BUFFERS AND pH LAB: A3

 

 

 

Introduction

The lab will explore how different biological substances from living organisms (liver, egg white) compare to non-biological substances (tap water and a commercial buffer compound), in their ability to resist changes in pH.

 

KEY POINTS OF THE LAB:

  • A cell’s chemical “tools” are called enzymes that are a necessary part of all biochemical reactions. 

  • Enzymes are an example of a type of protein.

  • A drastic change in pH and/or temperature can permanently damage the structure of proteins/enzymes. If the enzyme’s structure is damaged, so is its ability to function.

  • Cells die if their enzymes don’t work.

  • Ironically, most cellular activities produce a variety of acids and bases that in turn affect pH. The cell must resist these potential pH shifts.

  • Cells protect their enzymes by using buffering molecules also produced by the cells.  We will be using an acid called hydrochloric acid (HCl) and a base called sodium hydroxide (NaOH).

 

Materials:

 

  • 50 ml beaker

  • graduated cylinder

  • stirring rod

  • pH probe or pH paper

  • Egg whites

  • tap water

  • Buffer

  • Liver

  • 0.1M HCl

  • 0.1M NaOH

  • Safety Goggles

 

 

 

Procedure:

  • Fill a 50 ml beaker with 10ml tap water - Place the stirring rod in the tap water and stir.

  • Use the pH probe to measure the pH of the liquid and record the corresponding number as initial pH on your data chart. Keep the pH probe in the liquid at all times.

  • Add five drops of HCl and continue stirring.

  • Use the pH probe to measure the pH of the liquid and record the corresponding number

  • Record the pH after every fifth drop.

*** Be sure to continuously stir the sample! ***

 

Make sure you have all 7 measurements in the correct column on your data table before moving on.

  • Rinse the beaker and the probe with water.

  • Repeat the above procedure with each of the other substances: commercial buffer, egg whites, and liver.

  • After recording the data for each of the 5 samples using Hydrochloric Acid (HCl), repeat the procedure using the base, Sodium Hydroxide (NaOH). Fill in your second data chart accordingly.

  • Each substance should be tested once with the HCl and once with NAOH.

  • Clean up your station. Be sure to rinse the stirring rod with water and wash out all of the beakers.

 

Group members:    ________________________________________________________________________

 

Be sure to clean your lab station.

 

  • Wiped the lab station

  • Pushed stools under the table

  • Checked the floor around the station

  • Checked the sink

  • Replenished paper towels

  • Signature: __________________________

 

This portion may be completed for homework

 

Using your pH data table, create two line graphs to show changes in pH of the substances with addition of HCl and NaOH.

 

  • The Y axis should be pH (1-14).

  • The X axis should start with "initial" (Zero drops) and end with “Maximum” (30 drops).

  • Do not include the total change in the line graphs.

  • Be sure to label each line clearly with water, egg whites, liver, or buffer.

  • Use color pencils to make them distinct.

 

Drops of HCl added

pH of Tap Water

pH of Egg whites

pH of Liver

pH of Buffer

 

 

DATA COLLECTED:     (Data tables: 5 points each; Graphs: 20 points each)

 

 

Title: pH Changes of Different Substances After Adding HCl

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

            

 

 

Drops of HCl Added

 

 

 

 

 

 

Drops of NaOH added

pH of Tap Water

pH of Egg whites

pH of Liver

pH of Buffer

 

 

Title: pH Changes of Different Substances After Adding NaOH

 

 

 

 

                

 

 

Drops of NaOH Added

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Analysis Questions:

***Your answers must be in complete sentences to receive full credit. ***

 

1. How many total pH points (levels) did tap water change after you added all 30 drops of HCl?
    How many total pH points did tap water change after you added all 30 drops of NaOH?

 

 

 

 

 

 

2. How many total pH points (levels) did egg whites change after you added all 30 drops of HCl?
    How many total pH points did egg whites change after you added all 30 drops of NaOH?

 

 

 

 

 

 

3. How many total pH points (levels) did Liver change after you added all 30 drops of HCl?
    How many total pH points did Liver change after you added all 30 drops of NaOH?

 

 

 

 

 

4. How many total pH points (levels) did Buffer change after you added all 30 drops of HCl?
    How many total pH points did Buffer change after you added all 30 drops of NaOH?

 

 

 

 

 

 

5. Is the pH response of the buffer more like that of tap water, like that of egg whites, or liver?

 

 

 

 

 

6. Why do you think biological materials act differently than water?
    (Do they contain anything that allows them to behave differently?)

 

 

 

 

 

 

 

7. Do organisms need to resist shifts in pH? Why or why not?

 

 

 

 

8. Does tap water need to resist shifts in pH? Why or why not?

 

 

 

 

 

 

Conclusion:

Write one paragraph (4-5 complete sentences) that summarizes your results of the lab activity.  Describe the pH changes that each of the substances went through when you added HCl, and then when you added NaOH.

 

Statements like “interesting lab, I learned how to use pH paper or the pH probe, I learned a lot” are inadequate. Explain if the purpose of the lab was accomplished.  include answers for the following type questions.

  • Which of the substances were better at buffering than tap water? How do you know?

  • Which substances were the best at resisting pH? Which one was the worst?

  • Did they behave the same in acid and base?

Discuss possible sources of error.