Unit 3 (part 4): What is the pH Scale?
Concepts: Students will explore the concept of the pH scale and how pH is
measured.
Objective: The purpose of this lesson is to discuss the concept of pH and to
explore methods of measuring pH. Other objectives include formulating
hypotheses and drawing logical conclusions based on measurements.
Summary: pH is an important factor in aquatic systems. As algae generally exist
in such aquatic systems certain pH conditions must be met for taxa to
thrive/survive. As a part of our goal of cultivating the most algae possible, we are
experimenting with the effect of pH on algae. In this lesson we are gaining a better
understanding of what pH is and how we can measure it.
Useful Websites:
http://sv.berkeley.edu/showcase/flash/juicebar.html
http://www.miamisci.org/ph/
http://en.wikipedia.org/wiki/PH
http://www.epa.gov/acidrain/education/experiment1.html
Materials:
You may wish to ask each student to bring a solution from home for pH
testing
pH meter
pH indicator strips
Litmus paper
3-200 mL beakers for each group of 2-4 students
White Vinegar or Lemon Juice
© 2010, Ohio Northern University: Dr. Debra Gallagher, Dr. Robert Verb, Dr. Ken Reid and Ben McPheron
Household Ammonia or Baking Soda
Distilled Water
Glass stirring rod for each group
Activity Sheets:
pH typical value chart
pH Experiment Procedure
pH Quiz
Engage:
1.
2.
3.
4.
Ask the students to give a description of pH to test their knowledge.
Ask the students to describe the pH scale.
Show them the pH typical value chart.
See if students can identify neutral, acidic, and basic on the chart.
a. pH=7 is neutral, pH<7 is acidic, pH>7 is basic
Explore:
1. Explain material found in teacher notes section below.
2. Perform experiment found in the pH Experiment Procedure handout below.
Explain:
1. Ask the students the following questions about their experiment:
a. Is the vinegar/lemon juice acidic or basic?
b. Is the household ammonia/baking soda acidic or basic?
c. How does the pH of the distilled water compare to the acid and the
base?
2. Have the students describe the difference between an acid and a base.
a. Hydrogen ion activity, acid is low & base is high, etc. are acceptable
answers.
© 2010, Ohio Northern University: Dr. Debra Gallagher, Dr. Robert Verb, Dr. Ken Reid and Ben McPheron
Extend:
1. Ask students to measure the pH of the small tanks that are being used for the
experiment on nutrients and temperature.
2. Have the students determine if each tank is basic, acidic, or neutral.
3. Have the students formulate a hypothesis based on their measurements as to
what type of pH the algae that they are cultivating prefer.
a. The results of this hypothesis will be presented by the pH group at the
conclusion of the small scale experiments
Evaluate:
1. Give the students a short quiz based on what they learned in this lecture and
activity. The quiz and answer key can be found below.
Teacher Notes:
1. What is pH?
a. Simply put, pH is a measure of how acidic or basic a solution is.
b. The pH scale is normalized to values between 0 and 14, used to
represent acidity and basicity (alkalinity).
c. pH is really a measure of the hydrogen ion activity of a solution
d. A pH of ‘7’ is considered neutral
2. An acid is a solution with pH less than neutral or ‘7’
a. An acid has a hydrogen ion activity greater than that of pure water
b. That means that there are free hydrogen ions (H+) to be used in
reactions
3. A base is a solution with pH greater than neutral or ‘7’
a. A base has a hydrogen ion activity less than that of pure water
b. That means that there are free hydroxide ions (the opposite of a H+
ion: OH-) to be used in reactions
4. When an acid and its conjugate base react, the product is water
a. Use illustration below to explain this.
© 2010, Ohio Northern University: Dr. Debra Gallagher, Dr. Robert Verb, Dr. Ken Reid and Ben McPheron
5. Mathematical description of pH
a. The pH scale can be defined mathematically by the following
equation:
1
𝑝𝐻 = − log10 (𝑎𝐻 + ) = log10 (
)
𝑎𝐻 +
Where aH+ is the activity of hydrogen ions. Give examples of
calculating pH in this way.
b. Example 1: calculate the pH a solution with an aH+= 10-6 (or
0.000001)
i. Answer= 6
c. Example 2: calculate the pH a solution with an aH+= 10-12
i. Answer= 12
d. Example 3: calculate the pH a solution with an aH+= 10-7
i. Answer= 7
6. There are several methods of measuring pH
a. pH meter: commercially available electronic gadgets that often
display pH, temperature, water conductivity, among other things.
These meters are very accurate.
b. pH indicator: a liquid or indicator strip which changes color with
different pH levels. These come complete with a color key and allow
for a visual estimation of pH.
c. Litmus paper: a specific type of indicator useful for identifying if a
solution is acidic of basic.
i. Paper turns from blue to red in an acidic solution.
ii. Turns from red to blue in a basic solution.
iii. Of interest in that it is created from lichens (symbiotic
organisms consisting of a fungus and an alga)
© 2010, Ohio Northern University: Dr. Debra Gallagher, Dr. Robert Verb, Dr. Ken Reid and Ben McPheron
pH Typical Values for Common Solutions
© 2010, Ohio Northern University: Dr. Debra Gallagher, Dr. Robert Verb, Dr. Ken Reid and Ben McPheron
pH Experimental Procedure
1. Student groups of 2-4
2. Label each beaker: one for distilled water, one for ammonia, and one for
vinegar. If students have solution(s) from home for testing, label the
appropriate number of additional beakers.
3. Pour 50-100 mL of each solution into its designated beaker
a. Note that if you are using baking soda, you will need to start with
distilled water, then add a small amount baking soda and stir until the
baking soda dissolves
4. Test each solution with litmus paper. Observe and record the results in lab
notebooks.
5. Next, test each solution with the indicator strips. Determine the pH using
the color key and record results in the lab notebook.
6. Repeat using the pH meter. Record the data in the lab notebook.
7. Comment in the lab notebook on the accuracy of each measurement and
answer the following questions.
a. Is the vinegar/lemon juice acidic or basic?
b. Is the household ammonia/baking soda acidic or basic?
c. How does the pH of the distilled water compare to the acid and the
base?
© 2010, Ohio Northern University: Dr. Debra Gallagher, Dr. Robert Verb, Dr. Ken Reid and Ben McPheron
pH Quiz
Name_________________________________
1. What is pH? Put your response in sentence form. (2 points)
2. Answer ‘acid’, ‘base’, or ‘neutral’ for the following examples. (6 points)
a. pH=7
__________________
b. Ammonia
__________________
c. Distilled water __________________
d. pH<7
__________________
e. pH>7
__________________
f. Vinegar
__________________
3. Using the equation below and your calculator, find the pH of a solution with
an aH+= 10-6 (or 0.000001) (2 points)
1
𝑝𝐻 = − log10 (𝑎𝐻 + ) = log10 (
)
𝑎𝐻 +
BONUS: Which has a higher hydrogen ion activity (Circle one) (1 point)
Acid
Base
© 2010, Ohio Northern University: Dr. Debra Gallagher, Dr. Robert Verb, Dr. Ken Reid and Ben McPheron
pH Quiz
Name____KEY__________________________
1. What is pH? Put your response in sentence form. (2 points)
pH is a measure of the acidity or basicity of a solution. Also acceptable is that pH
is a measure of the hydrogen ion activity. 1 point for a correct answer, 1 point for
sentence form
2. Answer ‘acid’, ‘base’, or ‘neutral’ for the following examples. (6 points)
a. pH=7
___Neutral_________
b. Ammonia
___Base___________
c. Distilled water
___Neutral_________
d. pH<7
___Acid___________
e. pH>7
___Base___________
f. Vinegar
___Acid___________
1 point for each correct answer
3. Using the equation below and your calculator, find the pH of a solution with
an aH+= 10-6 (or 0.000001) (2 points)
1
𝑝𝐻 = − log10 (𝑎𝐻 + ) = log10 (
)
𝑎𝐻 +
pH= - log10 (0.000001)= 6
1 point for correct answer, 1 point for showing work
BONUS: Which has a higher hydrogen ion activity (Circle one) (1 point)
Acid
Base
© 2010, Ohio Northern University: Dr. Debra Gallagher, Dr. Robert Verb, Dr. Ken Reid and Ben McPheron