Detecting Carbon Dioxide

An oxygen and carbon dioxide exchange takes place in your bloodstream. When you breathe air into your lungs it brings in oxygen, which is carried from your lungs by red blood cells in your bloodstream. Cells of your body use the oxygen and carbon dioxide is produced as waste, which is carried by your blood back to your lungs. You exhale and release the C02. You will study this exchange in today’s lab.

You will be using a pH indicator known as bromothymol blue. When you exhale into a baggie, the carbon dioxide will react with water in the bag. This reaction produces carbonic acid, which starts to acidify the water. More breathes in the bag equal more carbon dioxide, which equal a lower (more acidic) pH. You will notice the bromothymol will turn green when the pH of the water is right about 6.8 and it will turn yellow when the pH drops further to 6.0 and lower.

Here’s what you need

    • 1 1 oz. bottle of bromothymol blue
    • 1 straw
    • 1 resealable baggie
    • 1 bottle of ammonia
    • 1 pipette
    • water


Here’s what you do

  1. Pour about 2 ounces of water into the baggie and add two capfuls of the bromothymol blue into it. Close the baggie well and swish the solution around inside it gently to mix. Note the color of the solution for your data record.
  2. Open the baggie a tiny bit and put the straw inside, but DO NOT drink the solution! It could make you sick. Close the bag tightly around the straw and gently blow into the solution. Again, be careful not to suck on the straw.
  3. Watch the color of the solution closely as you continue to blow into the solution and create bubbles of carbon dioxide gas. The color will change to a sea green color and then eventually it will change to bright yellow. Note each color change in your records.
  4. You can return the solution to blue by slowly adding a base – such as ammonia – to the solution in the bag. Bleach will also work. Please ask an adult to help with this. Add one drop at a time, shaking after each addition to mix the solution. You will be able to observe when the pH starts to change back by the color of the solution. It should turn back to green and then to blue.

What’s going on?

Bromothymol blue will change color in a pH range from 6.0 to 7.6.  It is an acid/base indicator. Its basic solution is at a pH of 7.6 or above – this is when it is blue. In acidic conditions, it will turn yellow – this is a pH of 6.0 or below. And when it’s in between the two, it will be the sea green color that you observed in your baggie.

Because carbon dioxide is a little acidic, when we breathe it out into the water and bromothymol blue solution its bubbles start to lower the pH. You saw a small change in pH with the sea green color, but as you continued to exhale and add carbon dioxide, the solution became more and more acidic. This eventually resulted in a pH at or below 6.0 and a bright yellow solution.

In order to exchange oxygen with carbon dioxide in your lungs, they have over 300,000,000 teeny little air sacs calls alveoli. In one minute, you breathe approximately 13 pints of air.


  1. What is pH and how it is useful?
  2. What does a yellow color indicate with bromothymol blue?
  3. Is CO2 acidic or basic?