Lab 17: Investigating pH

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We encounter acids and bases every day. Let’s find out more about them by doing some experiments.

Experimental Title: Lab 17:  Acids, Bases, and pH Indicators

Date of laboratory:  September 23, 2014

Purpose: The purpose of this laboratory is to investigate the use of pH indicators.

Introduction:

pH is a measurement of how basic or acidic a solution is based on the activity of hydronium ions (See lesson 17 for a discussion of acids and bases).

Certain molecules and substances change color when exposed to specific acidic or basic conditions, and thus can be used as pH indicators. There are over 150 different molecules that are used as pH indicators (see the excerpt from Handbook of Acid-Base Indicators by R. W. Sabnis at the bottom of this page). Examples of naturally-occurring pH indicators include litmus (a molecule extracted from certain lichens), anthocyanin (derived from red cabbage or berries), and curcumin (found in turmeric).

The pH value or range at which the pH indicator changes color is called the transition. Here are some of the known transitions:

Methyl red is red below a pH of 4.4 and yellow above 6.2. Methyl orange is red below a pH of 3.1 and orange/yellow above 4.4. Phenolthalien is pink in the range from 8.2 to 12.0. Thymol blue is red below 1.2, yellow from 2.8-8.0 and blue above pH 9.6. Turmeric is yellow/orange below 7.4 and red above 8.6.

 

Special safety concerns for Lab 17:

  • We will be using some strong acids and bases, so please bring and wear your goggles, gloves, long pants and closed-toe shoes.
  • If an acid spills, cover it with baking soda as we discussed. If a base spills, neutralize it with vinegar. If either spills on your skin, immediately wash with water.
  • If glass breaks, do not pick it up with your bare hands. Notify your instructor immediately.
  • Be sure to wash your hands when you are finished with this lab.

Materials:

  • Centrifuge tubes
  • Sharpie markers
  • Red cabbage
  • Blender
  • Distilled water
  • Acetic acid, 6M
  • Ammonia, 6M
  • Hydrochloric acid, 6M
  • Sodium hydroxide, 6M
  • Methyl red indicator
  • Methyl orange indicator
  • Phenolthalein
  • Thymol blue
  • Turmeric
  • Paper towels
  • Rubbing alcohol
  • Litmus paper
  • Wide range pH paper
  • 250 mL glass beaker
  • 100 mL graduated cylinder
  • Clear plastic cups
  • Household substances to test for pH

Procedures:

Note: I will prepare the red cabbage indicator and turmeric in advance using these simple steps:  For the red cabbage indicator, grind up fresh red cabbage in small batches with distilled water in a blender and then strain it in a colander to remove the bigger plant bits. For the turmeric, mix a little turmeric spice (used in curries) and rubbing alcohol in a small container and then dip in strips of paper towel. Allow the strips to dry (Protect the work surface, as turmeric stains).

Part 1. Acid and Base Standards

(Edit:  changed the numbering on Monday)

1. Label four centrifuge tubes, “0.1 M acetic acid”, “0.1 M ammonia”, “0.1 M hydrochloric acid”, and “0.1 M sodium hydroxide”. Place the tubes in the tray provided.
2. Measure 10 mL of distilled water to each of the four centrifuge tubes.
3. Using a pipette, transfer 0.25 mL of 6 M acetic acid to the “0.1 M acetic acid” centrifuge tube. Add additional distilled water to bring the total volume in
the tube to 15 mL. Cap the tube and swirl it gently to mix the contents.
4. Repeat that procedure in step 3 for each of the remaining centrifuge tubes, transferring 0.25 mL each of the 6 M ammonia, 6M hydrochloric acid, and 6M sodium hydroxide solutions. Be very careful and attend to any spills immediately.
5. Obtain a 24-well reaction plate and place it in the tray. Transfer 1.0 mL of 0.1 M acetic acid from the centrifuge tube into each well from A1 through A6. Recap the tube.
6. Transfer 1.0 mL of 0.1 M ammonia into each well from B1 through B6. Recap the tube.
7. Transfer 1.0 mL of 0.1 M hydrochloric acid into each well from C1 through C6. Recap the tube.
8. Transfer 1.0 mL of the 0.1 M sodium hydroxide into each well from D1 through D6. Recap the tube.

24-well-reaction-plate-template

Part 2. pH Indicators

Now we will test each type of pH indicator.

Part 2A:
1. Place 4 strips of wide-range pH paper on the lid of the 24-well plate. Using a stirring rod, obtain a drop of the fluid in well A1, acetic acid. Transfer it to a strip of pH paper. Allow the solution to react with the paper long enough so that it is no longer changing color. Record the final color and the pH that corresponds to that color in your notebook. Be sure to rinse the stirring rod carefully between samples.
2. Repeat step 1 completely, using the stirring rod to obtain a drop of the fluid in well B1, ammonia, and placing it on a strip of pH paper. Record the final color and the pH that corresponds to that color in your notebook.
3. Repeat step 1 completely, using the stirring rod to obtain a drop of the fluid in well C1, hydrochloric acid, and placing it on a strip of pH paper. Record the final color and the pH that corresponds to that color in your notebook.
4. Repeat step 1 completely, using the stirring rod to obtain a drop of the fluid in well D1, sodium hydroxide, and placing it on a strip of pH paper. Record the final color and the pH that corresponds to that color in your notebook.

Part 2B:
Now repeat part 2A using strips of red and blue litmus paper. Record your results.

Part 2C:
Repeat part 2A using the turmeric strips. Record your results.

Part 2D:
Add one drop of red cabbage indicator to each of the wells in column 2, A2 through D2. Look for color changes. If the changes are not conclusive, add another drop. Record the final color.

Part 2E:
Add one drop of methyl red to each of the wells in column 3, A3 through D3. Look for color changes. If the changes are not conclusive, add another drop. Record the final color.

Part 2F:
Add one drop of methyl orange to each of the wells in column 4, A4 through D4. Look for color changes. If the changes are not conclusive, add another drop. Record the final color.

Part 2G:
Add one drop of thymol blue to each of the wells in column 5, A5 through D5. Look for color changes. If the changes are not conclusive, add another drop. Record the final color.

Part 2H:
Add one drop of phenolthalein to each of the wells in column 6, A6 through D6. Look for color changes. If the changes are not conclusive, add another drop. Record the final color.

Now you have some idea what the range of pH values the pH indicators indicate. Keep these in the tray to use for comparison to household substances later.

 

Part 3. Phenolthalein “Titration”

Chemists use titration to determine the concentration of unknown solutions. Generally, doing a titration requires a burette, a piece of equipment that measures volume very accurately. Because we do not have access to a burette, we will simulate the process with a micropipette.

1. Place 50 mL of distilled water in a glass 250mL beaker using a 100 mL graduated cylinder.

2. Add 3 drops of phenothalein with a micropipette.

3. Using a clean micropipette, add drops of the 0.1 M ammonia from the centrifuge tube, stirring after each addition. Keep going until the solution turns pink and stays pink. Record the volume of ammonia you added.

4. Using a clean micropipette, add drops of 0.1 M acetic acid from the centrifuge tube, stirring after each addition. How much acetic acid is needed to make the solution go clear again?

 

Part 4. Determining the pH of common, household substances

Review Table 14.7 on page 507 in your textbook. We are going to check the pH of some common substances and create a similar table.

Leave space in your notebook for a table like this:

pH-table

Place substances in clear plastic cups provided. Test the pH with strips as we did in part 2 and then think about which indicator substances might verify your results. Mix indicator substances into the substances in clear cups. Try to find the transitions/color ranges for the red cabbage indicator. Record your results and the final pH values or ranges you obtain. We will compare our results.

Conclusions:

Once you have completed the four parts, sit down and write a sentence or two to explain the results of each part.

Discussion:

Record any thoughts you have about the experiments, including:

  • Possible improvements to the procedures or how to tweak techniques
  • How the results differed from your expectations
  • Suggestions for other experiments
  • What key concepts you learned about acids and bases

Excerpt from Handbook of Acid-Base Indicators by R. W. Sabnis:

Please leave a comment or send an e-mail if you have any questions before our meeting.

 

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