Can ferrous ions exist in an acidic solution?

Question

Can Fe²⁺ ions exist in an acidic solution or will they always pick up available protons and become Fe³⁺? My hunch is they can't exist but I'm not really sure.

If the latter, does that mean that dissolving iron in an acid will raise the pH since it ties up some of the H⁺? Or will the additional protons still be just as free to float around and interact with other stuff as they are before iron comes to the party?

If they can exist, however, is there some kind of ferric:ferrous equilibrium based on... something?

I ask because I have a solution of oxalic acid and water, with iron dissolved in it, and it is bright yellow-green (like a green or yellow highlighter), and the internet says Fe³⁺ ions are yellow-green. Also I'm pretty sure there can't be actual iron oxalate particles (which are also green) suspended in the solution (conclusion from this lesson). I'm trying to work out the actual contents of the solution on my own.

So I'm pretty sure I'm looking at ferrIC ions -- and it doesn't seem to make sense that ferrous ions could exist here -- but I just want a sanity check on that guess, since I'm still trying to wrap my head around this.

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As an aside: Now after a little more reading, I'm wondering if maybe my oxalic acid solution contains ferrioxalate ions too (or instead), although that's specific to my oxalic acid experiments rather than acids in general. They're also bright green, it seems. Apparently those are light sensitive though, so maybe my UV laser will do something if the wavelength (405nm) is right and so I can test for them? That's an experiment for tomorrow.

Answer 1

Can Fe²⁺ ions exist in an acidic solution or will they always pick up available protons and become Fe³⁺? My hunch is they can't exist but I'm not really sure.

I was intrigued by your query because there is a hidden misconception. There are no available free floating protons in solution. If you want $\ce{Fe^2+}$ accept a proton, you are asking to cause a nuclear reaction. Of course this is far from reality. Notationally, even if you wanted $\ce{Fe^2+}$ to accept a proton (=from an acid), like the way organic chemists talk about it, it would be written as $\ce{[HFe]^3+}$. You don't want that either, because such a species does not exist.

In short, if you have an acidic solution, free from dissolved oxygen and kept in an inert atmosphere, $\ce{Fe^2+}$, will sit swim happily for a long time.

When we talk about the color of the transition metal ions, usually we are talking about the free ion surrounded by water, for example, $\ce{Fe^2+}$, which is a light green solution, chemists are then actually talking about $\ce{[Fe(H2O)_6]^2+}$.

Iron(II) oxalate, as you can see from Wikipedia is yellow... just like the dry yellow highlighter ink. When the metal ions are complexed (=surrounded) by something which can hold the metal ion like a crab's claw (chelate), you cannot predict the color beforehand.

Another example is that iron(III) thiocyanate is blood red. This has nothing to do with the yellow color of Fe(III).

Answer 2

Ferrous salts are stable in acids, there are two places in nuclear chemistry where acidic ferrous salts are commonly used.

The solvated protons in an aqueous acid solution are unable to act as oxidants to convert Fe(II) to Fe(III). The only time I would regard the solvated proton as an oxidant would be when considering the chemistry of the solvated electron.

There the reaction

e- + H3O+ --> H. + H2O is important.

This reaction generates hydrogen atoms from solvated electrons but it is important to understand that this is a very strong reducing agent being converted into a another strong reducing agent (but slightly less strong reducing agent).

One common high dose doseometer is the Freike which is ferrous sulfate in acid, even when the solution is saturated with oxygen in the absense of radiation the iron stays as ferrous (iron(II)).

The other place I am thinking of is in the classical PUREX process as used in B205, there ferrous sulfamate (Fe(SO3NH2)2) is used as a reducing agent to convert a solution of plutonium(IV) into plutonium(III). What is done is that the iron(II) solution is added to a mixtrure of uranium(VI) and plutonium(IV) in nitric acid. While the tributyl phosphate at 30 % (v/v) in kerosene will extract Pu(IV) and U(VI) from nitric acid it is unable to extract Pu(III) or other trivalent f block elements.