Summary

In this lab, students will create a buffer that models the buffer system of human blood. They will then test the buffer system by adding lemon juice to simulate the consumption of acidic food. Analyzing the data collected in the lab will help students better understand the purpose of a buffer system. Additionally, students will evaluate the claim that the Alkaline Diet can make you healthier. This lab includes two student lab options—a general chemistry version and an AP chemistry version.

Grade Level

High School (AP option available)

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
• Scientific and Engineering Practices:
  • Planning and Carrying Out Investigations
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence
  • Obtaining, Evaluating, and Communicating Information

AP Chemistry Curriculum Framework

This lab supports the following unit, topics and learning objectives:

• Unit 8: Acids and Bases
  • Topic 8.8: Properties of Buffers
    • 8.8.A: Explain the relationship between the ability of a buffer to stabilize pH and the reactions that occur when an acid or a base is added to a buffered solution.

Objectives

By the end of this lab, students should be able to:

• Prepare a buffer solution with a pH of 7.4 using carbonic acid, H₂CO₃ and sodium bicarbonate, NaHCO₃.
• Compare changes in pH when adding an acid to a buffer solution and to a non-buffer solution.
• Analyze the health claim of the “alkaline diet” using data collected during the lab.

Chemistry Topics

This lab supports students’ understanding of:

• Acids and Bases
• Buffers
• pH

Time

Teacher Preparation: 45 minutes
Lesson: 60 minutes

Materials (per group)

• LabQuest or LoggerPro installed on computer
• pH probe
• Two 250 mL flasks
• 50 mL graduated cylinder
• Ring stand and clamp
• Magnetic stirrer and plate
• Pipet
• Wash bottle
• Sodium Bicarbonate
• 500 mL cold sparkling water (source of H₂CO₃)
• 500 mL distilled water
• 1 bottle lemon juice (contains citric acid – C₆H₈O₇)

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• The liquids in the lab should be considered a chemical not for consumption.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

• When preparing for the lab, keep the sparkling water cold until just before the start of the experiment to ensure that as much carbon dioxide, CO₂ stays dissolved as possible.
• Sparkling water contains, on average, about 6 to 8 grams of CO₂ in 1 L of water. The assumption was made for this lab that for every 1 L of water, there will be 7 grams of CO₂ dissolved. You can have your students set up a stoichiometry problem to determine the molarity of the acid, which is 0.159M H₂CO₃ for 7 grams of CO₂. This is the value used to determine the amount of NaHCO₃ needed.
• There are two separate student documents available for teachers to choose from. One is designed for AP chemistry students and the other is intended for general chemistry students. The main difference is that the AP chemistry lab handout requires that the students determine the amounts needed to make the buffer, whereas that information is given in the general chemistry student version. In the analysis section of the lab there are also a couple minor differences in the questions asked.
• Several companies make pH probes, such as Vernier and ThermoScientific. The instruments linked in this lab are from Vernier.
• The carbonic acid, H₂CO₃ needs to fully react with the sodium bicarbonate, NaHCO₃ before adding any lemon juice during the buffer step. Discuss with students how they will know the reaction has stopped (there will be no more bubbling, and pH is constant).
• If you are using this lab with AP Chemistry students, consider first using the Preparation and Evaluation of Buffers Lab beforehand.
• The pH probe has a glass tip that can be shattered. Make sure students are holding it firmly while clamping it onto the ring stand.
• You may need to calibrate individual pH probes. For more information, visit Vernier’s website.
• The ChemMatters article, Tooth Decay: A Delicate Balance, is a great companion to this lab experience.
• You can extend this lab by having students test how well the buffer works with other acidic or basic substances such as vinegar, soda, milk of magnesia, or Pepto-Bismol!
• References used in developing the lab:
  • The Alkaline Diet: What you need to know, MD Anderson Cancer Center
  • BioLibre: Buffers
  • ChemLibre: Blood Buffers
• Answer key documents for both student versions are available for teacher reference.