Materials Included In Kit

Additional Materials Required

Prelab Preparation

Prepare the buffer solutions (pH 2, 4, 6, 8, 10 and 12)

1.  Measure 100 mL of distilled or deionized water into a 250-mL beaker.
2. While wearing gloves, break open the pH 2 buffer capsule and add the pH 2 buffer to the water and stir.
3. Distribute into 3 test tubes or small beakers for class use.
4. Label the test tubes and add a pipet to each test tube.
5. Repeat steps 1–4 for buffers pH 4, 6, 8, 10 and 12.

Safety Precautions

The dilute hydrochloric acid and sodium hydroxide solutions used in this lab are slightly toxic by ingestion or inhalation and corrosive to skin and eyes. Buffers in low pH range are strongly acidic; those in high pH range are strongly alkaline. Use sodium carbonate or sodium bicarbonate to neutralize acid solutions. Use citric acid to neutralize base spills. Some household chemicals are toxic by ingestion or inhalation and are skin and eye irritants. Avoid contact of all solutions with eyes and skin and clean up spills immediately. All food-grade items that have been brought into the lab are considered laboratory chemicals and are for lab use only. Do not taste or ingest any material in the lab and do not remove any remaining food items after they have been used in the lab. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines. 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 may be neutralized with base and then rinsed down the drain with an excess of water according to Flinn Suggested Disposal Method #24b. Dilute sodium hydroxide solution may be neutralized with acid and then rinsed down the drain with an excess of water according to Flinn Suggested Disposal Method #26b. The liquid from the blue maize flour solution can be decanted and rinsed down the drain. Solid flour may be placed in the trash according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. This laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab.
• The following household substances are suggestions for testing with the blue maize indicator:

  Antacid tablet
  Baking powder
  Baking soda
  Club soda
  Contact lens solution
  Dishwasher detergent
  Drain cleaner
  Grapefruit juice
  Laundry detergent
  Lemon juice
  Liquid hand soap
  Mouthwash
  Shampoo
  Soft drinks
  Vinegar
  Vitamin C tablet
  Water, distilled and tap

• Dissolve solid substances with a small amount of distilled or deionized water. Tablets may be crushed with a mortar and pestle or between layers of waxed paper. It is not necessary for all of the solid particles to be dissolved in order to test the substance.
• Household solutions may be placed in labeled containers such as plastic cups, small jars, beakers or test tubes for student access. Set up several material dispensing stations around the room to avoid congestion.
• This laboratory can also be extended by using other natural products as indicators. A wide range of fruits and flowers contain natural acid–base indicators. Additional natural indicators that work well are red cabbage, blueberries and roses (red/pink). Flowers and fruits may be extracted with hot water. Instructors may find it convenient to prepare the natural indicators ahead of time.

Teacher Tips

• This activity is an excellent and fun way to introduce acids and bases along with their application in industry, health and the environment.
• The Indicator Sponge—A Discrepant Event Demonstration Kit (Flinn Catalog No. AP6160) is a great way to amaze your students and challenge their thinking skills regarding acids and bases.
• Another fun demonstration is the Upset Tummy? Mom to the Rescue!—Chemical Demonstration Kit, (Flinn Catalog No. AP5934).

Answers to Prelab Questions

1. Complete the following chart by identifying the substance listed as acidic, basic or neutral.
   {14105_PreLabAnswers_Table_1}
2. Describe the hazards and precautions associated with using hydrochloric acid and sodium hydroxide solutions.

   Hydrochloric acid and sodium hydroxide solutions are slightly toxic by ingestion or inhalation and corrosive to skin and eyes. Avoid contact of all solutions with eyes and skin and clean up spills immediately. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines.

3. Describe the precautions associated with food-grade items that have been brought into the laboratory.

   All food-grade items that have been brought into the lab are considered laboratory chemicals and are for lab use only. Do not taste or ingest any material in the lab and do not remove any remaining food items after they have been used in the lab.

Sample Data

Data Table 1

Data Table 2

Answers to Questions

1. Which tested solutions are acidic? What evidence supports this?

   Hydrochloric acid, lemon juice and seltzer water are acidic. They produced pink or pink/lavender colors after reacting with the blue maize indicator, representing pH values less than 7.

2. Which tested solutions are basic? What evidence supports this?

   Sodium hydroxide, liquid dish soap and drain cleaner are basic. They produced blue/green colors after reacting with the blue maize indicator, representing pH values greater than 7.

3. Are any of the tested solutions neutral or nearly neutral (pH 6–8)? If so, which one(s)?

   Liquid dish soap tested at a pH of 8.

4. Based on the results, which solutions would be considered strongly acidic? Which are strongly basic?

   The solutions with the lowest pH were hydrochloric acid and lemon juice, both had a pH of 2. Sodium hydroxide and drain cleaner had the highest pH of 12 and 12+.

5. Compare your results with those of other students.
   1. In what types of products are most household bases found? What characteristics do bases have in common?

      Most household bases are found in cleaning solutions such as soap and drain cleaner. Bases feel slippery and have a bitter taste. Bases have a pH greater than 7.

   2. In what types of products are most household acids found? What characteristics do acids have in common? Most household acids are found in juices and soft drinks. Acids taste sour and sting if they come in contact with broken skin. Acids have a pH less than 7.

References

Trujillo-de Santiago, G., Rojas-de Gante, C., Garcia-Lara, S., Ballescá -Estrada, A., & Moisés Alvarez, M. Studying Mixing in Non-Newtonian Blue Maize Flour Suspensions Using Color Analysis (2014). http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4234479/ (accessed July 2016)

Introduction

Indicators change color in acidic and basic solutions. Indicators have been around for centuries and can be made from natural materials like berries and vegetables. In this lab, investigate the indicator properties of blue maize flour, also known as blue corn flour. After discovering the various colors of the flour indicator, the blue maize indicator will be used to determine the pH of household liquids. Have fun exploring the pH of various solutions with this vibrant natural indicator!

Concepts

• Indicator
• Acids and bases
• pH scale

Background

Citric acid, stomach acid, acid rain—those are just a few acids in our everyday life! Acids are compounds that release hydrogen ions (H⁺, the same as a proton) in solution. Acids are corrosive, sting if they contact broken skin and taste sour. (Note: Taste should never be used to identify a lab chemical and chemicals should not be touched with the bare skin.) Bases are compounds that release hydroxide ions (OH^–) in solution.

Bases feel slippery as solutions, are corrosive and have a bitter taste. Remember, however, that touch and taste should never be used to test for the presence of a base or any chemical.

To express the concentration of hydrogen ions in solution, a term called pH (potential of hydrogen) is used. The pH scale ranges from 0–14. If the concentration of H⁺ ions is greater than the concentration of OH^– ions, then the substance is considered acidic and has a pH value lower than 7. Conversely, if the concentration of OH^– ions is greater than the concentration of H⁺ ions, then the substance is basic and has a pH value greater than 7. If the H⁺ and OH^– ion concentrations are equal (as in pure water, H₂O), the substance is neutral with a pH value of 7. Figure 1 provides pH values for some common substances.

Although acids as a group or bases as a group may have common characteristics, they are not all alike. Some weak acids, such as the citric acid found in fruit juices, are safe to consume, while stronger acids corrode metals and destroy human tissue. Likewise, some weak bases are used for bathing, while some strong bases are very caustic to living tissue. The strength of an acid or base depends on the amount of ions produced in solution. A strong acid, such as hydrochloric acid, HCl—found in the stomach—donates a greater number of H⁺ ions to a solution than a weak acid, such as the lactic acid found in milk. In a similar manner, strong bases donate more OH^– ions to a solution than weak bases. Household ammonia is a weaker base than sodium hydroxide, NaOH, which is found in drain cleaner.

Blue maize flour (also known as blue corn flour) contains a water-soluble pigment called anthocyanin. A solution prepared from blue maize flour acts as an indicator because it will react with various solutions and change color depending on the pH of the solution. Strongly acidic solutions will react with the anthocyanin to produce a pink color. The pigment will react with strongly basic solutions to produce a blue-green color, and with neutral solutions to produce a lavender color.

Experiment Overview

The purpose of this experiment is to explore the indicator properties of blue maize flour. The results will be used to identify the pH of several common household chemicals.

Materials

Prelab Questions

Read the Background and Safety Precautions sections, then answer the following questions on a separate sheet of paper.

1. Complete the following chart by identifying the substance listed as acidic, basic or neutral.
   {14105_PreLab_Table_1}
2. Describe the hazards and precautions associated with using hydrochloric acid and sodium hydroxide solutions.
3. Describe the precautions associated with food-grade items that have been brought into the laboratory.

Safety Precautions

The dilute hydrochloric acid and sodium hydroxide solutions used in this lab are slightly toxic by ingestion or inhalation and corrosive to skin and eyes. Buffers in low pH range are strongly acidic; those in high pH range are strongly alkaline. Notify your instructor and clean up all spills immediately. Use sodium carbonate or sodium bicarbonate to neutralize acid solutions. Use citric acid to neutralize base spills. Some household chemicals are toxic by ingestion or inhalation and are skin and eye irritants. Avoid contact of all solutions with eyes and skin and clean up spills immediately. All food-grade items that have been brought into the lab are considered laboratory chemicals and are for lab use only. Do not taste or ingest any material in the lab and do not remove any remaining food items after they have been used in the lab. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

1. Measure 5 g of blue maize flour and place the flour in a 100-mL beaker.
2. Measure 50 mL of distilled or deionized water and add it to the beaker. Stir.
3. Stir the mixture and use a clean, wide-stem pipet to transfer 10 drops of the blue maize flour suspension into each well on the reaction plate. 
4. Obtain the buffer solutions. Using a clean, thin-stem disposable pipet, place 3 drops of pH 2 buffer in well 1 (see Figure 2). {14105_Procedure_Figure_2}
5. Repeat step 4 with a clean, thin-stem disposable pipet for each buffer: pH 4 (well 2), 6 (well 3), 8 (well 4), 10 (well 5) and 12 (well 6).
6. Record the colors and pH in Data Table 1 on the Blue Maize Indicator Worksheet.
7. Obtain a sample of 0.1 M HCl and use a clean pipet to add 3 drops to well 7.
8. Using the data from Table 1 to determine the pH of the HCl solution and record the color and pH in Data Table 2.
9. Obtain a sample of 0.1 M NaOH and use a clean pipet to add 3 drops to well 8.
10. Record the color and pH in Data Table 2.
11. Choose four household items to test using the blue maize flour indicator. Record the items in Data Table 2.
12. Add 3 drops of one household item to well 9 and then record the color and pH in Data Table 2.
13. Repeat with three more household items in wells 10–12.
14. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

14105_Student1.pdf