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I need to make several Iron compounds but I am having a bit of trouble finding a decent way to determine the oxidation state of Iron. I'm starting with a Iron(II) compound that I try to oxidize using several methods. I can use raman to determine the compound but that requires me to create the compound I want to make and check that spectrum against my experimental sample. This is quite a bit of work and I was hoping someone could point me towards a better solution. I can make an EDS but since my compounds usually contain carbon or oxygen as counter ion so EDS isn't very accurate. I don't (currently) have access to an XPS and I don't have unlimited access to a mass spectrometer (I have to travel quite a bit to get to one).

Can someone help me find a better way to determine the oxidation state??

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  • $\begingroup$ I worked on a project years ago where we needed determine iron(II) and iron(III) in a lot of samples in a hurry. It was a UV-VIS absorbance technique. I think it was a form of phenanthroline (ortho?) that complexed with the iron(II) to give the colored species. There was then a reagent added to reduce iron(III) to iron(II), and one measured that and calculated iron(III) by difference. At least I've maybe given you some search terms. I'll try to find how we did that but no promises. Again, it was fast, cheap and easy, so long as you had a UV-VIS spectrometer...do you have one? $\endgroup$ – airhuff Jan 23 '17 at 23:24
  • $\begingroup$ I might be able to get one but phenanthroline is known to change color depending on the metal and its oxidation state. I'll try to use a solid state uv-vis and see if I can see any changes between an iron(II) and iron(III) complex but I am working with nanoparticles so I'm not very hopefull. $\endgroup$ – Arno van der Weijden Jan 25 '17 at 8:51
  • $\begingroup$ Yes, I believe what you said is part of the reason we were able to differentiate oxidation states; the iron(II) absorption max was distinct from that of iron(III). For one project in particular this distinction in the final analysis was absolutely critical. Since you are interested I'll try to get the method reference. I'm pretty sure it was in Standard Methods (for the Examination of Water and Wastewater). $\endgroup$ – airhuff Jan 25 '17 at 9:03
  • $\begingroup$ I still haven't made it into the lab to get a copy of our method, but I thought these links might be interesting regarding the theory and technique behind the VIS absorption with phenanthroline methods for iron determination. The biggest problems discussed are regarding total iron determination when some digestion is required to get the solid into solution, which doesn't seem to be applicable to your case. I don't have good (e.g. free) access to the full articles. 1) ncbi.nlm.nih.gov/pubmed/22349079 2) jstor.org/stable/23349093?seq=1#fndtn-page_scan_tab_contents $\endgroup$ – airhuff Jan 26 '17 at 21:10
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Here is a link to the full Standard Methods for the Examination of Water and Wastewater method 3500-Fe for determining ferrous, ferric and total iron in aqueous solutions.

Here is an excerpt describing the principle of the procedure:

"Principle:
Iron is brought into solution, reduced to the ferrous state by boiling with acid and hydroxylamine, and treated with 1,10-phenanthroline at pH 3.2 to 3.3. Three molecules of phenanthroline chelate each atom of ferrous iron to form an orange-red complex. The colored solution obeys Beer’s law; its intensity is independent of pH from 3 to 9. A pH between 2.9 and 3.5 insures rapid color development in the presence of an excess of phenanthroline. Color standards are stable for at least 6 months."

In this method, ferrous iron is measured directly and ferric is determined by difference between the total iron (measurement also described by method) and the ferrous iron. Obviously, when the ferrous iron concentration approaches the total the uncertainty in the the ferric iron concentration becomes large.

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In all the (physical) experimental literature I've read, at least for biomolecules (usually enzymes), the technique of choice is Mossbauer spectroscopy. Here is a good starting point.

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    $\begingroup$ One downside is the requirement that the sample be in the solid phase. $\endgroup$ – airhuff Jan 29 '17 at 2:19

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