U-Tube Demonstration

The electrolysis of an aqueous KI solution

Tim M. Su and James Armstrong

Department of Chemistry, City College of San Francisco


Materials Needed:
Glass U-tube (15-20 cm in length), C-clamp and ring stand, variable power supply, 0.1 M KI solution, phenolphthalein indicator, Nichrome wires (x2), wire leads with alligator clips.

Fill a glass U-tube with 0.1 M KI solution to approximately 1 cm from the top. Add 2 drops of phenolphthalein to each side of the tube, and stir briefly. Next, place a nichrome wire (an alloy of nickel and chromium which is very resistant to being oxidized) electrode into each side of the solution and clip them to the power source with wire leads. "Do not allow the clips to get wet!"

Turn on the power source and increase the voltage to approximately 3 volts. You should see a gas forming at one of the electrodes; if not, slowly increase the voltage until you do. "You have just created an electrolytic cell!" Observe the electrolysis reaction for a minute or two, carefully recording ALL your observations (changes in color, which side is connected to the positive terminal of the power source, etc.).

"How do you determine on which sides of the U-tube oxidation and reduction occur?"

The power source is controlling the flow of electrons. Electrons flow from the negative side of the power source into the U-tube, then from the other side of the U-tube into the positive side of the power source. "Can you figure it out now?"

The overall cell reaction has a negative potential. If you reverse this reaction, you will obtain a a reaction which has a positive potential, and therefore should be spontaneous.

To observe this reverse reaction, quickly disconnect the power supply from the wire electrodes, and connect the wire electrodes to a voltmeter. The positive terminal of the voltmeter should be connected to the side of the U-tube that was connected to the positive terminal of the power source. Observe the voltage reading. "Is it constant? If not, can you explain why?" Write the overall cell reaction for this cell; note that this is now a galvanic cell.

Disconnect the voltmeter, and reconnect the power source to the U-tube as before. Increase the voltage to its maximum setting. If the wire electrode is reasonably clean, you may see gases coming from both electrodes. "What is the new gas (the one that you did not observe earlier)?" Write a balanced half-reaction for its formation.