Acids, Bases, and Indicators

Chemistry Department

Minneapolis Community & Technical College

Intro to Chemistry Chem1020 Lab

1

Overview• Part IConcept review of pH

• Part IIIndicator

• Part IIIpH standard solutions

• Part IVHousehold products

• Part VpH paper

2

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.

3

Part II. Indicator

4

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.

5

Shredded red cabbage leaves are heated in distilled water in advance.

The filtered red cabbage juice is stored in a separa-tory 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.

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.

Part III. pH Solutions

6

1 32 4

5 6 7 8

9 10 11 12

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.

Part IV. Household Products

7

Household product A, vinegar, goes into well “A”.

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.

Part V. What if…..

8

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.

• 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.

9