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).
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.