Materials Included In Kit

Additional Materials Required

Prelab Preparation

Part A. Preparing the Red Cabbage 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 three smaller beakers with disposable pipets, for student use in lab.
5. Label the beakers.

6. Measure 100 mL of distilled or deionized water into a 250-mL beaker.
7. While wearing gloves, break open the pH 2 buffer capsule and add the pH 2 buffer to the water and stir.
8. Distribute into 3 test tubes for class use.
9. Label the test tubes and add a pipet to each test tube.
10. Repeat steps 6–9 for buffers pH 4, 6, 8, 10 and 12.

11. Measure 100 mL of distilled or deionized water into a 250-mL beaker.
12. While wearing gloves, break open the pH 3 buffer capsule and add the pH 3 buffer to the water and stir.
13. Distribute into 3 test tubes for class use.
14. Label the test tubes Unknown A and add a pipet to each test tube.
15. Repeat steps 11–14 for buffers pH 5 and 9 (pH 5 = Unknown C, pH 9 = Unknown B).

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. Buffers in low pH range are strongly acidic; those in high pH range are strongly alkaline. Avoid contact of all chemicals with eyes and skin. Avoid inhaling vapors. Keep spill materials on hand to neutralize acids and bases in case of spills. Use sodium carbonate or sodium bicarbonate to neutralize acid solutions. Use citric acid to neutralize base spills. Phenolphthalein and universal indicator are alcohol-based solutions and are flammable. Keep away from flames and other ignition sources. Methyl orange, phenolphthalein and universal indicator solutions are toxic by ingestion. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please consult 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. All of the solutions may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b. The used paper strips should be discarded according to Flinn Suggested Disposal Method #26a.

Lab Hints

• Enough test tubes are provided to allow for 3 test tubes for each pH and each unknown.
• Enough materials are provided in this kit for 30 students working in pairs. All parts of this laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab.
• To evenly split supplies, stagger groups by assigning them to start at different parts.
• Instead of all students sharing the pH paper color chart on the container, pass out copies of the color charts for the pH paper.
• Buffer capsules 7 and 11 are also included and can be used to create more unknowns.

Teacher Tips

• The focus of this experiment is to observe and identify unknowns using pH and indicators. This activity is a great qualitative introductory activity to acids, bases and indicators.
• Once students identify the pH of the unknown solutions, the activity can be extended by predicting color changes with the other indicators.
• Teacher and students can come back to this activity when discussing titrations and choosing an appropriate indicator. It can serve as an introductory discussion of indicators and equivalence points.

Answers to Prelab Questions

1. What is the pH range for acids?

   Acids have a pH range of 0–6.

2. What is the pH range for bases?

   Bases have a pH range of 8–14.

3. What are some of the materials used to make indicators?

   Indicators can be made from a variety of materials. Students can list moss, flowers, cabbage, berries, plants, fungi, algae and fruit.

4. Alizarin is an indicator that is yellow when the pH is < 5 and red when the pH is > 7. When the alizarin is added to solutions with a pH between 5 and 7, the indicator is intermediate in color between yellow and red, or various shades of orange. Table 1 shows the ranges for alizarin and bromthymol blue. The shaded areas indicate transition pH—intervals in which the color of the indicator changes from one form to another (in this case, where alizarin is orange). Note: Alizarin also changes color with the pH of 11–13. At pH values greater than 12, alizarin appears violet.

   A colorless household solution was tested with the two indicators shown above (see Table 1 on student handout). The solution was yellow with bromthymol blue and orange with alizarin. What is the pH of the solution? Be as specific as possible.
   The household solution has a pH of 5. Bromthymol blue is yellow with pH values 5 and below. Alizarin is orange with a pH range of 5–6. In order for both of the observations to occur, the pH of the unknown has to have a value of 5.

Sample Data

Data Table A. Universal Indicator

Answers to Questions

1. Predict the colors for the buffers, if a red cabbage indicator were used. Place your predictions in the the following table.
   {12240_Answers_Table_7}
2. How can phenolphthalein be used to tell whether a solution is an acid or a base? Be specific.

   Phenolphthalein is colorless in acidic or neutral solutions, pink or red-violet in basic solutions.

3. A colorless solution is tested with bromthymol blue, methyl orange, and phenolphthalein. It is colorless in phenolphthalein, yellow in bromthymol blue, and reddish orange in methyl orange. What is the pH of the solution?

   The pH of the solution would be 4. Phenolphthalein is colorless below 8, bromthymol blue is yellow below 6, and methyl orange is a red/orange at a pH of 4.

4. Construct a table to summarize the properties of the unknown solutions observed in Part E. Include the following in the table.
   1. An estimate of the pH value of each unknown
   2. Whether the solution is acidic or basic
      {12240_Answers_Table_8}
5. Use the data in the table from Question 4 to complete the following:
   1. Arrange the solutions in order from least acidic to most acidic.

      B, C, A

   2. Arrange the solutions in order from least basic to most basic.

      A, C, B

References

Cesa, I., Editor; Acids and Bases, Flinn ChemTopic^™ Labs Series; Flinn Scientific: Batavia, IL, 2002; Vol. 13.

Introduction

Indicators have been around for centuries and can be made from multiple materials. Moss, flowers, red cabbage, and berries are just a few of the materials that can be used for indicators. Indicators can be used to determine the pH of a chemical. Once the pH is known, chemicals can be classified as acidic, basic, or neutral. Explore the characteristic color changes of acid–base indicators and learn how to measure the pH of solutions using indicators!

Concepts

• Acids and bases
• pH scale
• Indicators

Background

An acid is a compound that increases the amount of H⁺ ions in a solution. Acids are corrosive, sting if they contact broken skin and taste sour. A base is a compound that increases hydroxide ions (OH^–) in a solution. Bases feel slippery as solutions, are corrosive and have a bitter taste. (Note: Taste should never be used to identify a lab chemical, and chemicals should not be touched with the bare skin). Acids and bases are found all over homes, schools and workplaces. Acids can be found in drinks, salad dressing and batteries. Bases can be found in household cleaners. How acidic or basic a chemical is can be determined using indicators and the pH scale.

Indicators are dyes or pigments that can be isolated from a variety of sources, including plants, fungi, and algae. One of the oldest known acid–base indicators is litmus, a natural dye obtained from lichens. Litmus paper, prepared by soaking paper in a solution of the dye, is often used as a general test for acids and bases. Almost any flower or fruit that is red, blue, or purple contains a class of organic pigments called anthocyanins that 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.

Indicators can change color at different points on the pH scale. The pH scale is a numerical scale that is related to the concentration of H⁺ ions in solution and is used to describe the relative acidity or basicity of a solution. The abbreviation pH stands for “power of hydrogen.” The pH scale ranges from 0 to 14, with 7 being neutral. Acids have pH values less than 7 while bases have pH values greater than 7 (see Figure 1).

Experiment Overview

The purpose of this experiment is to explore the reactions of indicators with acid and base solutions. After exploring indicators, the pH of unknown solutions will be analyzed and identified.

Materials

Prelab Questions

1. What is the pH range for acids?
2. What is the pH range for bases?
3. What are some of the materials used to make indicators?
4. Alizarin is an indicator that is yellow when the pH is < 5 and red when the pH is > 7. When the alizarin is added to solutions with a pH between 5 and 7, the indicator is intermediate in color between yellow and red, or various shades of orange. Table 1 shows the ranges for alizarin and bromthymol blue. The shaded areas indicate transition pH—intervals in which the color of the indicator changes from one form to another (in this case, where alizarin is orange). Note: Alizarin also changes color at pH 11–13. At pH values greater than 12, alizarin appears violet.
   {12240_PreLab_Table_1}
   A colorless household solution was tested with the two indicators shown in the table. The solution was yellow with bromthymol blue and orange with alizarin. What is the pH of the solution? Be as specific as possible.

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. Buffers in low pH range are strongly acidic; those in high pH range are strongly alkaline. 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. Phenolphthalein and universal indicator are alcohol-based solutions and are flammable. Keep away from flames and other ignition sources. Methyl orange, phenolphthalein and universal indicator solutions are toxic by ingestion. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part A. Creating a Color Chart for Universal Indicator

1. Obtain a 15-well reaction plate and place it on a piece of white paper as shown below in Figure 2.
   {12240_Procedure_Figure_2}
2. Place 1–2 drops of universal indicator into wells 1–6.
3. Place 1–2 drops of pH 2 in well 1, 1–2 drops of pH 4 in well 2 and 1–2 drops of pH 6 in well 3.
4. Place 1–2 drops of pH 8 in well 4, 1–2 drops of pH 10 in well 5 and 1–2 drops of pH 12 in well 6.
5. Record the color change for the corresponding pH in Data Table A on the pH Measurements Using Indicators Worksheet.
6. When finished with the observations, rinse the contents of the reaction plate down the drain with plenty of excess water.
7. Place the well reaction plate back on the white paper as shown in Figure 2.

Part B. Phenolphthalein

8. Place 1–2 drops of phenolphthalein into wells 1–6.
9. Place 1–2 drops of pH 2 in well 1, 1–2 drops of pH 4 in well 2 and 1–2 drops of pH 6 in well 3.
10. Place 1–2 drops of pH 8 in well 4, 1–2 drops of pH 10 in well 5 and 1–2 drops of pH 12 in well 6.
11. Record the color change for the corresponding pH in Data Table B.
12. Look at the data. Identify the transition for phenolphthalein (where both colors exist) and record on the worksheet.
13. When finished with the observations, rinse the contents of the reaction plate down the drain with plenty of excess water.
14. Place the well reaction plate back on the white paper as shown in Figure 2.

Part C. Methyl Orange

15. Place 1–2 drops of methyl orange into wells 1–6.
16. Place 1–2 drops of pH 2 in well 1, 1–2 drops of pH 4 in well 2 and 1–2 drops of pH 6 in well 3.
17. Place 1–2 drops of pH 8 in well 4, 1–2 drops of pH 10 in well 5 and 1–2 drops of pH 12 in well 6.
18. Record the color change for the corresponding pH in Data Table C.
19. Look at the data. Identify the transition for methyl orange (where both colors exist) and record on the worksheet.
20. When finished with the observations, rinse the contents of the reaction plate down the drain with plenty of excess water.
21. Place the well reaction plate back on the white paper as shown in Figure 2.

Part D. Bromthymol Blue

22. Place 1–2 drops of bromthymol blue into wells 1–6.
23. Place 1–2 drops of pH 2 in well 1, 1–2 drops of pH 4 in well 2 and 1–2 drops of pH 6 in well 3.
24. Place 1–2 drops of pH 8 in well 4, 1–2 drops of pH 10 in well 5 and 1–2 drops of pH 12 in well 6.
25. Record the color change for the corresponding pH in Data Table D.
26. Look at the data. Identify the transition for bromthymol blue (where both colors exist) and record on the worksheet.
27. When finished with the observations, rinse the contents of the reaction plate down the drain with plenty of excess water.
28. Place the well reaction plate back on the white paper as shown in Figure 2.

Part E. Identifying Unknowns

29. Obtain three unknowns from the instructor.
30. Write the three unknowns in Data Table E.
31. Tear the pH paper into small pieces and place one piece per well in wells 1–3.
32. Test each of the unknown solutions by placing 1–2 drops of the unknown into the designated well and observe the color.
33. Record all observations in Data Table E.
34. Compare the color in the well to the pH color chart. Record the pH for each unknown in Data Table E.
35. Using the data collected and a red cabbage indicator color chart, predict what colors the red cabbage indicator would change to using the unknowns.
36. Write the predictions in Data Table E.
37. In wells 4–6, place 1–2 drops of the red cabbage indicator.
38. Test each of the unknown solutions by placing 1–2 drops of the unknown into the designated well and observe the color. Record the observations.
39. When finished with the observations, use forceps to remove any leftover pH paper. Dispose of the paper in the trash. Rinse the contents of the reaction plate down the drain with plenty of excess water.

Student Worksheet PDF

12240_Student1.pdf