Unformatted text preview: Computer Determining the Phosphoric Acid
Content in Soft Drinks 35 Phosphoric acid is one of several weak acids that exist in carbonated beverages. It is a
component of all cola soft drinks. Phosphoric acid has a much higher concentration than other
acids in a container of soft drink, so its concentration can be determined by a simple acid-base
titration. py In this experiment, you will titrate a sample of a cola soft drink with sodium hydroxide solution
and determine the concentration of phosphoric acid, H3PO4. Hydrogen ions from the first
dissociation of phosphoric acid react with hydroxide ions from the NaOH in a one-to-one ratio in
the overall reaction: co H2O(l) + H2PO4–(aq)
H3PO4(aq) + OH–(aq) In this experiment, you will use a pH Sensor to monitor pH as you titrate. The region of most
rapid pH change will then be used to determine the equivalence point. The volume of NaOH
titrant used at the equivalence point will be used to determine the molarity of the H3PO4. tio In this experiment, you will n OBJECTIVES Use a pH Sensor to monitor pH during the titration of phosphoric acid in a cola soft drink. Using the titration equivalence point, determine the molarity of H3PO4. ua MATERIALS 0.050 M NaOH
various cola soft drinks, decarbonated
magnetic stirrer (if available)
stirring bar Ev al computer
Vernier computer interface
Vernier pH Sensor
50 mL buret
100 mL graduated cylinder
250 mL beaker PROCEDURE 1. Obtain and wear goggles.
2. Use a graduated cylinder to measure out 40 mL of a decarbonated cola beverage and 60 mL
of distilled water into a 250 mL beaker. Caution: Do not eat or drink in the laboratory. 3. Place the beaker on a magnetic stirrer and add a stirring bar. If no magnetic stirrer is
available, you need to stir with a stirring rod during the titration. Chemistry with Vernier 35 - 1 Computer 35
4. Connect the pH Sensor to the computer interface. Prepare the computer for data collection by
opening the file “35 Phosphoric Acid” from the Chemistry with Vernier folder of
5. Use a utility clamp to suspend a pH Sensor on a ring stand as shown in Figure 1. Position the
pH Sensor in the soda and adjust its position so that it is not struck by the stirring bar.
6. Obtain a 50 mL buret and rinse the buret with a few mL of the 0.050 M NaOH solution.
WARNING: Sodium hydroxide solution, NaOH: Causes skin and eye irritation. Dispose of
the rinse solution as directed by your teacher. Use a utility clamp to attach the buret to the
ring stand as shown in Figure 1. Fill the buret a little above the 0.00 mL level of the buret
with 0.050 M NaOH solution. Drain a small amount of NaOH solution so it fills the buret tip
and leaves the NaOH at the 0.00 mL level of the buret. Record the precise concentration of
the NaOH solution in your data table. Figure 1
7. You are now ready to perform the titration. This process goes faster if one person
manipulates and reads the buret while another person operates the handheld and enters
a. Start data collection.
b. Before you have added any NaOH solution, click
and enter 0 as the buret volume
in mL. Click
to store the first data pair for this experiment.
c. Add 0.5 mL of NaOH solution. When the pH stabilizes, click
and enter the current
buret reading in mL. Click
.You have now saved the second data pair for the
d. Continue to add 0.5 mL increments, entering the buret level after each increment. When
the pH has leveled off (near pH 10), click
to end data collection.
8. Examine the data on the graph of pH vs. volume to find the equivalence point—that is, the
0.5 mL volume increment that resulted in the largest increase in pH. As you move the
examine line, the pH and volume values of each data point are displayed to the right of the
graph. Go to the region of the graph with the large increase in pH. Find the NaOH volume (in
mL) just before this jump. Record this value in the data table. Then record the NaOH volume
after the 0.5 mL addition producing the largest pH increase.
9. Print a copy of the graph of pH vs. volume. Then print a copy of the NaOH volume data and
the pH data for the titration.
10. Dispose of the beaker contents as directed by your teacher. Rinse the pH Sensor and return it
to the storage solution.
35 - 2 Chemistry with Vernier Determining the Phosphoric Acid Content in Soft Drinks PROCESSING THE DATA
1. Use your printed graph and data table to confirm the volume of NaOH titrant you recorded
before and after the largest increase in pH values upon the addition of 0.5 mL of NaOH
2. Determine the volume of NaOH added at the first equivalence point. To do this, add the two
NaOH values determined above and divide by two.
3. Calculate the number of moles of NaOH used.
4. See the equation for the neutralization reaction given in the introduction. Determine the
number of moles of H3PO4 reacted.
5. Recall that you pipeted out 40.0 mL of the beverage for the titration. Calculate the H3PO4
concentration. DATA TABLE
Concentration of NaOH M NaOH volume added before the largest pH increase mL NaOH volume added after the largest pH increase mL Volume of NaOH added at equivalence point mL
Moles NaOH mol
Moles H3PO4 mol
Concentration of H3PO4 mol/L Chemistry with Vernier 35 - 3 Vernier Lab Safety Instructions Disclaimer
THIS IS AN EVALUATION COPY OF THE VERNIER STUDENT LAB. This copy does not include: Safety information Essential instructor background information Directions for preparing solutions Important tips for successfully doing these labs The complete Chemistry with Vernier lab manual includes 36 labs and essential teacher
information. The full lab book is available for purchase at Vernier Software & Technology
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