To Observe the Electrolysis of Water (aqueous sodium sulfate) and a Potassium Iodide Solution
To Write the Half-Reactions occurring at the Anode and Cathode for the above Reactions as well
as the Net reactions
Water containing sodium sulfate, potassium iodide solution, an indicator such as bromothymol
blue, universal indicator, or phenolphthalein, Petri dishes, electrolysis apparatus, carbon
electrodes (pencils), alligator clips, index card, scissors, and a 9 volt battery
1. Cut an index card so that it measures the diameter of the Petri dish. Place the cut index
card inside the Petri dish so that it divides the dish into two equal halves.
2. Fill a Petri dish with an aqueous solution of sodium sulfate.
3. To expose the carbon, sharpen two pencils at both ends. Connect the alligator clips to one
exposed carbon end of each pencil. Attach the other two ends of the alligator clips to the
terminals of the 9 volt battery. Place the pencils in the electrolysis apparatus and tighten
4. Arrange the two electrodes so that only the carbon is in contact with the solution in the
5. Add 1 drop of the chosen indicator to the solution above each electrode.
6. Observe the two electrodes for evidence of a reaction.
7. Write the anode and cathode half-reactions and finally the net reaction.
8. Repeat the above procedure using potassium iodide solution.
These experiments show the effect of passing an electric current through aqueous solutions of
sodium sulfate and potassium iodide. With the former experiment, bubbles form at both
electrodes. Color changes of an indicator at show that the solution at one electrode becomes
basic while the other electrode becomes acidic. When a direct current is passed through an
aqueous sodium sulfate solution, water is oxidized at the anode, and is reduced at the cathode.
The number of moles of hydrogen gas generated is twice that of oxygen. In addition, hydrogen
ions are formed at the anode and hydroxide ions at the cathode.
Passing a direct current through an aqueous solution of potassium iodide, iodide ions are
oxidized at the anode to molecular iodine. The iodine turns the colorless potassium iodide
solution brown. At the other electrode (the cathode), water is reduced to hydrogen gas and
hydroxide ions are produced which raises the pH of the solution. This increase in pH changes the
color of the indicator.
Half-Reactions and Net Reactions
Aqueous Sodium sulfate: Aqueous Potassium iodide:
Anode Reaction Anode Reaction
Cathode Reaction Cathode Reaction
Net Reaction Net Reaction