[Procrastinate]

I did an experiment the other day for Chemistry which involved the decomposition of Hydrogen Peroxide into Oxygen Gas and Hydrogen Gas using Ferric Nitrate (Fe(III)(NO₃)₃). One thing that stumped me is why the Ferric Nitrate changes colour during the reaction from dark orange to brown/black and then back to orange after about ten minutes.

I have been trying to figure this out and research around but no avail. I can't figure out why. It is a catalyst so it cannot partake in the reaction. Maybe it has something to do with pH? That is how other indicators change colours - a response to pH change so maybe that applies to ferric nitrate as well? However, none of the products of the reaction are acids or bases and what is even more confusing is the colour returning to its original form.

Could someone give me any ideas?

[JohnGraham]

It is a catalyst so it cannot partake in the reaction.

If it doesn't partake in the reaction, how can it have any effect?

A catalyst can indeed partake in the reaction, it just ends up chemically the same as it started. In this case, it is turned into something that is brown/black and then back to ferric nitrate.