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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!)


Source
 home.ku.edu.tr/~ldemirel/exp3.doc