Materials Included In Kit

Additional Materials Required

Prelab Preparation

Part A. Preparing the Red Cabbage Universal Indicator Solution

1. Prepare the red cabbage solution the day of the experiment. 
2. To prepare the red cabbage solution, measure 0.3–0.6 g of the red cabbage extract powder into a weighing dish.
3. In a 400-mL beaker, mix the red cabbage extract with 300 mL of distilled or deionized water.
4. Place the solution in smaller beakers with Beral pipets, for student use in lab.
5. Label the beakers.

Part B. Preparing the Buffer Solutions (pH 2, 4, 6, 8, 10 and 12)

6. Measure 100 mL of distilled or deionized water.
7. Pour it into a 250-mL beaker.
8. While wearing gloves, break open the pH 2 buffer capsule.
9. Add the pH 2 buffer to the water and stir.
10. Distribute into smaller beakers for class use.
11. Label the beakers and add pipets to the beakers (e.g., pH 2).
12. Repeat steps 6–11 for buffers pH 4, 6, 8, 10 and 12.

Part C. Preparation of the Birds 

(Note: Must be completed at least 4–5 hours in advance of the lab.)

13. Cut the blotting paper along the dotted lines to make 30 rectangular pieces, each with an outline of a bird. 
14. Fill a 400-mL beaker with 300 mL of distilled or deionized water.
15. Measure a small amount (0.3–0.6 g) of red cabbage extract powder into a weighing dish.
16. Stir the red cabbage extract powder into the beaker of water.
17. Pour the red cabbage indicator solution into a plastic container.
18. Place no more than three rectangular pieces of blotting paper into the plastic container. Wait about 30 seconds to allow the blotting paper to absorb the indicator solution.
19. Wearing rubber gloves or using tongs, carefully remove the blotting paper pieces from the solution and lay them out on paper towels to dry.
20. Repeat steps 18–19 for the remainder of the blotting paper pieces. Note: You may use the included margin strip of blotting paper for instructor testing.
21. Allow the treated blotting paper birds to completely dry, about 4–5 hours.

Safety Precautions

All of the acids and bases used in this lab are corrosive to eyes, skin and other body tissues. They are toxic by ingestion. Avoid contact of all chemicals with eyes and skin. Avoid inhaling vapors. Notify your teacher and clean up all spills immediately. Use sodium carbonate or sodium bicarbonate to neutralize acid solutions. Use citric acid to neutralize base spills. Universal indicator is alcohol-based and is flammable. Keep away from flames and other ignition sources. Universal indicator is toxic by ingestion. Wear chemical-splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information. 

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Dilute hydrochloric acid solution may be neutralized with base and then disposed of down the drain with an excess of water according to Flinn Suggested Disposal Method #24b. Dilute acetic acid solution may be neutralized and then disposed of down the drain with excess water according to Flinn Suggested Disposal Method #24a. Dilute sodium hydroxide and ammonium hydroxide solutions may be neutralized with acid and then disposed of down the drain with an excess of water according to Flinn Suggested Disposal #10. Any leftover dilute HCl and NaOH solutions may be combined, and then the final product checked with pH paper. If the final product is near neutral (pH 5–9), it can then be discharged down the drain with an excess of water. Red cabbage indicator solution may be disposed of down the drain with an excess of water according to Flinn Suggested Disposal Method #26b. Used cotton swabs, scrap blotting paper and paper towels may be disposed of in the regular trash.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. All parts of this laboratory activity can reasonably be completed in two 50-minute class periods. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• You may stagger the starting points for lab groups. Some groups could start with Part A and the other groups could start with Part B.
• For household materials, you can also encourage students to bring items (e.g., juices, soaps) from home to test. All materials must be reviewed and approved by the teacher before testing.

Teacher Tips

• Once students identify the unknowns’ pH, the activity can be extended by predicting color changes with the other indicators.
• The difference between strong and weak acids is frequently misunderstood by students, who may confuse strong acids with concentrated solutions and weak acids with dilute solutions. In order to avoid misunderstanding, the solutions in this experiment all had the same concentration. Strong and weak acids (or bases) can be distinguished by their pH values if the solutions all have the same concentration. If two acids have the same concentration, then a stronger acid solution will have a lower pH value than a weaker acid. Thus, 0.1 M hydrochloric acid has a pH of 1 and 0.1 M acetic acid has a pH of 3. If two bases have the same concentration, then a stronger base will have a higher pH value than a weaker base. Thus, 0.1 M sodium hydroxide has a pH of 13 and 0.1 M ammonia has a pH of 11, while 0.1 M sodium bicarbonate would have a pH value of about 10. The pH value of an acid (or base) solution depends on both the strength of the acid and its concentration.
• Teacher and students can come back to this activity when talking about titrations and choosing an appropriate indicator. It can serve as an introduction discussion to indicators and equivalence points.

Sample Data

Data Table 1

Data Table 2

Data Table 3

Data Table 4

Answers to Questions

1. Calculate the [H⁺] values for the chemicals used in this experiment. Use the data from Data Table 4 for your calculations.

Sample calculations
[H⁺] = 10^–pH
Hydrochloric acid solution, 0.1 M       [H⁺] = 10^–2 = 1 x 10^–2 M
Baking powder solution                       [H⁺] = 10^–10 = 1 x 10^–10 M

2. Calculate the pOH values for the same items in Data Table 4, assuming 25 °C.

Sample Calculations
pH + pOH = 14 at 25 °C
Hydrochloric acid
pOH = 14 – pH = 14 – 2 = 12
Acetic acid
pOH = 14 – pH = 14 – 2 = 12
Sodium hydroxide
pOH = 14 – pH = 14 – 12 = 2

3. Sodium hydroxide and ammonium hydroxide solution both have a molarity of 0.1 M. Why do the two solutions have different pH numbers?

While both solutions have the same molarity, the bases vary in strength. Sodium hydroxide is a stronger base than ammonium hydroxide. Therefore, there is more OH^– in a 0.1 M sodium hydroxide solution than a 0.1 M ammonium hydroxide solution. The pOH value will be smaller with sodium hydroxide than ammonium hydroxide. Therefore, the pH value for sodium hydroxide will be higher than the ammonium hydroxide solution.

Introduction

With this lab activity, you will become more familiar with the pH scale and acid–base indicators. Acids and bases are all around us! They are in the foods we eat and the soaps and cleaners used in our homes and schools. This activity will familiarize you with the amazing world of acids and bases. Have fun exploring!

Concepts

• Acids and bases
• pH scale
• Indicators

Background

The pH scale can be used to determine if a substance is acidic, basic or neutral. A pH of 7 at 25 °C is considered neutral. Acids have a pH of less than 7. Bases have a pH of greater than 7.

pH is a representation of how many hydronium ions (H₃O⁺) are in solution and is based on a logarithmic scale. Therefore, when you see a pH of 7, it means that there are 1.0 x 10^–7 hydronium ions in solution. Hydronium ions are represented as H₃O⁺ or are also abbreviated as H⁺. To derive the pH of a solution from the hydronium concentration, you must use the following equation:

Other useful equations with the pH scale are as follows:

If given a pH and asked to calculate the [H⁺] concentration, use Equation 4.

The same can be done, if the pOH is given and [OH^–] is desired with Equation 5.

In this lab, you will use indicators to test for the pH of various solutions. Indicators are organic dyes that change color in acidic or basic solutions. Almost any flower or fruit that is red, blue or purple contains a class of organic pigments called anthocyanins, which change color with pH. The use of these natural dyes as acid–base indicators originated during the middle ages—painters made watercolor paints by combining flower and fruit extracts with vinegar, an acid, and limewater, a base.

Figure 2 shows pH values for more everyday items.

Experiment Overview

First, you will create a color scale with universal indicator at a variety of pH levels. After creating your pH/indicator color chart, you will test some additional acids and bases. Then, you and your classmates will be able to test household items. Lastly, as a fun extension, you can make your very own colorful red cabbage indicator bird!

Materials

Safety Precautions

All of the acids and bases used in this lab are corrosive to eyes, skin and other body tissues. They are toxic by ingestion. Avoid contact of all chemicals with eyes and skin. Avoid inhaling vapors. Notify your teacher and clean up all spills immediately. Use sodium carbonate or sodium bicarbonate to neutralize acid solutions. Use citric acid to neutralize base spills. Universal indicator is alcohol-based and is flammable. Keep away from flames and other ignition sources. Universal indicator is toxic by ingestion. Wear chemical-splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory. 

Procedure

Part A

1. Obtain a 24-well reaction plate, and place it on a piece of white paper, as shown.

2. Place 1–2 drops of universal indicator into wells 1–6.
3. Add 1–2 drops of pH 2 buffer in well 1. Record the color in Data Table 1.
4. Add 1–2 drops of pH 4 buffer in well 2. Record the color in Data Table 1.
5. Add 1–2 drops of pH 6 buffer in well 3. Record the color in Data Table 1.
6. Add 1–2 drops of pH 8 buffer in well 4. Record the color in Data Table 1.
7. Add 1–2 drops of pH 10 buffer in well 5. Record the color in Data Table 1.
8. Add 1–2 drops of pH 12 buffer in well 6. Record the color in Data Table 1.
9. Test the various solutions from your instructor using wells 7 and up. Place 1–2 drops of universal indicator and then add 1–2 drops of the solution to be tested. Record your results in Data Table 2.
10. Rinse the contents of the reaction plate down the drain with plenty of water.

Part B

11. Place the well plate back on a piece of white paper.
12. Place 1–2 drops of the red cabbage indicator solution into wells 1-6.
13. Add 1–2 drops of pH 2 buffer in well 1. Record the color in Data Table 3.
14. Add 1–2 drops of pH 4 buffer in well 2. Record the color in Data Table 3.
15. Add 1–2 drops of pH 6 buffer in well 3. Record the color in Data Table 3.
16. Add 1–2 drops of pH 8 buffer in well 4. Record the color in Data Table 3.
17. Add 1–2 drops of pH 10 buffer in well 5. Record the color in Data Table 3.
18. Add 1–2 drops of pH 12 buffer in well 6. Record the color in Data Table 3.
19. Test the various solutions from your instructor using wells 7 and up. Place 1–2 drops of red cabbage indicator and then add 1–2 drops of the solution to be tested. Record your results in Data Table 4.
20. Rinse the contents of the reaction plate down the drain with plenty of water.

Part C

21. Obtain a bird from your instructor. All the birds have been soaked in red cabbage indicator and allowed to dry.
22. Place the bird on a paper towel.
23. Cut the excess blotting paper away from the outline of the bird. The excess blotting paper may be used for testing different solutions and their colors.
24. Dip cotton swabs into the different solutions (one solution at a time) and lightly touch the blotting paper.
25. Use your earlier data from the red cabbage indicator test for an indication of the various colors the solutions will create.
26. Have fun making your colorful bird! You can use a variety of dots, lines and other patterns. Always us a fresh cotton swab for each solution to avoid contamination.
27. Rinse the contents of the reaction plate down the drain with plenty of excess water. Consult your instructor for appropriate disposal procedures for leftover solutions. Used cotton swabs, scrap blotting paper and paper towels may be disposed of in the regular trash.

Student Worksheet PDF

14149_Student1.pdf