deltaH
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Potassium ferrate(VI) from Fe3O4 and KBrO3 in molten KOH/KNO3 eutectic
PLEASE NOTE: This is a discussion of a hypothetical methodology for synthesising an extremely powerful and dangerous
oxidant. Only people with the required means and prodigious knowledge, skill and suitable safety equipment should EVER attempt this and they would do
so entirely at their OWN risk!
[Note to moderators, please consider carefully before merging this with the thread on potassium ferrate as I believe that thread to have
become too broad]
Potassium ferrate(VI) is an extremely powerful oxidant that could be very useful as a 'green oxidant'.
Unfortunately, it's hard to prepare in good yields, see:
(i) synthetic methodologies for ferrates on SM's library
(ii) general SM thread on potassium ferrate
According to Tan et al (2013), potassium nitrate and potassium hydroxide form a eutectic that melts at a 'mere' 225°C with the composition
KOH/KNO3 = 63.8:36.2.
The lower temperature AND strongly basic nature of this eutectic might make this a useful media for preparing potassium ferrate(VI)!
The notion of preparing high oxidation state compounds in eutectic media has precedence in the literature. For example, in section 5 of his review,
Kerridge (1975), states that a Neptunium (VII) state was prepared from a eutectic of potassium nitrate, sodium nitrate and sodium hydroxide at 300°C,
using potassium bromate as oxidant.
Nota bene, Kerridge goes on at length to discuss a study that showed that while potassium permanganate (a related compound to potassium
ferrate) quickly decomposes in hot nitrate eutectic melts to form oxygen gas, the study shows that this can be prevented in the
presence of small amounts of potassium bromate or iodate.
On my thread about burning iron wool, we appear to have reached consensus that this primarily yields Fe3O4, otherwise known as magnetite.
Now could KBrO3 be sufficiently a strong oxidant to form Fe(VI) from the easily prepared magnetite? In this thread I am deriving the E values for the half reactions. Currently my values for them are (which might change):
BrO3-(aq) + 3H2O(l) + 6e- <=> Br-(aq) + 6OH-(aq) E° =
+0.583V
Using the same method and data from wiki's table of standard electrode potentials, one can also show that:
Fe3O4(s) + 16OH-(aq) <=> 3FeO42-(aq) + 8H2O(l) + 10e- E° =
-0.772V
Overall reaction (in a melt) is:
3Fe3O4(s) + 5KBrO3(s) + 18KOH(l) => 9K2FeO4(s) + 5KBr(s) + 9H2O(g)
The difference is a mere 0.189V at standard conditions and in solution, which aught to be correctable by employing a strongly alkaline medium, like
the potassium hydroxide eutectic, as the overall reaction consumes large amounts of OH- and can thus we can shift the Ecell slightly by working in
high hydroxide concentrations (see Nernst equation).
Also, large amounts of steam are formed, this also helps to shift the reaction to the right at high temperatures!
Accordingly, I conclude that there is an excellent case for oxidising Fe3O4 to K2FeO4 in an excess
eutectic melt of KOH/KNO3 = 63.8:36.2 as molten salt SOLVENT at greater than 225°C using KBrO3 in stoichiometric amounts (+
slight excess) as oxidant, whose presence should also retard the decomposition of K2FeO4 to oxygen in this nitrate melt.
The work up of the product would be relatively simple and consist of dunking the cooled melt into ice water slurry to dissolve the potassium nitrate
and hydroxide, followed by filtering out potassium ferrate if this is less soluble under the conditions.
References
Kerridge, David H., Recent Advances in Molten Salts as Reaction Media, Pure Applied Chemistry, 1975, 41(3), pp. 355-371.
Tan, J. et al, Facile Preparation of Mn-doped CeO2 Submicrorods by Composite-Hydroxide-Salt-Mediated Approach and Their Magnetic Property,
Materials Research, 2013, 16(4), pp. 689-694.
Edit(woelen): corrected an error in reaction equation after request from deltaH
[Edited on 5-8-15 by woelen]
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blogfast25
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Quote: Originally posted by deltaH  |
The work up of the product would be relatively simple and consist of dunking the cooled melt into ice water slurry to dissolve the potassium nitrate
and hydroxide, followed by filtering out the less soluble potassium ferrate.
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At which point your ferrate VI will probably fall apart.
Procedures for oxidising Fe to Fe (VI) are a plenty, procedures for isolating the ferrate intact... not so many.
Np (VII), you've gotta love that!
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deltaH
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My apologies blogfast25, I made an edit as you typed that! After reviewing my text, I realised that I couldn't be sure that the
potassium ferrate would be less soluble in the ice water, but I think if you use just enough, this might be the case.
| Quote: | | At which point your ferrate VI will probably fall apart. |
I don't understand why you say this, because I was
under the impression that ferrate(VI) is stable enough (temporarily) in strongly basic and cold water?
| Quote: | | Procedures for oxidising Fe to Fe (VI) are a plenty, procedures for isolating the ferrate intact... not so many. |
The reason I started working on this is because of (i) poor yields of the known methods plus the very large amounts of oxidant
required when it's hypochlorite based. I wanted to use an oxidant with much more available oxygen and conveniently discovered that bromate also
prevents decomposition of permanganate to O2 gas in nitrate melts.
The other important aspect here is working with eutectic melts to bring the temperatures way down and so also favour keeping the ferrate in tact.
The Kerridge paper claims that the reason bromate or iodate prevents O2 decomposition in those melts is because they quickly scavenge any nitrite
formed.
[Edited on 17-10-2013 by deltaH]
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blogfast25
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deltaH:
Going by my (admittedly very old) A.F. Holleman, K2FeO4 can indeed be crystallised from very alkaline conditions. But the compound will be very
unstable in any but these conditions, I think, capable of oxidising water.
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deltaH
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Thanks blogfast25, I think very little would even dissolve if you use the correct (minimum) amount of ice slurry... just enough to
dissolve the nitrate and hydroxide. Also, the temperature would drop dramatically because as the salt dissolves, it would depress the freezing point
of the ice slush... probably could go down to -10C even? Anyhow, it just needs to survive filtration of the powder immediately afterwards.
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deltaH
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I've calculate some numbers on the discussed basis and have assumed the authors worked on a mass basis for their eutectic composition.
My calculation yield a fusing composition consisting of:
6.9g Fe3O4
10.0g KBrO3
12.1g KOH
6.8g KNO3
This provides a 20% excess of KBrO3 AND 20% excess KOH, as well as an initial mass ratio of KOH:KNO3 = 64:36 as per the eutectic
requirement.
The reaction should produce about 2 scdm (standard cubic decimeters) of steam in total (about twice that volume at the temperatures employed), so
clearly it should be bubbling rather profusely if successful.
While the eutectic is meant to begin melting at 225C, this might be a little higher because of the KBrO3 present. As the reaction proceeds,
the melt should solidify more and more because of the consumption of KOH.
The reaction may be endothermic (to be confirmed) and if so the bubbling would depend on the heating, which if true, would be a convenient way of
controlling it (simple move the crucible in or out of the flame with long tongs) while swirling.
The reaction is deemed done when no more bubbling occurs/slows down a lot upon heating.
The crucible should ideally be made of zirconium to resist the extreme oxidation. Zirconium crucibles are, for example, the standard employed for
mineral digestions in molten sodium peroxide (peroxide fusion method).
NOTE: The safety of fusing such a mixture is yet to be established! This is for discussion only!
From wiki's solubility table I find:
KNO3: 22.4g/100g @ 273K
Assuming the residual of KOH and KBrO3 is very small, the amount of water+ice required is 30 g.
To be on the safe side, I would suggest 40 g ice and no water is added straight to the cooled crucible, I use more than 30 g because the temperature
might drop sub zero once the salts dissolve.
Furthermore, there should be 2 g KOH in the residual melt, so in 40ml water, that gives a concentration of KOH = 0.89M, so it might be a good idea to
add another gram of KOH before adding the ice to make sure the K2FeO4 survives the dissolution intact.
I am unsure if ordinary filter paper could be used to filter such a strong oxidant? Another option would be to use a glass microfibre filter paper.
The final problem is what to use to wash the product at the filter? Pure water, being insufficiently alkaline would degrade it as the
K2FeO4 would oxidise the water. One option is to simply filter under suction as best you can and dry in a desiccator and simply
tolerate the small amounts impurities coating the particles, besides this being alkaline, may even help stabilise the K2FeO4!
[Edited on 18-10-2013 by deltaH]
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froot
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What about this eutectic?
We salute the improvement of the human genome by honoring those who remove themselves from it.
Of necessity, this honor is generally bestowed posthumously. - www.darwinawards.com |
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deltaH
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Thanks froot
The lithium containing eutectics are fab, but I was wanting to avoid them for simplicity of reagents, but if you have lithium hydroxide or nitrate,
then certainly one could use them.
Also, if it turns out that one HAS to do this at T<200C, then you have no choice but to use a lithium containing melt.
The downside is that you have introduced a second cation type to the system and so you might worry about lithium ferrate forming (not sure about its
solubility?).
[Edited on 18-10-2013 by deltaH]
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