52-53. Retrieved from http://search.proquest.com.ezp-prod1.hul.harvard.edu/scholarly-journals/bromothymol-blue-demo-we-all-do/docview/214616197/se-2
Full text
A solution with bromothymol blue indicator changes from blue to yellow as the instructor blows into the liquid with a straw.
Video of Professor's Acid Breath can be found here https://youtu.be/hfhNZEzBEtg
What it Shows
A person’s breath contains carbon dioxide, which reacts with water to form carbonic acid in solution. The carbonic acid then converts the acid-base indicator bromothymol blue to its acidic yellow form.
How it Works
Bromothymol blue is an acid-base indicator with a pKa of 7.2. It is blue in solutions with a pH ≥ 7.6, yellow in pH ≤ 6.0, and greenish in between. A person’s breath contains carbon dioxide which reacts with water to produce carbonic acid according to the following equilibrium reaction:
CO2 (g) + H2O (l) ⇆ H2CO3 (aq)
Initially, the solution is made basic using sodium hydroxide, and the solution is blue. As a person blows into the solution, more and more CO2 dissolves in the solution, producing carbonic acid in solution. After several breaths, the solution turns more and more yellow as the pH of the solution reaches 6.0. The color change is striking, and students will enjoy watching the instructor puff away into the solution.
Safety
Wear eye protection when preparing and performing the demonstration. Only blow into the solution - do not inhale!
Setting it Up
Equipment needed: 500 mL flask with a stopper, Acidified bromothymol blue solution (0.04%, can also use plain bromothymol blue and add some acid to it), 1 M NaOH, DI water, two straws taped together, safety glasses. A light box can be used to illuminate the flask, or it can be placed under a document camera.
Put about 250 mL of DI water in the flask and add a squirt or two of bromothymol blue. Add drops of 1 M NaOH until the soluiton is a blue color. Stopper the flask immediately - if you don't, the CO2 in the air will start acidifying the solution!
Presenting to the class: Take off the straw wrappers and place the long straw into the solution. Start blowing into the solution until it turns yellow. You will probably need at blow in at least 2 or 3 breaths before the color changes.
Disposal: The solution can be poured down the drain.
Comments
This demo can also be done with lime water (saturated solution of calcium hydroxide). Other indicators would work also, such as phenolphthalein, red cabbage juice, universal indicator, though the SDS on these indicators should be consulted, except for the cabbage juice. Best done at the start of a lecture as a puzzler or for a lecture on acids and bases, ocean acidification, climate change, etc. Do not give away the color change - just tell students to carefully observe what happens as you blow into the solution. Ask students for observations of the solution before and after you blow into it. Revisit the demo after students have learned more about acids, bases, and indicators.
References
Rutherford, S., & Coffman, M. (2005). Bromothymol blue: The demo we all do! The Science Teacher, 72(8), 52-53. Retrieved from http://search.proquest.com.ezp-prod1.hul.harvard.edu/scholarly-journals…