pH and Acid Content
Subject Area: Chemistry
Grade Level: High School Chemistry
Lesson Title: pH and Acid Content
National Science Education Standards
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Science as Inquiry, 9–12
Physical Science, 9–12
Science and Technology, 9–12
Science in Personal and Social Perspectives. 9–12
History and Nature of Science, 9–12
Suggested Prior Knowledge: concepts of acids and bases, pH; lab technique of acid–base
titration
Purpose: To understand pH indicators, pH and properties of acids and bases, and titration of an
acid.
Key Vocabulary:
acid—substance that has a pH less than 7; produces hydrogen ions in solution
base—substance that has a pH greater than 7; produced hydroxide ions in solution
equivalence point—the point in an acid–base titration at which the moles of OH− equal the
moles of H+
extract—to draw out a substance from or to separate a substance from a mixture
indicator—compound that changes color in different conditions; in this case we are looking
at pH indicators, which are compounds that give different colors in solutions of different pH
molarity- concentration of a solution in moles of solute/liters of solution
pH—measure of the acidity of a substance; a pH of less than 7 is acidic (lower number =
more acidic) and a pH of greater than 7 is basic, in general the scale goes from 1 (very
acidic) to 14 (very basic)
phosphoric acid—H3PO4, weak acid commonly found in cola soft drinks
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titration—lab process used to determine the concentration of a solution (often an acid or
base) in which a solution of known concentration is added to a measured amount of the
solution of unknown concentration to determine the concentration of this unknown solution
Objectives:
1. Students will determine the acid content of a solution
2. Students will design and carry out an investigation titrating a weak acid solution (cola
soft drink) with sodium hydroxide, NaOH
3. Students will calculate the molarity of the weak acid solution from titration data
4. Students will compare the acid content of various cola soft drinks or other acidic
beverages
Materials:
- Safety goggles
- Phenolphthalein solution (if no pH sensor used)
- 0.050 M NaOH solution
- pH sensor (optional)
- 50 ml buret
- 100 ml graduated cylinder
- 250 ml beaker
- Various cola soft drinks, decarbonated
- Distilled water
- Ring stand
- Utility clamp
- Magnetic stirrer (optional)
- Stirring rod (if no magnetic stirrer)
Procedure:
1. Phosphoric acid is one of the weak acids found in carbonated soft drinks. It is commonly
found in colas. Its concentration can be determined by a titration with a basic solution of known
concentration. Discuss with students the properties of acidic solutions and the process of
titration. Review acids and bases and the neutralization reactions involved in an acid–base
titration. Acidic drinks have been linked to erosion of tooth enamel. Can students design an
investigation to determine the acid content of a soda?
 What is the difference between an acid and a base?
 What pH values do acids have?
 What pH values do bases have?
 What is a titration?
 What is a neutralization reaction? (acid + base  water and salt; properties of acids
and bases are decreased, forming a more neutral solution)
 How can we determine the amount of acid in a solution?
 What kind of investigation can we do to determine the acid content of various beverages
such as cola soft drinks?
2. Lab protocols will incorporate safety equipment. Goggles must be worn at all times. Remind
students that they should never drink or taste substances in the laboratory..
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3. Guide students to design an investigation that is valid and that will allow them to determine
the acid content in a cola soft drink or other acidic solution. Phosphoric acid reacts with NaOH
according to the following reaction:
OH−1 + H3PO4  H2O + H2PO4−1
Other acidic solutions can also be used, such as household vinegar.
4. Some questions to help focus students’ plans and guide them include the following:
 Does brand-name cola contain more or less acid than store brands?
 Does diet cola contain more or less acid?
 Does temperature alter the acid content?
 Does a noncola soda contain less acid?
 Do noncarbonated drinks contain as much acid?
 Do organic brands contain less acid?
 Does the amount of carbonation change the acid content?
 Does soda fountain cola contain the same amount of acid as bottled soda?
5. Students should have carried out a basic titration before this investigation to learn the
techniques and how to analyze the data.
6. General procedure (written for titration of a soft drink but can also be done with any acid
solution; use 0.10 M NaOH for other acids):
a. Obtain and wear goggles at all times.
b. Measure out 40.0 ml of decarbonated (flat) cola beverage and place this in a 250 ml beaker
with 60.0 ml of distilled water.
c. Set up the pH sensor to measure the pH of the beverage solution. If there is no pH sensor, add
2–3 drops of phenolphthalein to the solution. Students will need to be alert to pink coloring in the
colored solution. If the beverage solution is still very dark after dilution, add more distilled water
so that it is lighter colored.
d. Obtain a clean and dry 50 ml buret and use the utility clamp to attach it to the ring stand. Rinse
the buret with a small amount of the 0.050M NaOH solution and allow several mls to flow
through the valve to eliminate any bubbles. Fill it with NaOH solution and record the initial
volume of the NaOH solution.
e. Place the beaker containing the beverage solution under the buret so that you are ready to
begin titration of the solution. If you are using a pH sensor, record the pH of this solution along
with the volume of NaOH solution added (0.0 ml).
f. If you are using a pH sensor:
 Add 0.5 ml of NaOH at a time and stir. Record the pH and total volume of NaOH added
after each addition.
 When the pH starts to increase by more than 1 with addition of 0.5 ml of NaOH, slow
your rate of addition to 0.2 ml each time.
 Continue adding NaOH and recording the volume and pH until the pH reaches at least 10.
 Graph pH versus volume of NaOH added. Analyze this graph to identify the equivalence
point. This is the point at which the graph is steepest, when addition of a very small
amount of NaOH produced the largest increase in pH.
 Record the volume of NaOH added at the equivalence point.
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g. If you are using phenolphthalein:
 Add 0.5 ml of NaOH at a time and stir. Watch after each addition for any pink color.
 Once the pink color begins to appear, add NaOH more slowly (0.2 mL at a time).
 Once the pink color is visible and lasts for over 30 seconds of stirring record the final
volume of the NaOH in the buret. This is the equivalence point. Subtract to find the
volume of NaOH that has been added.
h. The concentration of the acid solution in the soft drink can be determined using the following
reaction equation and calculations.
OH−1 + H3PO4  H2O + H2PO4−1
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Moles of NaOH = [volume (L) of NaOH added] × [0.050 M]
Based on the neutralization equation above, moles of phosphoric acid = moles of NaOH
Concentration of phosphoric acid = moles of phosphoric acid/0.040 L
7. After the students have finished their investigations, they may present their findings to their
classmates using a slide show or poster.
8. Encourage students to compare their results with those gathered from different beverages or
even to develop a further hypothesis to test. Some interesting extension questions might include
the following:
 Can these beverages erode tooth enamel?
 How much soda solution does it take to damage a tooth?
Additional Resources:
 http://www.flinnsci.com/Sections/Safety/safety.asp
 http://chemistry.about.com/od/acidsbases/Acids_Bases_and_pH.htm
 http://www.erowid.org/archive/rhodium/chemistry/equipment/ph-indicator.html
 http://en.wikipedia.org/wiki/Titration
 http://chemistry.about.com/od/acidsbases/a/aa082304a.htm
 http://chemistry.about.com/od/workedchemistryproblems/a/titrationexampl.htm
 http://www.livescience.com/7198-acids-popular-sodas-erode-tooth-enamel.html
 http://alkalinedietnetwork.com/ph-levels-of-different-bottled-waters-sodas-and-ice-tea/
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Student Worksheet for pH and Acid Content
Experiment Title: _____________________________Date: __________Name: _____________
Student Hypothesis: _____________________________________________________________
Materials: _____________________________________________________________________
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Procedure:
Wear safety goggles for all lab work and follow lab safety protocol. Before proceeding with lab,
have procedures approved by instructor.
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Data and Observations:
Quantity
Measurement
Concentration of NaOH solution
M
Initial volume of NaOH in buret
ml
Volume of NaOH in buret at equivalence point
ml
Volume of NaOH added at equivalence point
ml
Moles of NaOH
moles
Moles of acid
moles
Concentration of acid
M
Observations: __________________________________________________________________
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Analysis of Data: _______________________________________________________________
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Conclusion: ___________________________________________________________________
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