Patriotic Colors Electrolysis Demo Materials
- 500 mL 1M potassium nitrate, KNO3 (make this)
- 1 mL thymolphthalein indicator solution (make this)
- 2 mL phenolphthalein solution (make this)
- approximately 2 mL 0.1M sodium hydroxide, NaOH (make this)
- approximately 1 mL 0.1M sulfuric acid, H2SO4 (make this)
- 3 250-mL beakers
- 3 8-mm x 200-mm carbon rods
- 25-cm uninsulated 14-ga copper wire
- 10-cm rubber tubing, approximately 5-mm outside diameter
- #6 rubber stopper, 1-hole
- 2 U-tubes, 100-mm, 13-mm outside diameter
- 4 cotton balls
- 3 20-cm glass stirring rods
- adjustable DC power supply that can produc 1 amp at 10 volts (e.g., automotive battery charger)
- clip leads
- Pour 150 mL of 1.0M KNO3 into each of the three beakers.
- Line the beakers up in a row. Place a carbon electrode in each beaker.
- Wrap one end of the copper wire around one the carbon electrodes at the end of the row. Slip rubber tubing over the copper wire to cover the exposed wire that will be between the electrodes. Wrap the other end of the copper wire around the third carbon electrode, at the end of the row of beakers. Skip the center carbon rod and be sure no exposed copper touches it.
- Fill the two U-tubes with 1M KNO3 solution. Plug the ends of each tube with cotton balls. Invert one of the U-tubes and hang it over the rim of the left and center beaker. The arms of the U-tube should be immersed in the liquid. Repeat the procedure with the second U-tube and the center and right beakers. There should not be an air bubble in either U-tube. If there is, remove the tube and re-fill it with KNO3 solution.
- Place a glass stirring rod in each beaker.
- Make certain the power supply is off and then connect the positive (+) terminal to the central carbon electrode and the negative (-) terminal to one of the outer carbon electrodes.
- Add 1 mL of thymolphthalein solution to the beaker on the right and 1 mL of phenolphthalein indicator to each of the other two beakers.
- Add 1 mL of 0.1M NaOH solution to the middle beaker. Stir the contents of each beaker. From left to right, the solutions should be: clear, red, clear.
- These solutions may be stored in sealed containers and may be re-used to repeat the demonstration. If the colors become faint, more indicator solution may be added.
Perform the Demonstration
- Turn on the power supply. Adjust it to 10 volts.
- Wait 15 minutes. Turn off the power supply and stir each solution.
- At this point, the solutions should now appear red, colorless and blue. You may wish to place a white sheet of paper or posterboard behind the beakers to display the colors. Also, this makes the center beaker appear white.
- You can return the solutions to their original colors by reversing the connections to the power supply adjusting it to 10 volts, and allowing 20 minutes before turning off the power and stirring the solutions.
- Another way to return the solutions to their original colors is to add 0.1 M H2SO4 to the beakers on the end until the liquids turn colorless. Add 0.1 M NaOH to the middle beaker until the liquid turns from clear to red.
DisposalWhen the demonstration is complete, the solutions may be rinsed down the drain with water.
How It WorksThe chemical reaction in this demonstration is simple electrolysis of water:
The color change is a result of the pH shift accompanying electrolysis acting on the pH indicators, which were selected to produce the desired colors. The anode is located in the center beaker, where water is oxidized to produce oxygen gas. Hydrogen ions are produced, decreasing the pH.
2 H2O(l) → O2(g) + 4 H+(aq) + 4 e-
Cathodes are located on either side of the anode. In these beakers, water is reduced to form hydrogen gas:
4 H2O(l) + 4 e- → 2 H2(g) + 4 OH-(aq)
The reaction produces hydroxide ions, which increase the pH.
Other Patriotic Chem DemosRed, White and Blue Density Column
Colored Fireworks Demonstration
Fireworks in a Glass - Safe Demo for Kids
ReferencesB. Z. Shakhashiri, 1992, Chemical Demonstrations: A Handbook for Teachers of Chemistry, vol. 4, pp. 170-173.
R. C. Weast, Ed., CRC Handbook of Chemistry and Physics, 66th ed., p. D-148, CRC Press: Boca Raton, FL (1985).