CHEMISTRY NOW: pH and Acid Content (Grades 9-12) Print

Objective:

Students will determine the acid content of a solution. Students will design and carry out an investigation titrating a weak acid solution (cola soft drink) with sodium hydroxide, NaOH. Students will calculate the molarity of the weak acid solution from titration data. Students will compare the acid content of various cola soft drinks or other acidic beverages.


Introduction Notes:

 

CHEMISTRY NOW: pH and Acid Content

 

Subject Area: Chemistry

Grade Level:  High School Chemistry

Lesson Title:  pH and Acid Content

National Science Education Standard

  •  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 pointthe 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 – 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

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 à 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.

 

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 à 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 colas contain more or less acid than store brands?
  • - Does diet cola contain more or less acid?
  • - Does temperature alter the acid content?
  • - Does a non-cola soda contain less acid?
  • - Do non-carbonated 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 de-carbonated (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 in color.

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.

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 à H2O + H2PO4−1

 

  • - 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/

 

 


Student Worksheet for pH and Acid Content

 

 

Experiment Title: ____________________Date: __________Name: ______________

 

Student Hypothesis: _____________________________________________________

 

Materials: 

________________________________________________________________

 

_________________________________________________________________

 

Procedure:

Wear safety goggles for all lab work and follow lab safety protocol. Before proceeding with lab, have procedures approved by instructor.

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________


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:

 __________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

 

Analysis of Data:

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

 

Conclusion:

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

 

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