Battery Blue

Concept: Oxidation and Reduction, Electrolysis, Acids and Base Indicators

Electrical energy can be used to produce a chemical change.

Materials:

• 3 beakers
• water
• stirring rod
• 1-9V battery
• cloth with Congo red indicator
• spatula spoon or 2 g potassium iodide
• 10 mL of 3 M sulfuric acid
• 5 mL of starch solution
• spatula spoon or 2 g of ascorbic acid
• 30 mL of 6 M NaOH

Directions:

Fill the first beaker with 600 mL of water. Add the solid potassium iodide to the beaker and stir until dissolved, and then add 10 mL 3 M sulfuric acid (have 5 mL of starch handy until ready to be put in). The second d beaker should only contain the solid ascorbic acid. Fill the third beaker with 500 mL of water and add 30 mL of 6 M NaOH.

When all set up, add starch to beaker #1 and act disappointed as if the experiment did not work. Drop in the battery. When the liquid is black/blue color, pour it in beaker #2. Soak the cloth in beaker #2, and then in beaker #3.

Introduction:

My favorite color is blue but all I have are these clear solutions. Let us see if I can pour this first chemical in my beaker and make a blue color. Did you see anything? Me neither. How do you think I can make this work? What do you use to make your toys work at home? Everything needs a battery? Can a battery make a chemical reaction? Well let me try this battery. There, I got my blue solution, but how do I get the battery out without getting my hands wet? (Pour beaker # 1 into beaker # 2)

Well now my blue is gone again. You do not see any blue color?? Well, I think the blue color is still there whether you can see it or not. To prove that it is still there I am going to pull the blue color out of the solution. Do you see this red cloth? How about if I dip it into the colorless solution and....pull the blue color out of solution on the cloth. See this experiment was successful after all.

Now I want to rinse this cloth so that I can preserve the blue color. (Dip the cloth into beaker # 3). Oh, no I lost the blue color again. I guess I will have to give up on the blue color.

Explanation:

The first part of the demonstration is in reality an electrolysis reaction with the battery supplying the electrons for the chemical reactions.
These are the electrode reactions:
Cathode: 2 H₂O + 2 e^- ====> H₂ + OH^-

At the cathode the battery supplies electrons to water to make hydrogen gas seen as tiny bubbles in the reaction.

Anode: 2 I^- ====> I₂ + 2 e^-

At the anode, the iodide ion is converted to iodine element and gives away two electrons.

The iodine element reacts with the starch that is present to produce the blue color.
Iodine, I₂ + starch ===> blue-black color

As the blue starch/iodine solution is dumped into the second beaker which contains ascorbic acid, Vitamin C, another redox reaction occurs to change the iodine element back into colorless iodide ions. Therefore the blue color with the starch disappears. The second series of reactions are not redox but acid/base. The red cloth is dyed with Congo red acid/base indicator. The original solution contains sulfuric acid which changes the Congo red to a blue color. Finally the third beaker contains sodium hydroxide, a base, which changes the Congo red indicator back to red color.

Safety Precautions:

Take care in avoiding the spilling of either the sulfuric acid or the NaOH base on your hands, eyes, face as they can cause burns.

Waste Disposal:

Dispose all liquids down the drain. The battery can be saved for several more uses.

Reference:
Hand out - Road Show, p. 10, 11
A Chemical Demonstration Road Show for High School Students
Otis Rothenburger and James Webb, Department of Chemistry, Illinois State University, Normal, Illinois
about 1988-90