Kamikaza
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Obtaining "pure" H3PO4/NaH2PO4/Na2HPO4/Na3PO4 from mixed fertilizer
The last couple of days I tried to obtain a reasonably pure phosphate (for a fungus culture) from a mixed fertilizer. As I couldn't find a write up, I
thought I might as well post it here.
First I dissolved the fertilizer and filtered off the insoluble stuff. When the solution was clear, a solution of FeSO4 was added (which should form
insoluble Fe3(PO4)2) until no further precipitate was formed. Any metal salt can be used, as long as it has soluble sulfates and nitrates, and
insoluble hydroxides and phosphates. This way the phosphates are filtered from the sulfates and nitrates and god knows what kind of stuff there is in
there  .
The precipitate was then filtered and dissolved in hydrochloric acid. This was then neutralized with dilute NaOH until no more brown precipitate (most
likely a form of iron hydroxide) was formed.
This should leave a reasonable pure phosphate solution. At this step I stumbled upon some problems: first of all, when testing the pH of this solution
it turned out to be acidic (around pH 3 or 4). This greatly suprised me because the pH was high enough to form iron hydroxide. Maybe the Fe2+ is a
stronger acid then whatever kind phosphate was produced? Secondly, I boiled the solution down from 40 ml to 5 ml, and still no crystals have formed,
while weighing the liquid with a very faulty balance showed that there should be around 8 grams of salt in solution. And as a last point, I have no
idea of how to test if it is P2O5 or PO4.
So the questions are:
1.) How can the solution be acidic while it was basic enough for iron hydroxide to form?
2.) How to obtain Na(x)H(x)PO4 crystals from the final solution?
3.) How to test if it is P2O5 or PO4?
Tomorrow I can do the phosphate test as I have found a source of MoO  .
Here is an overview of the reactions:
3 Fe(2+) + 2PO4(3-) --> Fe3(PO4)2
6 HCl + Fe3(PO4)2 --> H3PO4 + Fe(2+) + 2Cl(-)
?
__________________________
Fe(2+) + OH(-) --> iron hydroxide
H3PO4 + (x)OH(-) --> H(x)PO4 + H2O
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blogfast25
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Any phosphorus in fertilizer is present as P (+V), so as P2O5 in its various forms (mainly PO4(3-), HPO4(2-) or H2PO4(-)). No “PO4”.
Brown precipitate point to Fe (+III), precipitating as Fe(OH)3.2H2O (red-brown). Fe (+II) readily oxidises in the presence of air:
4 x [Fe2+ === > Fe3+ + e-]
O2 + 2 H+ + 4 e- === > 2 OH-
4 Fe2+ O2 + 2H+ === > 4 Fe3+ 2 OH-
So even if you started with Fe (+II) it looks like you ended up with Fe (+III).
Did you stir during addition of NaOH? Fe(OH)3 is very insoluble and local points of higher pH would precipitate it.
What’s your basis for the 8 g claim?
MoO: you mean molybdenum oxide? What source?
[Edited on 24-7-2011 by blogfast25]
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Kamikaza
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Quote: Originally posted by blogfast25  | Any phosphorus in fertilizer is present as P (+V), so as P2O5 in its various forms (mainly PO4(3-), HPO4(2-) or H2PO4(-)). No “PO4”.
Brown precipitate point to Fe (+III), precipitating as Fe(OH)3.2H2O (red-brown). Fe (+II) readily oxidises in the presence of air:
4 x [Fe2+ === > Fe3+ + e-]
O2 + 2 H+ + 4 e- === > 2 OH-
4 Fe2+ O2 + 2H+ === > 4 Fe3+ 2 OH-
So even if you started with Fe (+II) it looks like you ended up with Fe (+III).
Did you stir during addition of NaOH? Fe(OH)3 is very insoluble and local points of higher pH would precipitate it.
What’s your basis for the 8 g claim?
MoO: you mean molybdenum oxide? What source?
[Edited on 24-7-2011 by blogfast25] |
I recently found out that the brown/orange could be an iron oxychloride, as it is the same colour, and I didn't really stir that much.
As for the presence of the presence of P, I know that it is in the form of phosphate ions.
I found the molybdenum oxide at a local pottery store.
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blogfast25
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Yes, an iron (III) (hydr)oxychloride is really plausible.
Mo oxide in a pottery store? Good find… I assume that you want to make the blue hetero phosphorus polymolybdate (P[M12O40]3-) as proof that
phosphate is present? With pottery grade Mo oxide that’ll be a hat trick: these pottery grades tend to be very inert, so you’ll probably have to
fuse the oxide with NaOH or KOH to get it to solubilise.
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