Cabbage Juice Indicator

Description

Red cabbage juice is mixed with a series of buffers to obtain a set of color standards. Various household substances are added to red cabbage juice. By comparing the color of the resulting solution to that of the color standards, the approximate pH of the household materials is determined.

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Some products (e.g., Drano^®, bleach) are extremely corrosive and may cause blindness. Other products may be hazardous. Burns are possible.

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Eye protection must be worn. Follow manufacturers' precautions for other commercial products. Use caution with hot objects.

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1. Add 4-5 mL of the buffer solutions of pH 2, 4, 6, 7, 8, 10, and 12 to separate test tubes. Use 1.0 M NaOH for pH 14, 1.0 M HCl for pH 0, and pure distilled water for pH 7.0
2. Add 4-5 drops of cabbage juice to each of the test tubes.
3. Shake well.
4. Record the colors in Data Table I. You may not have a buffer for each pH. (If the color is not pronounced, add 4 or 5 drops more cabbage juice.)
5. Add 5 drops of one of the solutions to be tested to 1 mL of distilled water in a clean, labeled test tube. Add enough cabbage juice to give a pronounced color. Stir, compare the color to the bank of standards, and record your results in Data Table II.
6. Not all of the materials listed may be available; alternatives may be provided. Test all of the materials provided.
7. Record your observations.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 099 Cabbage Juice Indicator

Test household solutions.

Closure Questions:

1. What characteristics do most cleaning products have in common? Identify the ion that is probably responsible.
2. Which pH values will handsoaps and detergents for fine fabrics have?
3. Use balanced chemical equations to show that OH^- ions may be produced in solutions of salts such as Na₂CO₃ and Na₃PO₄.
4. Account for the difference in pH between 0.1 M HCl, 0.1 M HC₂H₃O₂, and 0.1 M H₃BO₃?

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Name ___________________________ Class ________

Teacher__________________________

DoChem 099 Cabbage Juice Indicator

Watch the movie, and record observations of color and pH from the movies and slides.

Use the slides on the movie to record the pH of the following solutions.

Closure Questions:

1. What characteristics do most cleaning products have in common? Identify the ion that is probably responsible.
2. Which pH values will handsoaps and detergents for fine fabrics have?
3. Use balanced chemical equations to show that OH^- ions may be produced in solutions of salts such as Na₂CO₃ and Na₃PO₄.
4. Account for the difference in pH between 0.1 M HCl, 0.1 M HC₂H₃O₂, and 0.1 M H₃BO₃?

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Purpose

• To prepare a standard set of colors for an indicator
• To use the standard set to determine the pH of several solutions

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(for 10 students working in pairs)

• 100 mL red cabbage juice
• 100 mL 1.0 M HCl (8.6 mL of concentrated HCl/ 100 mL solution)
• 100 mL 1.0 M NaOH (4.0g NaOH/ 100 mL solution)
• 100 mL each buffer solutions: pH = 2, 4, 6, 7, 8, 10, and 12. Purchase capsules of chemicals which, when dissolved in distilled water, provide solutions of known pH. These capsules are available from many chemical supply houses.
• 5 250-mL beaker
• 100 13 mm x 100 mm test-tubes
• 5 10-mL graduated cylinder
• household chemical products (colorless) such as: bleach (NaClO); Spic and Span^® (Na₃PO₄); borax (Na₂B₄O₇); vinegar (CH₃COOH); baking soda (NaHCO₃); Drano^® (NaOH); eye wash (H₃BO₃); liquid soap (sodium stearate, NaC₁₇H₃₅COO); and others.

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• Prepare the cabbage juice fresh. Use caution. Break up about 100-200 cm² of red cabbage leaves and place them in a 250-mL beaker. Add enough water to cover the leaves. Boil until the color of the solution is deep purple.
• Filter the juice through a glass wool plug. The solution may be filtered hot or cold.
• Keep the cabbage juice refrigerated or on ice until used.
• Use buffer capsules for these experiments (e.g., Frey Scientific). It is possible to make solutions of known pH (directions below), but this is very time consuming.
• Make several sets of standards for the students to share. This will save time and test tubes.
• Check the pH of the distilled water. Use freshly distilled water if the pH is outside of the range 6.5-7.3.

Preparation of pH Standards

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Teacher preparation: 20 minutes

Class Time: 30-35 minutes

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The materials used in this experiment may be disposed of safely at the sink after neutralization. Follow special disposal procedures recommended by manufacturers, if any.

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Closure Questions:

1. What characteristics do most cleaning products have in common? Identify the ion that is probably responsible.
2. Which pH values will handsoaps and detergents for fine fabrics have?
3. Use balanced chemical equations to show that OH^- ions may be produced in solutions of salts such as Na₂CO₃ and Na₃PO₄.
4. Account for the difference in pH between 0.1 M HCl, 0.1 M HC₂H₃O₂, and 0.1 M H₃BO₃?

Answers to Closure Questions:

1. Cleaning solutions are usually strongly basic and contain OH^- ions.
2. For cleaners, the higher the pH the more harmful to fine fabrics and sensitive hands. The closer to pH 7, the less harmful.
3. Na₂CO₃ --> 2Na⁺ + CO₃^2-

   CO₃^2- + H₂O --> HCO₃^- + OH^-

   Na₃PO₄ --> 3Na⁺ + PO₄ ^3-

   PO₄ ^3- + H₂O --> HPO₄ ^2- + OH^-

4. HCl is a strong acid, and is fully dissociated in water solution. Acetic acid (HC₂H₃O₂) and boric acid (H₃BO₃) are weak acids, and are only partially dissociated in water solution. Acetic acid is stronger than boric acid, so it is more dissociated, the [H⁺] is larger, and the pH is lower.

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• Indicators are weak acids that can be represented symbolically by the formula HIn. The dissociation for the indicator is:
  • HIn H⁺ + In^-
• The acid form of the indicator has one color, and the base or conjugate base form another. Addition of acid leads to the formation of HIn. Addition of base leads to the formation of In^-.
• Buffers are solutions that maintain a nearly constant pH upon addition of acids or bases (within well-defined concentration limits).
• By adding indicators to buffer solutions, a set of color standards for the indicator may be prepared. When the indicator is added to an unknown, the pH of the unknown may be ascertained by comparing the resulting color with that of the standards.
• With red cabbage juice, there are many different indicator materials present which accounts for the wide variation of color and allows this one indicator source to be used for the entire pH range.

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Household substances on movie

See closure answers for the questions.

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• acid
• base
• pH indicator
• buffer
• color change
• hydrolysis of a salt
• acid strength

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