Intro to Chemistry Chem1020 Lab
Acids, Bases, and Indicators
Chemistry Department
Minneapolis Community & Technical College
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Overview
• Part I
Concept review of pH
• Part II
Indicator
• Part III
pH standard solutions
• Part IV
Household products
• Part V
pH paper
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Part I. pH
• pH is used to indicate the acidity of a solution.
• The lower the pH value, the more acidic the solution is.
• Solutions can be classified based on their pH values:
pH < 7.0, the solution is acidic
pH = 7.0, the solution is neutral
pH > 7.0, the solution is basic
The goal of this experiment is to determine if each of the
12 given household products is acidic, neutral, or basic
after measuring their individual pH values.
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Part II. Indicator
There are many different ways of determining pH values of
solutions. One easy way is to use an indicator.
Indicator is a substance that changes its color based on the
pH of its environment. A substance present in the juice from
this lovely red cabbage will be used as an indicator in this
experiment.
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Shredded red cabbage
leaves are heated in
distilled water in advance.
The filtered red cabbage
juice is stored in a separatory funnel for dispense.
Obtain 40 mL of juice.
Now it seems that you are ready to explore with this wonderful indicator juice!
But wait! How do you know what color of the indicator is associated with
what pH value?
So before you work on the household products, you need to test the red
cabbage indicator on a series of standard solutions with known pH values.
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Part III.
pH Solutions
1. Get a well-plate marked
with numbers 1-12.
2. Add 10 drops of a buffer
solution of a specific pH
into the corresponding well.
Examples are shown on the
right with red arrows. Be
careful and do not spill or
cross-contaminate!
3. To each of the well, add 15
drops of red cabbage juice.
Observe and record the
characteristic colors of the
indicator at different pH
environments.
4. Don’t discard the contents
in this well-plate. You still
need them.
1
2
3
4
5
6
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10
11
12
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Part IV. Household Products
1. Get a different well-plate marked with letters A-L.
2. To each of the well, add 15 drops of red cabbage juice.
3. Add a certain household product into the corresponding well: 2 drops if it is a liquid
and a match-head amount if solid. An example is shown below with the red arrow. Be
careful and do not spill or cross-contaminate!
4. Compare the color in a specific well with those of 12 wells in the well-plate labeled
with numbers (see the previous slide). For example, if the color in well “A” is
somewhere in between the colors of wells “3” and “4”, record the pH of vinegar as 3.5.
Household product A,
vinegar, goes into well “A”.
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Part V. What if…..
a color in the well of a household product does not match any one of those
12 colors of the pH standard solutions?!
• We will have to use pH paper which is coated with a different indicator.
The correlation between the different colors of this indicator and the pH
values can be easily found on the cover of the container.
• Use a pair of tweezers to pick up the pH papers as many as needed. Add
that household product directly onto the paper. Do not add the red
cabbage juice when using the pH paper!
• Compare the color on the paper with the colors on the cover of the
container. Record the pH value of that household product.
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• Dispose of the used pH paper into the regular trash.
• Dispose of the contents in the well-plates and the beaker in sink.
•
Wash all the glasswares, including the well-plates, beaker,
medicine dropper, glass stirring rods, and watch glass, thoroughly
with tap water. Return them to your bench for drying.
• Wipe the bench with wet sponge…..ask your instructor to sign
you out….then you are ready to leave.
• Finish the lab report (including both data sheet and postlab
questions) after the lab. Feel free to ask instructors questions, but
never ever copy other students’ answers.
• Turn in the lab report the next time when you are expected to
come to the lab.
• Don’t forget to prepare for the next lab before you come next
time.
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