# How do I prevent oxygen interfering when I prepare a FeCl2 solution from anhydrous FeCl2 powder?

I have a bottle of anhydrous $$\ce{FeCl2}$$ powder (from here; it is 98% pure and was kept in a glovebox after opening it for the first time to minimize reactions with oxygen) and want to make this into a solution. I work in a glovebox, but what I have done prior is that I prepared the reagents outside of the glovebox and then put them inside. I let nitrogen gas bubble through water and then add this water to a certain amount of the anhydrous $$\ce{FeCl2}$$ powder. After this, I put the solution into the glovebox.

However, I get nowhere near to the green color of the $$\ce{FeCl2}$$ solutions that I find online. My solution has a slight green hue, and after some time I find a little bit of orange precipitate at the bottom of my reagent. So I fear that there is some reaction between the $$\ce{FeCl2}$$ and $$\ce{O2}$$. What can I change about my method to prevent this?

The purpose of my research is to produce the mineral tochilinite ($$\ce{6Fe_{0.9}S\cdot5[Mg,Fe](OH)_2}$$). I mix the $$\ce{FeCl2}$$ reagent with $$\ce{NaOH}$$, $$\ce{MgCl2}$$ and $$\ce{Na2S}$$. I do not want any $$\ce{Fe^{3+}}$$ in my $$\ce{FeCl2}$$ reagent, because it is also not present in the mineral tochilinite. If it is present in the reagent, other minerals might start to form (such as magnetite).

• Is it possible your solid FeCl2 already contains some Fe(III) ? What is purpose of solution ? Can a reduction agens be applied, or Fe(III) hydroxide filtered out ? Include in the question all eventual relevant circumstances to prevent wrong assumptions and requests for clarifications. – Poutnik Nov 11 '20 at 11:37
• I've added some information to the text. How would I go about filtering out Fe(III) hydroxides? – Soof_fie Nov 11 '20 at 13:06
• Try searching for filtering techniques, e.g. using columns with solid sorbents and consider also centrifuging. Be aware than depending on pH, there can be significant amount of dissolved $\ce{Fe^3+}$. Consider also using FeCl2 . 2 or 4 H2O instead. See en.wikipedia.org/wiki/Iron(II)_chloride – Poutnik Nov 11 '20 at 13:22
• You could try mixing it with Fe(0) powder. It will reduce the Fe(III) ions. Also if it does not matter, which I am not sure you could add excess HCl to your solution, since it increases the stability of Fe(II). Best thing you can do if you don't want to add iron powder or HCl is to work in an inert atmosphere, still then there is a chance that there are Fe(III) ions present. – Andrew Kovács Nov 12 '20 at 6:07
• I would add some grains of metallic iron powder on the ground of the flask. Metallic iron reacts with ferric ions, according to $$\ce{Fe + 2 Fe^{3+} -> 3 Fe^{2+}}$$ – Maurice Nov 12 '20 at 20:29

See if your group or an other (e.g., organic / organic catalysis) has a Schlenk line. With some training, this allows you to reduce the interference of oxygen greatly, because you may evacuate your reaction flasks and to replace the air by an inert gas such as nitrogen or argon. Nitrogen is relatively cheap because it equally is used for welding (as gas) or for cooling (think e.g., NMR spectrometers). Argon gas, on the other hand has the advantage of a density greater than the one of air, thus you may cover and protect your substance under a blanket of argon.

Then, search for Schlenk flasks. You identify these quickly for their connector to said Schlenk line. Besside round bottom flasks and tubes (a gallery, a second one), there equally are Schlenk flasks to perform a pressure filtration while working under argon enclosing a filter plate (example).

Then, degas your solvent. In HPLC, bottled solvents for example are placed in the basket of the ultrasound bath while purging them with a steady, slow flow of argon to remove dissolved oxygen. Do this for your deionized water.

Weigh in $$\ce{FeCl2}$$ in your dry Schlenk flask, and add the deionized water prepared under the inert gas.

My variation on the suggested synthesis is not the direct addition of, say, the metal iron (has already been suggested), but instead the partitioned introduction into the reaction chamber of an oxygen scavenger.

For example in one possible embodiment, in place of the direct intervention of Fe in the reactive solution mix, just place a small amount of freshly prepared fine powdered magnesium metal in the chamber, but not in direct solution contact.

My logic is that magnesium metal acting as an oxygen scavenger will anodically corrode (to MgO) with oxygen, thus mitigating the further possible action of oxygen on ferrous ions.

The procedure is then mixing of the FeCl2 reagent directly with NaOH, MgCl2 and Na2S, in the presence of a potential separated side surface reaction between Mg and any present oxygen.

If this fails, try a layer of Mg powder on the reactive mix. However, as in the case of suggested employment of Fe, the resulting chemistry may or may not favor the targeted formation of the mineral tochilinite ($$\ce{6Fe_{0.9}S⋅5[Mg,Fe](OH)2}$$).