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National Chemistry Week - Experiments - Electrolysis of KI

(Please note that phenolphthalein in ex-lax has been banned in the US and Canada due to suspected carcinogens.  It was available at the time this experiment was published.)

Objective:

To electrolyze a potassium iodide solution. Electrolysis is a process by which a chemical reaction is carried out by means of the passage)of an electric current. As the reaction proceeds iodide is oxidized at the anode (negative) while water is reduced at the cathode (positive).
2I--->(aq) I2(s) + 2e-
2H2O(l) + 2e- --> 2OH-(aq) + H2(g)
net reaction: 2I-(aq) + 2H2O(l) -->I2(s) + 2OH-(aq) + H2(g)
The cathode will be recognized by bubbles (H2) on the electrode and pink phenolphthalein indicator from hydroxide production. Phenolphthalein turns pink in the presence of base and clear in the presence of acid. A slight yellow/brown colour to the water around the anode may be observed. This is due to a small amount of iodine being dissolved in the Water.

Materials:

  • phenolphthalein indicator
  • alligator clips (4)
  • 9V battery
  • insulated wire
  • solid potassium iodide
  • spoon
  • carbon electrodes (two pencils sharpened at both ends)
  • shallow bowl (clear glass or white plastic)
  • white paper

Procedure:

  1. Add some KI (about the size of your baby fingernail) to the bowl, fill it half full with water and stir until the KI is dissolved. Put a white piece of paper under the bowl if it is a glass bowl.
  2. Add 5 drops of phenolphthalein indicator (instructions below).
  3. Connect the wire to the alligator clips. Clip one alligator clip to the tip of the pencils.
  4. Attach the other ends of the wires to the battery.
  5. Place the electrodes in the bowl so that the pencil tip is in the solution.
  6. Observe the electrodes for evidence of reaction. A few drops of the phenolphthalein indicator near the cathode will increase the pink colour if the solution is too dilute.

Phenolphthalein Indicator

Materials:

  • Ex-Lax tablets (2)
  • glass jar
  • 2-propanol (rubbing alcohol)
  • measuring spoons
  • hammer or heavy blunt object (heavy book)

Procedure:

  1. Wrap the tablets in some wax paper and crush them.
  2. Add 30 mL 2-propanol to the jar.
  3. Add the powdered tablets to the jar and stir.
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