If you think the Oxygen absorber is Iron Oxide you then basically have 3 situations to test...it could be either Iron (III) Oxide - Fe2O3, Iron (II) Oxide - FeO, or Iron (II, III) Oxide - Fe3O4.
Iron (II) Oxide and Iron (II, III) Oxide is back while Fe2O3 is reddish-brown. Fe3O4 has both Fe+2 and Fe+3 ions in the compound while the others are either only contain Fe+2 or only Fe+3 cations.
What you can do is dissolve the Iron Oxide in an acid, preferably Sulfuric acid, to place the Iron ions in solution. What you can then do is try to oxidize all the ions to Fe+3 using the Permanganate ion, MnO4-, from Potassium Permanganate. KMnO4 is a powerful oxidizer and is also especially useful in a situation like this since it has an easily detectable color (purple) in solution, so it can be used as its own indicator to tell when the reaction is complete.
You can titrate your Iron ion solution with a KMnO4 solution. Measure how much KMnO4 is required to oxidize all the Iron ions to Fe+3...the end point should come as the solution turns from clear to purple.
If your solution contained entirely Fe+3 ions, it shouldn’t take any KMnO4 to oxidize it.
If your solution contained entirely Fe+2 ions or mixture of Fe+3 and Fe+2 (in a 2:1 ratio), you should be able to calculate out the amount of KMnO4 required to oxidize it all to Fe+3.
Compare your actual measurements with the theoretical values for the three types of Iron Oxides to determine which type you originally had.
