If you bubble sulphur dioxide gas into an aqueous solution of iron(III) ions, a chemical reaction takes place

During this reaction a transient red colour is observed which is also seen when aqueous solutions of sulphur based salts are added to aqueous iron(III) ions

Equipment

Chemicals

20 cm3 of 1 mol dm–3 solutions of each of: 

  • Sodium sulphide (Na2S)
  • Sodium thiocyanate (NaSCN)
  • Sodium sulphate (Na2SO4)
  • Sodium sulphite (Na2SO3
  • Sodium thiosulphate (Na2S2O3)
  • Sodium metabisulphite (Na2S2O5)
  • Sodium dithionite (Na2S2O4)
  • Sodium pyrosulphate (Na2S2O7)
  • Sodium tetrathionate (Na2S2O6
  • Sodium dithionate (Na2S4O6)
  • Sodium persulphate (Na2S2O8)
  • Iron(III) chloride (FeCl3)

Apparatus

  • Test tubes
  • Test tube racks
  • Spatulas
  • Dropping pipettes
  • Aqueous 0.5% (w/v) iron(III) chloride solution
  • Safety glasses

Health, safety and technical notes

  • Read our standard health and safety guidance here.
  • Wear eye protection.
  • The test tube samples can be washed to waste with plenty of running water. Larger quantities of most should be kept for disposal.
  • This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
  • Hazards are given for the solids – where no mention is made of the solution, the classification is the same.
  • Sodium sulphide (Na2S) is corrosive to skin and eyes, harmful if swallowed, toxic in contact with skin, contact with acids liberates toxic gas. Toxic to aquatic organisms.
  • Sodium thiocyanate (NaSCN) is harmful if swallowed, inhaled or in contact with the skin; Contact with acids liberates toxic gas. (1 mol dm–3 solution is of low hazard) 
  • Sodium sulfate (Na2SO4) is of no significant hazard.
  • Sodium sulfite (Na2SO3) is harmful if swallowed, causes serious eye damage, contact with acids liberates toxic gas.
  • Sodium thiosulfate (Na2S2O3) is of no significant hazard.
  • Sodium metabisulfite (Na2S2O5) is harmful if swallowed, causes serious eye damage, contact with acids liberates toxic gas. (1 mol dm–3 solution causes serious eye damage and in contact with acids liberates toxic gas).
  • Sodium dithionite (Na2S2O4) is a self-heating solid – it may catch fire. Harmful if swallowed. (1 mol dm–3 solution is of low hazard).
  • Sodium pyrosulfate (Na2S2O7) is of no significant hazard. 
  • Sodium tetrathionate (Na2S2O6) is a skin, eye and respiratory irritant.
  • Sodium dithionate (Na2S4O6) is of no significant hazard. 
  • Sodium persulfate (Na2S2O8) is a skin/respiratory sensitiser, skin/eye/respiratory irritant, harmful if swallowed. (1 mol dm–3 solution is a skin/respiratory sensitiser and a skin/eye irritant).
  • See CLEAPSS Hazcard HC088 for more information on sodium.
  • Iron(III) chloride (FeCl3) is corrosive to skin/eyes, harmful if swallowed, Hazardous to the aquatic environment. 

Commentary 

Students should be encouraged to draw out the structures of the sulphur-based anions and consider how the anions might interact with the Fe3+ ion.

If 3 cm3 of the sulphur anion solution is mixed with the same volume of iron(III) solution, then the solutions in bold type will give a transient red colour: 

  • Sodium sulfide (Na2S)
  • Sodium thiocyanate (NaSCN)
  • Sodium sulfate (Na2SO4)
  • Sodium sulfite (Na2SO3)
  • Sodium thiosulfate (Na2S2O3)
  • Sodium metabisulfite (Na2S2O5)
  • Sodium dithionite (Na2S2O4)
  • Sodium pyrosulfate (Na2S2O7)
  • Sodium tetrathionate (Na2S2O6)
  • Sodium dithionate (Na2S4O6)
  • Sodium persulfate (Na2S2O8)

Notes

This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.

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