This experiment will allow students to find out some interesting chemical reactions of sodium thiosulphate, record, observe, and understand this compound

Students will induce reactions between sodium thiosulfate and other chemicals. This practical takes place in three parts, with each part showing learners a new side of this complex substance.

This experiment should take 20 minutes.



  • Eye protection
  • Student worksheet
  • Clear plastic sheet (eg OHP sheet)


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.

  • Sodium thiosulphate, 0.1 mol dm–3
  • Silver nitrate, 0.1 mol dm–3
  • Sodium chloride, 0.1 mol dm–3
  • Potassium bromide, 0.2 mol dm–3
  • Potassium iodide, 0.2 mol dm–3
  • Iron(III) nitrate, 0.1 mol dm–3
  • Copper(II) sulfate, 0.2 mol dm–3
  • Iodine solution, 0.05 mol dm–3 in 0.2 mol dm–3 KI

Health, safety and technical notes

  • Read our standard health and safety guidance.
  • Wear eye protection for part B and splash resistant goggles to BS EN166 3 for part C.
  • Silver nitrate, AgNO(aq), 0.1 mol dm–3 is an eye irritant (see CLEAPSS Hazcard HC087). Keep separate from organic waste containers.
  • Copper(II) sulfate 0.2 mol dm–3 causes eye damage and is toxic to aquatic life (see CLEAPSS Hazcard HC027c).
  • Iron(III) nitrate, Fe(NO3)3.9H2O(aq), 0.1 mol dm–3 (see CLEAPSS Hazcard HC055c), potassium bromide, KBr(aq), 0.2 mol dm–3, and potassium iodide, KI(aq), 0.2 mol dm–3 (see CLEAPSS Hazcard HC047b), are low hazard. As is Iodine solution 0.05 mol dm–3, but this is also toxic to aquatic life (see CLEAPSS Hazard HC054).
  • Sodium thiosulphate, 0.1 mol dm–3 is low hazard (see CLEAPSS RB087 for preparation and Hazcard HC95a).


Part A — The reaction between thiosulfate ions and iodine solution:

  1. Cover the worksheets with a clear plastic sheet.
  2. Put one drop of iodine solution in the box provided on the worksheet.
  3. Add two drops of thiosulfate solution.
  4. Observe, comment and write an equation for the reaction.

Question A

What type of reaction are you observing?

Part B — The reaction between thiosulfate and silver halide:

  1. To form the silver halides, first put one drop of silver nitrate solution into each of the empty boxes provided on the worksheet, then add one drop of potassium bromide solution and potassium iodide solutions into the appropriate boxes. Observe and comment.
  2. Add three drops of sodium thiosulfate solution to each box and stir with the end of a pipette.
  3. Observe and comment.

Question B

What explanations can you give for your observations?

Part C — The reaction between thiosulfate ions and transition metal ions:

  1. Put two drops of iron(III) solution in the first box provided on the worksheet.
  2. Put two drops of iron(III) solution and one drop of copper(II) solution in the second box provided.
  3. Put two drops of copper(II) solution in the third box provided.
  4. Add one drop of thiosulfate solution to each box and observe carefully, especially the second box.

Question C

What explanations can you give for your observations?


Part A

The brown colour of iodine is discharged as it is reduced by thiosulfate ions:

I2(aq) + S2O32–(aq) → 2I(aq) + S4O62–(aq)

Part B

The addition of halide ions to the silver nitrate solution produces precipitates of the silver halides – pale yellow (silver bromide) and deeper yellow (silver iodide). Silver bromide dissolves readily in sodium thiosulfate solution, whereas silver iodide is less soluble. This could be used as a test to distinguish a bromide from an iodide.


 Ag+(aq)  +  X-(aq) AgX(s)
 Silver    Halide   Silver
 Ion    Ion   Halide

The dissolution of silver bromide in thiosulfate solution is used in the fixing stage in photographic developing. Here, thiosulfate is used to dissolve unreacted silver bromide through the formation of soluble complexes such as Ag(S2O3)23–(aq).

Part C

The reaction of iron(III) with thiosulfate produces a deep violet complex anion, Fe(S2O3)2. This decomposes slowly with the fading of the violet colour:

Fe(S2O3)2(aq) + Fe3+(aq) → 2Fe2+(aq) + S4O62–(aq)

The presence of copper(II) ions catalyses the decomposition reaction, and the violet colour fades more rapidly.

Thiosulfate reduces Cu(II) to Cu(I) and complexes the Cu(I):

2S2O32– + 2Cu2+(aq) → 2Cu+(aq) + S4O62–(aq)

2Cu+(aq) + 2S2O32– → Cu2(S2O3)22–(aq)

The characteristic blue colour of copper(II) fades, leaving a colourless solution containing the complex ion Cu2(S2O3)22–(aq).