Acid Base Chemistry
WHAT MAKES A SOLUTION BUFFERED?
(Adapted from Dan D. Holmquist, J. Randall, and D. L. Volz. "The Buffer in Lemonade," Chemistry with CBL;
Vernier Software: 1995. Modified for the MCL by Dr. Patricia Amateis, Virginia Tech)
Introduction:
Many times, you may have heard of a solution or chemical being “buffered”. Buffers prevent a solution from
changing drastically in pH when a small amount of acid or base is added. For example, a popular pain reliever is
advertised as “buffered”, so that it will not upset your stomach.
Today, we will titrate two acid/ base systems. In the first system, you will titrate citric acid (the acid in lemons
and other citrus fruit) with sodium hydroxide. Then, you will titrate lemonade (which contains citric acid and a
citrate salt) with sodium hydroxide. When you have completed both titrations, you will compare the graphs.
Goals: The student will titrate two citric acid containing solutions (0.01 M citric acid and lemonade) and determine
the effects of buffering on the pH titration curve.
Prelab:
1. What is the formula of citric acid?
2.
Buffers are made of weak acids and their (weakly basic) salts. This way, if a small amount of acid is added, the
basic salt can neutralize the acid. If a small amount of base is added, the weak acid can neutralize the base.
What is the formula for the sodium salt of citric acid?
Safety:
Wear eye protection in the lab.
!
Procedure:
1.
Use a graduated cylinder to measure 40 mL of lemonade drink and 100
mL of distilled water into the 250 mL beaker. Place the beaker on the
stirring hot plate, as shown in the figure on the right.
2.
Your buret will be filled with 50.00 mL of 0.1 M NaOH by the MCL
staff.
3.
Turn on the stir plate to 3. Adjust the clamp if necessary so that the pH
probe will remain immersed in solution, but will not be struck by liquid
from the buret and will not be struck by the stirring bar.
4.
You are now ready to begin the titration. This process goes faster if one
person manipulates and reads the buret while another person operates the
computer and enters volumes.
5.
Click Collect. After the pH reading has stabilized, click Keep, type
"0.00" in the window that appears, and hit Enter.
6.
Dispense 2.00 mL of solution from the buret into the beaker. After the
pH reading has stabilized, click Keep, type 2 for the total volume of base
added, and hit Enter.
7.
Buret
to computer
pH meter
hotplate/stirrer (set to 3)
Continue in 2.00 mL increment, Be sure to click Keep, enter to total volume NaOH added, and hit Enter after
each 2 mL increment. PLEASE NOTE that the volume of base added refers to the TOTAL volume of base
added, so your entries would be 2.00, 4.00, 6.00, 8.00, etc. If you make an error entering the volume,
remember where the error occurred and do not panic—it can be corrected when you are done collecting data.
Also, if you inadvertently add (say) 2.5 mL, don’t worry about it; but be sure to enter the amount you have
added. When you click keep, enter (say) 2.5 to show that’s how much base you added.
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What Makes a Solution Buffered?
Acid Base Chemistry
8.
Continue until you have used all 50 mL of NaOH solution or until the pH has reached 12, whichever comes
first.
9.
Click Stop when you finish your titration. (If you entered any volumes wrong during the data collection, make
your corrections by clicking the incorrect volume cell in the table window and entering the correct value.)
10. Choose Data, then Store Latest Run. This will keep the data from the lemonade on the graph while you titrate
the citric acid solution so that you can compare them. On your data table, double click on the header of the pH
column. In the window that opens, change the Point Protector and the Column Color for your graph. Hit OK.
11. Empty your beaker into an approved waste container and rinse with distilled water. Use a graduated cylinder to
measure out 40 mL of 0.010 M citric acid solution and 100 mL of distilled water into a 250 mL beaker. Repeat
Steps 3-8 above.
12.
Print your data by first clicking in the graph window to select this window. The band above the graph
should turn blue Click on the printer icon in the upper left. Enter your name(s) in the appropriate box, and the
name of the experiment below your name(s). Click okay. The printer box will appear; print a copy for each
member of your group.
13. When you are finished, dispose of the beaker contents as directed by your teacher. Rinse the pH electrode and
return it to the storage solution.
Postlab:
1. Write the equation for the reaction of citric acid (H3C6H5O7) with sodium hydroxide.
2.
Compare the two titration curves and answer the following questions.
(a) Based on your pH readings, compare the initial concentrations of free acid (H+) in the two solutions.
(b) Compare the amount of base required to react with the lemonade compared to the citric acid?
(c) Which solution is the better buffer and why?
3.
The "acid" can exist as free acid (H+ ions) or molecular acid (H3C6H5O7) in solution. Calculate the total acid
concentration (free acid plus molecular acid) by completing the following calculations:
(a) Mark the point on each graph where the pH changes the most. (This is called the equivalence point, or end
point, and represents the point where the volume of base added equals the amount of acid originally in the
solution.) Record the volume of 0.1 M NaOH used at this point for each graph.
Lemonade___________ mL NaOH needed
Citric Acid___________ mL NaOH needed
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What Makes a Solution Buffered?
Acid Base Chemistry
(b) Find the total number of moles of NaOH required to neutralize each solution.
Lemonade_______________ moles NaOH
Citric Acid_______________ moles NaOH
(c) Find the total acid concentrations in the citric acid and the lemonade in moles/L (M).
Lemonade_________________ M
Citric Acid_________________ M
4.
Based on your calculations above, which solution contained more total acid?
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What Makes a Solution Buffered?