By using potassium iodide, iodate, and manganate(IV), plus a full-range indicator students discover the mysteries of sulfur dioxide

In this experiment, students will discover the reactions of sulfur dioxide. By using potassium manganate(IV) to create SO2, learners will explore the reactions that create this gas.

This experiment should take 20 minutes.



  • Eye protection
  • Student worksheet
  • Clear plastic sheet (eg OHP sheet)
  • Plastic Petri dish (base + lid), 9 cm
  • Plastic pipette
  • Scissors


Solutions should be contained in plastic pipettes. See the accompanying guidance on apparatus and techniques for microscale chemistry, which includes instructions for preparing a variety of solutions.

  • Hydrochloric acid, 1 mol m–3
  • Potassium iodide, 0.2 mol dm–3
  • Potassium iodate(V), 0.1 mol dm–3
  • Potassium manganate(VII), 0.01 mol dm–3
  • Full-range indicator solution diluted 1:1 with deionised water
  • Sulfuric acid, 1 mol dm–3
  • Sodium sulfite powder

Health, safety and technical notes 

  • Read our standard health and safety guidance.
  • Students must wear eye protection.
  • Sulfur dioxide is toxic and is a particular problem for asthmatics. Only use a very small amount of sulfite and acid to keep the sulfur dioxide production to a minimum. A risk-assessment should include any individual sensitivities (see CLEAPSS Hazcard HC097).
  • Hydrochloric acid 1 mol m–3, potassium iodide 0.2 mol dm–3, potassium iodate(V) 0.1 mol dm–3 and potassium manganate(VII) 0.01 mol dm–3 are all of low hazard (see CLEAPSS Hazcards HC047a, HC047b, HC080, HC081).
  • Sulfuric acid 1 mol dm–3 is a skin/eye irritant (see CLEAPSS Hazcard HC098a).
  • Sodium sulfite powder is a skin, eye and respiratory irritant. Depending on its formulation, full range indicator can still be flammable when diluted 1:1 with water. Keep away from sources of ignition (see CLEAPSS Hazcard HC092).


  1. Cover the worksheet with a clear plastic sheet.
  2. Place the base of the Petri dish directly over the circle provided. Place the reaction vessel in the centre.
  3. Place two drops of full-range indicator solution where shown.
  4. At another corner of the triangle, place two drops of ammonia solution. Place the lid on the Petri dish and wait for the indicator drop to change colour.
  5. Remove the lid from the Petri dish and, using a piece of tissue, mop up the drop of ammonia.
  6. At the two remaining corners of the triangle, add the two other test solutions.
  7. Add a small quantity of sodium sulfite powder to the reaction vessel, followed by three drops of hydrochloric acid. Quickly replace the lid.
  8. Record all your observations over the next 15 minutes.


Sodium sulfite + hydrochloric acid generates sulfur dioxide:

Na2SO3(s) + 2HCl(aq) → 2NaCl(s) + SO2(g) + H2O(l)


The iodide/iodate mixture turns black due to liberation of iodine:

IO3(aq) + 5I(aq) + 6H+(aq) → 3I2(g) + 3H2O(l)

If sufficient sulfur dioxide is produced and the solution contains excess acid, the potassium manganate(VII) solution is decolorised:

8H+(aq)+ 5e + MnO4(aq) → Mn2+(aq) + 4H2O(l)

However, with less sulfur dioxide and therefore less acid, the brown manganese(IV) oxide is formed:

4H+(aq) + MnO4(aq) + 3e → MnO2(s) + 2H2O(l)

Full-range indicator turns from green to yellow, indicating that sulfur dioxide is an acidic gas.


  1. What explanations can you give for your observations?