Blood And Buffers

Biology, Chemistry:	Buffering Agents, Homeostasis
Grade Range:	Middle School, High School
Format:	Hands-on, Stage

This demonstration on buffering agents will interest most students, especially when the connection is made to their own bodies!

Materials

• Bromothymol Blue Indicator
• Water (DI Water ideal, but will work with Tap Water)
• Dry Ice
• Calcium Carbonate
• Phosphate Buffer Solution
• Three 500mL (Stage) or Four 100mL (Hands-on) Beakers
• Optional: Poster Board

Safety Precautions

Please read the Dry Chemical, Liquid Chemical, and Cryogen sections of the Demonstration Safety page before performing this demonstration. The solutions for this demonstration can be poured down the drain, and do not need to go into a waste container.

This demonstration requires: Cryogen Gloves, Goggles, Disposable Gloves

Demonstration

Preparation

• Make the Phosphate Buffer Solution: In a 600mL beaker, add 13.8g of NaH₂PO₄ to 500mL of DI water, and stir until completely dissolved. In a separate 600mL beaker, add 14.2g of Na₂ HPO₄ to 500mL of DI water, and stir until completely dissolved. Pour both solutions into a 1L bottle and mix. Label the solution as a 0.2M Phosphate Buffer Solution and write the date it was made. Be sure to test the pH of the solution to make sure it stays within two decimals of 7.0. If it is below, add a small amount of NaOH. If it is above, add a small amount of HCl.
• Setup the demonstration: In the first and last beakers, pour 100mL/500mL of water in, add 20 drops of the Bromothymol Blue indicator and stir. In the center beaker(s), add 100mL/500mL of the Phosphate Buffer Solution, then add 20 drops of the indicator and stir. All the solutions should be green, and should be the same shade as each other. Keep the buffer solution(s) in the center.

Presentation

1. Show the Audience that the solutions look the same, and that the indicator you added shows that they are at or close to the same pH. Ask the audience what might happen to the first two solutions if you were to add a small amount of dry ice to them. What do they think it will do?
2. Place a piece of dry ice into each solution, and watch what happens. The water solution will start turning yellow, while the buffer solution will stay unchanged! For the stage show, remove the dry ice from the buffer solution once the color change happens.
3. Ask the audience what they think might happen if you add calcium carbonate to the last two solutions. What do they think it will do to the solutions?
4. Add the Calcium Carbonate to both solutions, and watch what happens. The water solution will start turning blue, while the buffer solution still stays unchanged!
5. For a hands-on show, use two beakers with the buffer solution so that one can have a piece of dry ice and the other can have the calcium carbonate. You can leave all the solutions with their solvents; the buffer solutions should hold up to any changes for 30 minutes to an hour.

Why This Works

Short Explanation

Buffering Agents are able to resist a change in the pH, or acidity, which keeps the solution they are in from becoming too acidic or too basic. In our blood, we have buffering agents that work to keep the blood in a stable pH range between 6.8 and 7.4, which is very narrow. One reason why our blood has these buffering agents is because our bodies get energy from sugars and oxygen. When sugar reacts with oxygen, it breaks down into carbon dioxide (CO₂) and water. CO₂ can react with water to make carbonic acid (H₂CO₃), which would make our blood more acidic, lowering the pH. We see this happen with the dry ice in the beaker with water, and how the indicator changes to yellow because of the acidity. The buffer prevents this by slowly reacting with the carbonic acid as it forms, and breaking it back down into CO₂ and water so that our lungs can breathe it out. Another reason why our blood has these buffering agents is because the carbonic acid can break down into bicarbonate (HCO₃^-). Bicarbonate can make the blood more basic, which increases the pH. We see this happen with the calcium carbonate in the beaker with water, and how the indicator turns blue because of it. The buffer prevents this by latching onto the extra bicarbonate so it doesn't react, and helping our kidneys filter it out.

Full Explanation

COMING SOON

Additional Information

• Buffer solutions are a complicated topic, so do not be surprised if students are having trouble understanding the demonstration.
• This demonstration pairs well with the Blood Clotting demonstration.