Red Cabbage Pigment pH Indicator (Sarah Ishida)

Principles Investigated:

· Electrophoresis to split water molecules into H⁺ and OH^- ions

· Using natural pigments in red cabbage as a pH indicator

· Acid and base equilibrium

Chemistry Standards:

3a: Students know how to describe chemical reactions by writing balanced equations.

5a: Students know the observable properties of acids, bases, and salt solutions.

5b: Students know acids are hydrogen-ion-donating and bases are hydrogen-ion-

accepting substances.

5d. Students know how to use the pH scale to characterize acid and base solutions.

5g: Students know buffers stabilize pH in acid–base reactions.

Materials:

· Red cabbage (one handful, chopped)

· Blender

· Water (400ml)

· Salt (3 tablespoons)

· Gel box

· +/- Charge power supply

· Cheese Cloth

· Beaker

· Test tubes (4)

· Eye droppers (4)

· Lemon (1)

· Ammonia 10ml

· Buffer- Tris Acetate EDTA (optional)

Procedure:

1. Cut a handful of red cabbage into small pieces.

2. Mix cabbage in blender with 400 ml of water and 3 tablespoons of salt.

3. Strain liquid into beaker with a cheese cloth.

4. Pour liquid into gel box and turn on the power supply.

5. After the solution on the negative side is green and positive side is pink, take two samples from each end and put into four test tubes.

6. Add lemon juice to one acid sample and one alkaline sample to observe color change.

7. Add ammonia to one acid sample and one alkaline sample to observe color change.

Student Prior Knowledge:

· Students must understand the differences between an acid and a base.

· Students must understand that ammonia is a base, lemon juice is an acid.

· Students must understand what happens to solutions when they go through electrophoresis.

Explanation:

The anthocyanin pigments in red cabbage react differently to various wavelengths of light depending on the amount of H⁺ or OH^- ions present, therefore allowing us to test whether a solution is an acid or a base.

The chemical reaction that was occurring as electricity was running through the gel box is explained in #1 of the Questions below. The gel box was responsible for breaking down water molecules into H⁺ and OH^- ions, and creating a gradient of acidic to alkaline solutions, but ultimately yielded Oxygen and Hydrogen gas which was released on either end of the gel box as bubbles.

The H⁺ and OH^- ions recombined to form water and neutralized the pH (therefore becoming purple again) when the liquid was mixed together evenly.

Questions:

1. Write the chemical equation for the occurring reaction.

A: ½ reaction occurring on alkaline side. (Reduction)

4H₂O + 4e^- --> 4OH^- + 2H₂

½ reaction occurring on acidic side. (Oxidation)

2H₂O --> O₂ + 4e^- + 4H⁺

Combined:

4H₂O + 4e^- + 2H₂O --> O₂ + 4e^- + 4H⁺ + 4OH^- + 2H₂

2H₂O --> O₂ + 2H₂

2. Why were there more hydrogen bubbles than oxygen bubbles produced?

A: Both Hydrogen gas and Oxygen gas are composed of two atoms: H₂ and O₂. Two molecules of water (H₂O) can yield two Hydrogen gas molecules but only one Oxygen gas molecule. Therefore, as electrophoresis occurs, double the Hydrogen gas is produced and you will see double the bubbles.

3. What would happen if the voltage was increased or decreased during electrophoresis?

A: The time it would take for the reduction and oxidation to occur in the solution would take longer at a lower voltage and take less time at a higher voltage. (But if the voltage is too high, it will burn out the fuse, so be careful).

Applications to Everyday Life:

· Students may do a simple version of this experiment at home, by putting the cabbage juice into many cups and pouring in samples of household foods/chemicals to observe the color changes.

· As they test different samples of acids and bases, students will learn which bases may be used to neutralize acids and vice versa. For example, if someone spills car battery acid on themselves and/or their clothes, they may neutralize it by rinsing it off with Lysol.

· When the dentist suggests that a person stops consuming acidic foods and drinks such as soda and many meats because it is corroding their teeth, they will understand that they need to incorporate alkaline foods into their diet such as brown rice and many vegetables.

References for Ideas:

Mr. John McLaughlin- Canoga Park High School

http://chemistry.about.com/od/acidsbase1/a/red-cabbage-ph-indicator.htm

http://www.ncmls.org/visit/campus-and-exhibits/exhibits/investigate-health/lab/cabbage