3.2. Homogeneous catalysis
Thiosulfate ion [S2O3]2- is oxidised to tetrathionate ion [S4O6]2- by Fe3+:
2Fe3+ + 2[S2O3]2- <----> 2Fe2+ + [S4O6]2-
Fill two test tubes with 4 ml 0.1 M Fe(NO3)3 solution and add to one of them 1 drop of 0.1 M Cu2+ solution. Finally, add 5 ml of 0.1 M Na2S2O3 solution to both test tubes and compare the time required for the deep color of the solutions to disappear.
(Note: Fe3+ forms with thiosulfate ions a deep brown colored complex!)