Homogeneous Catalysts

The reaction between peroxodisulphate(VI) ions and iodide ions

Peroxodisulphate (VI) ions and iodide ions react together in aqueous solution to form sulphate(VI) ions and iodine

S2O82-(aq) + 2I-(aq)

2SO42-(aq) + I2(aq)

Transition metal catalysts

This reaction is catalysed by a number of transition metal ions including Fe2+ and Fe3+.

If a small amount of Fe2+ ions are added to the reaction mixture, they will reduce the peroxodisulphate (VI) ions to sulphate (VI) ions and will be oxidised to Fe3+ ions in the process.

2Fe2+(aq) + S2O82-(aq)

2Fe3+(aq) + 2SO42-(aq)

The Fe3+ ions produced will oxidise iodide ions to iodine and will be reduced back to Fe2+ ions in the process.

2Fe3+(aq) + 2I-(aq)

2Fe2+(aq) + I2(aq)

The Fe2+ ions produced can reduce more peroxodisulphate ions and so the catalysed reaction can continue.