What is the easiest way to concentrate a solution without causing it to hydrolyze?

I’ve been working with iron acetate solutions in dilute acetic acid (vinegar) and I’ve been running into a problem. Every time I try to boil off the solvent the solution hydrolyzes to form iron oxides. I’ve given up trying to dry it completely because supposedly iron acetates can’t exist outside of solution, but is there a way of concentrating the solution?




Try adding Na2SO4 to the mix and freeze forming Na2SO4.7H2O. Slow, but at least, no oxygen-related chemistry, discussed below.


As a sidebar, you likely have formed basic ferric acetate from boiling, which adds oxygen, to your acidic (vinegar) and aqueous ferrous acetate mix. 


Likely reaction mechanics are complex, see, for example, [Table S1 here](https://www.atmos-chem-phys.net/13/11259/2013/acp-13-11259-2013-supplement.pdf) . Per the source my take of a possible reaction sequence:


$\ce{ O2 (aq) + Fe^{2+} → Fe^{3+} + .O2− }$ ( moderately fast) )


$\ce{ . O2− + Fe^{2+} + 2 H+ → Fe^{3+} + H2O2}$ (fast )


$\ce{ H2O2 + Fe^{2+} → Fe^{3+} + .OH + OH− }$ ( slow)


$\ce{ .OH + Fe^{2+} → Fe^{3+} + + OH− }$ ( fast )


Net radical reaction: 


$\ce{ O2 + 4 Fe^{2+} + 2 H+ → 4 Fe^{3+} + 2 OH-}$ 


The creation of FeOOH is also possible in less acidic conditions per his source: ['Air Oxidation of Ferrous Iron in Water'](https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.jieas.com/fvolumes/vol081-5/3-5-11.pdf&ved=0ahUKEwiQk7-fsOHMAhVK6iYKHXWmDVcQFghlMBI&usg=AFQjCNGeU9yaSSIfaQ51Tcclo0sik8pjfA&sig2=Uqlsoeiu4FunJXZlyZC9Yg).


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Some good news here as the redox potential of iron (III) to iron (II) is very favorable:


Fe +++ + 1 e - ----> Fe ++ + 0.77 V


Almost as strong as silver 1 to metallic silver


Ag + + 1 e - ----> Ag. + 0.80 V



> 
> You may try to extract [iron (II) acetate](https://en.m.wikipedia.org/wiki/Iron(II)_acetate) into ether, which can be much more easily removed by vacuum distillation (under nitrogen for safety)$^1$.
> 
> 
> 


It is very difficult to "boil off" water from vinegar$^3$ (acetic acid) because their boiling points are very close (100 C and 118C). With vinegar, you are probably boiling off all of the acetic acid$^4$. [Iron (II) hydroxide](https://en.m.wikipedia.org/wiki/Iron(II)_hydroxide) is almost insoluble in water, and could be formed during attempts at distillation as:


2 CH3COO- + Fe ++ + 2 H2O ----> CH3COOH (volatile) + Fe(OH)2 (solid)


And finally, unfortunately, O2 is strong enough to oxidize even Iron (II) to Iron (III) under acidic conditions:


O2 + 4 H+ + 4 e- ----> 2 H2O. + 1.23 V


or even: H2O2 + H+ + 2 e- ----> 2 H2O. + 1.76 V. $^2$


But in basic solution:


O2 + 2 H2O + 4 e- ----> 4 OH- + 0.39 V. (no reaction)


So, one really would not want to acidify iron (II) and heat it in the presence of oxygen$^5$.


$^1$ this may also reduce unwanted oxidation of Fe (II) to Fe (III)
  
$^2$ [what is this colorless ferric solution?](https://chemistry.stackexchange.com/questions/5025/what_is_this_colorless_ferric_solution)
  
$^3$ vinegar, being *dilute* acetic acid, boils much closer to 100 C.
  
$^4$ acetic acid would have significant vapor pressure at 100 C.
  
$^5$ If a reducing agent such as iron wool is used, conceivably, fractional distillation could be employed, or, *one could use more concentrated acetic acid* and [iron carbonate](https://en.m.wikipedia.org/wiki/Iron(II)_carbonate) in the synthesis process.


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