Writing with Electricity

Introduction

Electricity is the flow of electrons. Following the direction of electron flow during an electrochemical reaction can be confusing for students. In this demonstration electrolysis will be used to write messages with either iodine or phenolphthalein, illustrating the reactions occurring at the anode and cathode when an aqueous solution of potassium iodide is electrolyzed.

Concepts

• Electrochemistry
• Acid–base indicators
• Anodic and cathodic reactions
• Consumer chemistry

Materials

Safety Precautions

Phenolphthalein indicator solution contains ethyl alcohol which is a flammable liquid, moderately toxic, and a fire risk; keep away from heat and open flame. Observe precautions when working with live electrical equipment and when completing the electric circuit. Avoid contact of all chemicals with skin and eyes. Follow all laboratory safety guidelines. Wear chemical splash goggles and chemical-resistant gloves. Remember to wash hands thoroughly with soap and water before leaving the laboratory. 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. The used paper can be thrown away in the trash, Flinn Suggested Disposal Method #26a. Leftover potassium iodide and starch solutions can be disposed of down the drain with excess water according to Flinn Suggested Disposal Method #26b. Phenolphthalein can be stored for further use or treated according by Flinn Suggested Disposal Method #18a.

Prelab Preparation

Part A. Potassium Iodide Solution

1. Dissolve 1.6 g of potassium iodide into 20 mL of distilled or deionized water. Prepare the solution fresh within 1–2 days of use.

Part B. Starch Solution

2. Prepare a fresh starch solution. To make 50 mL of a 1% starch solution:
   1. Place 50 mL of distilled or deionized water in a 150-mL beaker and bring to boiling on a hot plate.
   2. Make a smooth paste with 0.5 g of soluble starch and a small volume (several milliliters or so) of distilled or deionized water.
   3. Once the water is boiling, carefully remove the beaker containing the boiling water from the hot plate. Pour the starch paste into the boiling water and stir until all of the starch is dissolved. Note: The resulting solution may be somewhat cloudy.
   4. Allow the starch solution to cool to room temperature before use.
   5. Use the starch solution within 1–2 days for best results.

Part C. Writing Surface

3. Obtain the 8½" x 11" white paper and cut the paper to the same size as the 20 cm dia. filter paper.

Part D. Writing Instrument

4. Set up the copper writing instrument by pinching the top of the pipet bulb and cutting a very small hole in the top using a pin.
   1. Feed the copper wire from the top of the pipet down through the tip until there is enough copper exposed to write with (approximately 0.5 cm).
   2. Fold the copper wire tightly around the pipet bulb once or twice to make it secure (see Figure 1).
   3. Leave a longer exposed section of copper wire (approx. 20 cm) available for attaching an alligator clip.
   4. Caution: The demonstrator should hold onto the pipet and not directly onto the copper wire when writing.

Procedure

1. Obtain the working solution of potassium iodide made during Prelab Preparation step 1.
2. Add 5 mL of the 1% starch solution and 5 mL of the 1% phenolphthalein solution. Mix well. If the solution turns dark, it can be decolorized with a few drops of dilute sodium thiosulfate solution. Note: The solution will work fine even if it has a “dirty dishwater” color.
3. Center one piece of 20 cm diameter filter paper inside the aluminum pan.
4. Place the cut circle of white paper on top of the filter paper.
5. Place another piece of filter paper on top of the copy paper. The end result is a “sandwich” consisting of three layers, (filter paper–white paper–filter paper).
6. Add approximately 15 mL of the combined KI/starch/phenolphthalein solution to thoroughly wet the sandwich of papers inside the aluminum pan without creating excess solution in the pan. Add a few mL to each layer—starting with the layer closest to the aluminum pan works best.
7. Assemble the battery connection by placing the battery end cap with alligator clip leads on the battery.
8. Connect one alligator clip lead from the battery to the edge of the aluminum foil pan containing the saturated papers (see Figure 2).
   {12786_Procedure_Figure_2_Pan and battery assembly}
9. Connect the other alligator clip lead to the copper wire which runs out from the pipet bulb (see Figure 2).
10. Grasping the pipet insulator of the copper wire/pipet “pen” assembly, carefully write a message on the sandwich of papers with the copper wire pen. The writing will either be brown/black or pink in color, depending on which battery terminal is attached to the copper wire.
11. Disconnect and reverse the leads: Attach the lead previously connected to the foil pan to the copper and the lead previously attached to the copper to the foil pan.
12. Repeat step 10. The new writing will appear in the opposite color than before, either brown/black or pink.
13. With a gloved hand pick up the first piece of filter paper and the copy paper to let the students observe that the writing also appears on the first piece of filter paper touching the aluminum pan, but the colors are reversed from those on the top layer.

Student Worksheet PDF

12786_Student1.pdf

Teacher Tips

• This kit contains enough chemicals and consumable materials to perform the demonstration as written 7 times: 11.6 g of KI, 1 g of starch, 35 mL of 1% phenolphthalein, aluminum pan, 14 pieces of 20-cm filter paper, 9V battery, battery clip with alligator leads, copper wire and 7 pipets.
• Starch solutions have a poor shelf life and will deteriorate quickly. Therefore, a fresh starch solution should be prepared within 48 hours of the lab. The mixed KI/starch/phenolphthalein solution may turn dark if the KI solution is not fresh due to any oxidation of iodide to iodine. Adding sodium thiosulfate (see Procedure, step 2) will reduce any iodine impurity and decolorize the solution.
• Starch solutions are often used as indicators for detecting the presence of iodine. Generally, a 1% starch solution will produce a nice, deep-blue color in the presence of iodine. The more concentrated the starch solution, the deeper blue in color is the resulting solution. If the starch solution is too dilute, a color change will still be observed in the presence of iodine; however, the color produced is more of a brown color. If this brown color is observed and a darker color is desired, simply prepare a fresh 1% starch solution.
• A slurry of the starch solution can be used, however the color will be more brown than the traditional blue-black. For a slurry add 0.1 g of starch directly to the potassium iodide solution from Preparation Step 1 in lieu of the 5 mL of the 1% starch solution. This will give a brown color.
• The brown/blue color of the writing from the starch indicator will remain for quite some time. The pink writing of the phenolphthalein however will fade after a few minutes. This is due to the absorption of carbon dioxide present in air.
• If desired, dissolve a tablet of vitamin C in a small amount of water. Draw some of the vitamin C solution into a pipet and dispense this solution onto the drawing. This solution will “erase” all the lettering as well as perhaps clear up any discoloration in the paper made by the original solution. Writing on the areas where the Vitamin C has been applied will result in the color quickly fading.
• When writing with electricity, do not let the bare copper wire rest on the aluminum pie plate. This would cause a short circuit situation to occur.

Answers to Questions

1. The battery acts as an “electron pump” in this demonstration. What do the positive and negative signs on the battery suggest about the flow of electrons? 

   Electrons flow from the negative battery terminal to a conducting metal touching an aqueous solution containing ions. Electrons flow from a second conducting metal surface to the positive terminal on the battery.

2. What color writing was produced when the copper “pen” was attached to the positive end of the battery? Write a chemical equation for the oxidation reaction responsible for this color.

   The writing appeared blue-black due to a starch–iodine complex. Iodide ions were oxidized to iodine, which reacted with the starch.

   2I^–(aq) → I₂(aq) + 2e^–

   {12786_Answers_Equation_5}
3. What color writing was observed when the copper wire was attached to the negative end of the battery? Write a chemical equation for the reduction reaction responsible for this color.

   The writing appeared pink due to the indicator color change of phenolphthalein in the presence of a base. Water molecules were reduced to produce hydrogen gas and hydroxide ions.

   2H₂O(l) + 2e^– → 2OH^–(aq) + 2H₂(g)

4. Why does the pink color fade in air after just a short time? Hint: Think of what the pink color means and what else could be around the demonstration that may influence the results.

   There is a reaction with the surrounding air. Phenolphthalein has turned pink due to the hydroxide ions. Carbon dioxide from the air forms carbonic acid, lowering the pH to the point where phenolphthalein becomes colorless.

Discussion

This demonstration shows the electrolysis of potassium iodide solutions using standard acid–base and redox indicators in such a way that students can easily follow the flow of electrons during the reaction. In any electrolysis reaction, when an electric current is passed through a solution containing an electrolyte, oxidation occurs at the positive electrode, called the anode, and reduction occurs at the negative electrode, called the cathode. When the copper pen is the anode during the electrolysis, iodide ions in the electrolyte solution are oxidized, resulting in the formation of molecular iodine (Equation 1).

The iodine reacts with the starch indicator in the solution to produce dark blue or black writing due to the formation of a starch–iodine complex. The blue-black writing appears when the copper wire is attached to the positive terminal on the battery.

When the copper pen is the cathode of the electrolysis, water in the solution will be reduced, forming hydrogen gas and hydroxide anions (Equation 2). The hydroxide anions react with the phenolphthalein indicator in the solution to produce the pink lettering. The pink writing appears when the copper wire is attached to the negative terminal on the battery. Remember that phenolphthalein is pink in the basic region and colorless in the acidic region. This pink lettering will fade over time as the paper reacts with the carbon dioxide from the surrounding air. The carbon dioxide from the air dissolves in water to form carbonic acid (Equation 3). The aqueous carbonic acid forms hydronium ions and biocarbonate ions. If water with a pH of 7 is left open to the surrounding air, including carbon dioxide, in time the pH will drop as the solution becomes more acidic due to the reaction of carbonic acid (formerly CO₂) with the water. These same reactions occur as the filter paper is exposed to the surrounding air. The pink writing formed by phenolphthalein and hydroxide ions becomes more acidic changing the phenolphthalein from pink back to colorless. If desired this process can be slowed by limiting the air source, for example putting the system into a resealable plastic bag.

Since the aluminum foil pan always acts as the other electrode in this electrolysis setup, any writing done on the top layer will be “copied” onto the filter paper layer closest to the aluminum. The writing will appear in the opposite color than that on the top layer as the opposite reaction occurs at the aluminum electrode.

The use of Vitamin C, or ascorbic acid, to decolorize the paper after the reaction illustrates that Vitamin C is both an acid and a reducing agent. Vitamin C reduces iodine back to iodide ions and so the blue-black color disappears. Observations of its properties can immediately lead to discussions on the role of Vitamin C in the body.

References

Special thanks to Paul D. Price, Trinity Valley School, Fort Worth, TX, for bringing this demonstration to the attention of Flinn Scientific, Inc.

Harris, D. C. Quantitative Chemical Analysis, 6th Ed.; W. H. Freeman and Co.: New York, 2003.