Chemical Forums
Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: eprigge on October 26, 2006, 03:41:04 PM
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I am very confused about a simple reaction I have read about and done. In a very acidic solution (pH<1) I oxidize ferrous sulfate to ferric sulfate using hydrogen peroxide. On addition of more sulfuric acid the Fe+3 ion is reduced back to Fe+2. I just don't understand what is donating the electron to make this happen...
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The H2O2 probably does. H2O2 can act as reductor and as oxidizer. E.g. when you add 30% H2O2 to 30% HCl, then Cl2 is formed (oxidation), but after a while, O2 is formed and Cl2 disappears (reduction). This reaction can even oscillate.
A similar thing is known for oxidation of ferrocyanide and reduction of ferricyanide with H2O2. So, it would not surprise me when it also shows similar behavior with Fe(3+) ion.
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Hmm could be. I actually formed one batch of Fe+3 via electrolysis of ferrous sulfate solution and I think it had the same behavior. Also, I think the reduction reaction with H2O2 is supposed to occur under basic conditions:
2 Fe3+ + H2O2 + 2 OH? ? 2 Fe2+ + 2 H2O + O2
And there was no release of O2. But what do I know. ???
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OK, that is important additional information. No release of O2. The only other thing I then can think of is that the iron (III) ions become totally hydrated. The ion [Fe(H2O)6](3+) is almost colorless. The yellow color, we usually observe, is partly hydrolysed hexaaqua iron (III) ion, something like [Fe(H2O)5(OH)](2+).
Have a look at my webpage, where I explain this. Look at the section on oxidation state +3.
http://woelen.scheikunde.net/science/chem/solutions/fe.html
Iron ions are almost colorless, when fully present as hexaaqua ion. When you add extra acid, then the hydrolysis products will be protonated again, and you get the true hexaaqua ion.
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Very nice page! I would not have guessed that lowly iron was so complex. What I had to start with was maybe 3/4 saturation solution of ferrous sulfate in sulfuric acid originally and both electrolysis and H2O2 gave a deep red amber like what your Fe+4 pic shows, I thought this was Fe+3. Could barely see through a glass thimble full of it.
So it's likely shifting states of hydration I'm seeing instead, rather than reduction of Fe+3, and could even be Fe+4 hydrated states. Would KSCN be a definative test for Fe+3? Thanks for you *delete me*
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Thiocyanate is a sensitive indicator for ferric ions, but only for iron in the +3 oxidation state. Iron at higher oxidation states are reduced to the +3 state, so using excess KSCN certainly will show up any iron in oxidation state +3 or higher, and also in any hydration state.