[Mr.X]

Hello, it's nice to write here for the first time!

I have a little chemical problem at work. My task is to analyse the crystal structures and physical properties of the oxides TiO2, Al2O3, MgO, Cr2O3, FeO3 dissolved in Na3AlF6 with conditions of rapid decrease in concentrations of Na3AlF6 and stable temperature of 1 000 C°. That is the reason why precipitation reaction is perfect because the process happens fast and at stable temperature. Now I have to find a chemical that reacts with Na3AlF6 to lower the concentrations of it and to make supersaturated solution of the desired oxides. The problem is that I can't figure out or find a chemical that reacts only with the solvent and not with the oxides. All of the chemicals that come to my mind react with both fluoride and oxide ions.

Any ideas of what chemical to use?
It must react only with Na3AlF6 and not with the oxide ion.
Any suggestions will be really helpful! I really don't know what to do!

[Hunter2]

I think there is no solution for that problem. But why you need the Oxides remain. Maybe addition of Calcium- or Bariumoxide will form CaF₂ or BaF₂. I am not sure this helps.

[Mr.X]

The purpose is to make supersaturated solution to able to crystalize the oxides but it can't be done with temperature reduction method because it must happen in stable temperature. But I can't figure out what chemical I need. May be it's not practacly possible or it's just really hard. I don't know that is why I'm asking 

[Arkcon]

It's hard for me to wrap my mind around your problem as well.  You want to make a supersaturated solution of those particular transition metal oxides in liquid Na₃AlF₆.  That's a little outside my experience.  But let me try to think of it analogously with aqueous crystal growing.

Say I'm growing crystals of aqueous soluble salts, but instead of evaporating the water, I decompose it instead.  Well, that's plausible, but if I add something to the mix, aren't I contaminating the mix with that reagent, and with potential products?

Interesting idea 'tho.  Say I wanted pure crystals of an aqueous solute.  Suppose I electrolyse the water away instead of evaporating.  Hydrogen and oxygen fly away, and the solute crystallizes out.  You can't do that -- or can you?  If you electrolytically decompose the cryolte, even if aluminum metal precipitates at an electrode, won't the crystals grow?  Does that happen in commercial aluminum production?