Name _________________________________ Period ___________ Date ___________
Partner ________________________________
Acid-Base Buffer Lab
Much of the acid that invades the human bloodstream comes from the carbon dioxide that cells give off
as they oxidize glucose. Carbon dioxide reacts with water in the blood to produce carbonic acid, H2CO3,
which then forms H+ and HCO3-.
CO2(g) + H2O(l) ⇋ H2CO3(aq) ⇋ H+(aq) + HCO3-(aq)
The amount of carbon dioxide dissolved in the blood determines the concentration of carbonic acid.
Other acids like phosphoric and lactic acids are also in our blood so how does the body maintain its pH?
A buffer solution is one that can neutralize the addition of acid (H+) or base (OH-) to it, thus resisting
changes in pH.
The blood’s primary buffer system is made of carbonic acid (H2CO3) and sodium bicarbonate, NaHCO3.
The addition of acid or base to this system shifts the equilibrium and results in these reactions:
H+(aq)
acid
added
OH-(aq)
base
added
Materials
0.1 M NaOH
straw
+
+
HCO3-(aq)
from
buffer
H2CO3(aq)
from
buffer
0.1 M HCl
well plate
⇋
⇋
H2CO3(aq)
carbonic
acid
HCO3-(aq) +
bicarbonate
ion
0.1 M NaHCO3
universal indicator
H2O(l)
water
distilled water
Procedure: Record data in the table below.
1. Place a wellplate on a white sheet of paper. Add 20 drops of distilled water to one well of the
wellplate. Add one drop of universal indicator to that well and record the color of the resulting
solution in your data table.
2. Add 20 drops of 0.1 M sodium bicarbonate (baking soda) solution to another well. Place the tip of
the straw into the NaHCO3 solution. Create a buffer solution by blowing gently (not too much so
you get dizzy) through the straw for 2-3 minutes, keeping the straw in the solution.
3. Remove the straw from the solution and throw it in the trash. Add one drop of universal indicator to
the buffer solution and record the resulting color.
4. Add 5 drops of 0.1 M NaOH to the distilled water well used in step 1. Record the color.
5. Counting the number of drops, add 0.1 M NaOH dropwise to the well containing the buffer solution
and universal indicator until its color matches the color (not the intensity) in the well from Step 4.
Record the number of drops required.
6. Repeat steps 1, 2 & 3 using clean, dry wells. No need to record data again.
7. Add 5 drops of 0.1 M HCl to the new well containing distilled water and universal indicator. Record
the color.
8. Counting the number of drops, add 0.1 M HCl drop-wise to the well containing the buffer solution
and universal indicator until its color matches the color in the well from Step 7. Record the number
of drops required.
9. Dispose of the solutions in the sink. Wash the wellplate.
10. Wash your hands thoroughly.
Step #
Starting Material
1
Distilled water
3
Buffer Solution
Distilled water +
Universal Indicator
Buffer Solution +
Universal Indicator
Distilled water +
Universal Indicator
Buffer Solution +
Universal Indicator
4
5
7
8
Data Table
Added
Color
Universal
Indicator
Universal
Indicator
0.1 M NaOH
Approx. pH
# of Drops
5 drops added
0.1 M NaOH
0.1 M HCl
5 drops added
0.1 M HCl
Questions:
1. Did your bicarbonate buffer solution perform as a buffer? ________ What observations support
your answer? __________________________________________________________________
_____________________________________________________________________________
2. Compare the number of drops of base needed to change the water and the buffer to the same pH.
_____________________________________________________________________________
Compare the number of drops of acid needed to change the water and the buffer to the same pH.
_____________________________________________________________________________
3. Write an equation showing how bicarbonate ions (HCO3-) would prevent the pH of the blood from
rising if a small quantity of base (OH-) were added.
Name _________________________________ Period _________ Date ________________
Buffers Pre-Lab Questions
1. If you held your breath, how would this affect the [CO2] in the blood? _____ increase [CO2] or
_______ decrease [CO2] The pH of the blood? _____ increase pH or _______ decrease pH.
2. Hyperventilation is a condition of forced heavy breathing that typically occurs when an individual is
frightened or near physical exhaustion. How would hyperventilation affect the [CO2] in the blood?
_____ increase [CO2] or _______ decrease [CO2].
How would this affect the pH of the blood? _____ increase pH or _______ decrease pH.
One first aid treatment to get blood back to the proper pH is to have the person breathe into a paper
bag for a short time. Comment on how this treatment works chemically to re-establish blood pH.
______________________________________________________________________________
______________________________________________________________________________
3. Strenuous muscle activity, such as long distance running, can cause lactic acid to accumulate in the
blood, resulting in a wobbly feeling in leg muscles. How would the accumulation of lactic acid affect
blood pH? _____ increase pH or _______ decrease pH.
What condition of acid-base imbalance might be caused by a large overdose of aspirin
(acetylsalicylic acid)?
4. Cardiac arrest occurs when the heart stops beating, blood circulation is halted and breathing stops,
but cellular respiration (production of CO2 by the body) continues. When this occurs, doctors
sometimes inject NaHCO3(aq) directly into the heart muscle, even before trying to restart the heart.
What affect would cardiac arrest have on blood pH? _____ increase pH or _______ decrease pH.
Show the equilibrium equation that proves your answer.
How would the injection counteract this affect?
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________