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My lab has been using solution of FeCl2 for an assay that was mixed about three years ago. Recently the assay has failed twice. I was wondering if it is possible that the iron ions in the solutions have changed oxidation over time?

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    $\begingroup$ Yes, that's quite probable. $\endgroup$ – Ivan Neretin Sep 5 '17 at 15:17
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    $\begingroup$ @IvanNeretin any guesses on the self-life of 1 liter of 1M FeCl2? $\endgroup$ – James Draper Sep 5 '17 at 15:19
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    $\begingroup$ Depends on how you store it, etc. but Fe(II) is oxidised fairly easily by oxygen. $\endgroup$ – orthocresol Sep 5 '17 at 15:35
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    $\begingroup$ I agree with Ortho but how you didn't notice color change? $\endgroup$ – Mithoron Sep 5 '17 at 15:50
  • $\begingroup$ @Mithoron so I'm just the lab comp-bio/bioinformatics guy I try to stay out of the wet-lab. This came up in discussion and I was wondering how an ionic solution would go bad. It's been a couple of years since I took general chemistry lol. $\endgroup$ – James Draper Sep 5 '17 at 15:58
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Water evaporates. Water dissolves the fixed gases ($\ce{N2}$, $\ce{O2}$, $\ce{CO2}$). A well-sealed bottle of water will last for years - that's assuming that we understand that 'bottles' are made out of heavy glass*, and seals are permanent (in the sense that opening the bottle is irreversible). That's not a particularly useful way to store stuff you need access to. (In other words, most bottles allow only limited storage life.) $\ce{O2}$ can oxidize $\ce{Fe(II)}$. Unless working under (and stored under) inert atmospheres, the titer will inevitably change over time. Even when working in inert atmosphere, there will be evaporative losses. All that said and assuming that there wasn't any accident involving the reagents, nor changes to their storage conditions (temperature, location, etc.) then the decline in $\ce{Fe(II)}$ should be roughly proportional to the number of times the bottles are accessed. Assuming that that is regular, with no recent upsurge, then the change in titer should be gradual over time; no sudden change should have occurred.(*please don't tell me that they're storing reagents in plastic! yet still any changes should be gradual, not sudden.)

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Generally, ferrous chloride gives ferric chloride and iron(III) oxide on heating in air (pubchem) but on long standing, ferrous chloride forms other Fe(III) species like iron hydroxide($\ce{Fe(OH)3}$) and iron oxide-hydroxide($\ce{FeOOH}$), iron oxychloride($\ce{FeOCl}$) etc. all present in the solution in equilibrium. The type of species present in the solution depends on the color of solution. Orange-red reveals ferric chloride, propably hydroxide and the oxide-hydroxide. More darker color like brownish reveals iron(III) oxide(magnetic?).

Giving a cursory search on Google, numerous references can be found which discuss on ferrous chloride oxidation. I am showing 2 of them:

  1. Source 1 - propose formation of a binuclear complex having the composition $\ce{Cl2FeOOFeCl2}$
  2. Source 2 - gives details explaination about the mechanism of this oxidation
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