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rstar
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[*] posted on 15-3-2013 at 05:24
FePO4 blue !?


Hi
I made FeCl2 solution. In another container i dissolved some BAP(biammonium phosphate, that i bought frm fertilizer store) in water and filtered it.

When i added FeCl2 to BAP , i instantly saw white ppt. I believ that it was FePO4. I allowed that to settle down.

After a few hours, i saw that the precipitate now looks bluish. I filtered that and allowed to dry. Now some parts of the ppt look even darker blue. I'm going to upload a photo of it.

Now my question is, Wth have i made ?




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[*] posted on 15-3-2013 at 05:26


Photo, with my low quality camera

Ch2.jpg - 28kB




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[*] posted on 15-3-2013 at 09:01


An orthophosphate with a Fe(II) salt could never yield Fe(III)PO4, without prior oxidation of the ferrous ions to ferric ions.

Fertiliser diammonium phosphate is likely to seriously contaminated.

Fe3(PO4)2 does appear to exist though and is reported as insoluble.



[Edited on 15-3-2013 by blogfast25]




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AJKOER
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[*] posted on 15-3-2013 at 09:55


Perhaps "Ferrous ammonium phosphate" ?

Pure dry FeNH4PO4 is a greyish-green powder. See http://www.fao.org/ag/agn/jecfa-additives/specs/monograph7/a...

One route to preparation:

"Ferrous ammonium phosphate is manufactured by first combining
iron powder and phosphoric acid in deionized water with stirring
and heating the mixture to get ferrous hydrogen phosphate as a
slurry. Ammonia is added to get ferrous ammonium phosphate."

[EDIT] Your possible reaction:

(NH4)2HPO4 + FeCl2 --> FeNH4PO4 (s) + NH4Cl + HCl

and the presence of HCl limits the hydrolysis of the FeCl2:

FeCl2 + 2 H20 <---> Fe(OH)2 + 2 HCl

[Edited on 15-3-2013 by AJKOER]
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[*] posted on 15-3-2013 at 12:45


Quote: Originally posted by AJKOER  
Perhaps "Ferrous ammonium phosphate" ?

Pure dry FeNH4PO4 is a greyish-green powder. See http://www.fao.org/ag/agn/jecfa-additives/specs/monograph7/a...

[Edited on 15-3-2013 by AJKOER]


Not very likely, IMHO. The dissociation of diammonium phosphate:

(NH4)2HPO4(s) === > NH4(+)(aq) + HPO4(2-)(aq)

The trouble is that there really isn’t anything in there that can the deprotonate HPO4(2-), except for insolubility of FeNH4PO4.


[Edited on 15-3-2013 by blogfast25]




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[*] posted on 15-3-2013 at 12:50


Quote: Originally posted by blogfast25  


The trouble is that there really isn’t anything in there that can deprotonate HPO4(2-)


If FeNH4PO4 is sufficiently insoluble, then

Fe2+ + NH4+ + 2 HPO42- --> FeNH4PO4(s) + H2PO4-

will be favourable.

Anyone know the Ksp of FeNH4PO4?

If we use K2 = 7e-8 and K3 = 4e-13 for phosphoric acid, then the disproportionation reaction would have a Keq of 5.7e-6. So if the Ksp for FeNH4PO4 is 6e-6 or less (and most Ksp values are), then this reaction will have a Keq of greater than one.

[Edited on 15-3-2013 by DraconicAcid]

[Edited on 15-3-2013 by DraconicAcid]




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[*] posted on 15-3-2013 at 18:03


That means my BAP is contaminted and i made an iron complex. But what kind of iron complex could have bluish color ?



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[*] posted on 15-3-2013 at 19:32


That thing now looks greyish black. Here's a photo

Che3.jpg - 53kB




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[*] posted on 15-3-2013 at 19:52


I would assume that your wet precipitate is oxidizing, and you're getting a mixed iron oxide/phosphate.



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[*] posted on 16-3-2013 at 08:22


OK, some related research (see bottom of page 11 at http://click.infospace.com/ClickHandler.ashx?du=http%3a%2f%2... ). To quote:

≡FeOH0 + PO43- + 3H+ ↔ ≡FeH2PO4 + H2O log K1 = 32.08 (1)
≡FeOH0 + PO43- + 2H+ ↔ ≡FeHPO4- + H2O log K2 = 26.39 (2)
≡FeOH0 + PO43- + H+ ↔ ≡FePO42- + H2O log K3 = 20.73 (3)
12
How much ferric is needed to remove phosphorus?
Traditionally the ferricphosphate precipitant FePO4(s) was thought to be the primary precipitant in wastewater treatment. However, recent research (Smith et al., 2008) showed a complex structure, with several fundamental reactions occurring simultaneously as iron reacts to form ferric hydroxides and phosphates.
The amount of phosphate that binds to the metal hydroxide is still a topic of discussion. Current research suggest that the stoichiometric ratio of metal:phosphorus (Me: P) in the precipitant depends on many factors, including the phosphate concentration in the liquid, chemical dose, age of the hydroxyl complex, mixing, and many other factors (Szabo et al., 2008, de Haas et al., 2000, Yang et al., 2006)."

Now, this somewhat current (2008) 'topic of discussion' among researchers is before anyone has introduce NH4+. With respect to the precise chemistry involved in the cited procedure, I am even more indefinite without addressing impurities.
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[*] posted on 16-3-2013 at 10:38


I doubt strongly that the conditions decribed in that infospace paper can apply to what 'rstar' was doing.

These 'FeOH values' for instance are for surface reactions on hydrated ferric oxide. Not greatly relevant here, IMHO...

'rstar' should repeat his experiment using stoichiometric amounts of reagents and assuming FeNH4PO4 is formed. Then see what colour he gets.

Assuming FeNH4PO4 was precipitated and that there was an excess Fe(II) available, the latter would have started to oxidise to Fe(III). Having Fe in two different oxidation states could possibly lead to strange results (see e.g. magnetite and Berlin Blue).


[Edited on 16-3-2013 by blogfast25]




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[*] posted on 17-3-2013 at 00:21


Hey ! now the colour actually looks greyish-green




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[*] posted on 17-3-2013 at 06:30


Quote: Originally posted by rstar  
Hey ! now the colour actually looks greyish-green


You mean the blue has now changed colour to greyish-green? Did any drying take place?




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[*] posted on 17-3-2013 at 07:22


Quote: Originally posted by blogfast25  
Quote: Originally posted by rstar  
Hey ! now the colour actually looks greyish-green


You mean the blue has now changed colour to greyish-green? Did any drying take place?


Oh yes...
There was drying...
I guess it may be a Fe - NH4 complex. :)




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[*] posted on 17-3-2013 at 10:50


Quote: Originally posted by rstar  

I guess it may be a Fe - NH4 complex.

Can't be. Ammonia can form complexes with transition metals because it has a lone pair it can donate to the metal. The ammonium ion doesn't have a lone pair, and can't act as a Lewis base (i.e., a ligand).




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[*] posted on 17-3-2013 at 11:11


any chance there is a bit of copper in that fertilizer?
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[*] posted on 17-3-2013 at 21:34


What if this is FeNH4PO4, that also looks greenish, right?
is there any test for it ?




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[*] posted on 18-3-2013 at 06:37


It should dissolve in concentrated sulphuric acid, displacing the weaker phosphoric acid, into a greenish solution of FeSO4 and (NH4)2SO4. But that's not a definitive test...



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[*] posted on 18-3-2013 at 08:43


I havnt stored it in a vessel yet, it's outside in the watch glass. Does it react with O2 in air ?



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[*] posted on 18-3-2013 at 10:22


Quote: Originally posted by rstar  
Does it react with O2 in air ?


If it is an Fe(II) salt it probably will oxidise to Fe(III), especially when wet. But that oxidation is compound dependent: Mohr's Salt [(NH4)2Fe(SO4)2 hydrate] for instance keeps well without oxidation, in dry conditions.




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[*] posted on 18-3-2013 at 15:20


Okay I got very "pure" mohr salt here, gonna try and mix it down with phosphoric and see what happens, reporting soon with a edit,.
EDIT: the precipitation of iron II out of mohr's salt takes a little while, so I'll refer to see photo that might be of interest: it is a sample from a local excavation which florished this amazing red clay which I believe is composed of iron, along this other green stuff inside the bigger bucket which I believed could be chromium but I'm more inclined for it be green rust. It came in larger chunks that were smashed into a powder!
(current reproduction of the original experiment coming..)
EDIT 2: yea ammonium iron phosphate is really grey-green. But when it was first prepared starting through iron hydroxide, thats green in color, and phosphoric acid, it first dissolved completely and turn red-brown? Then uppon addition of ammonium hydroxide it turned to the final color.
I think its just hydration, no big enigma.
Perhaps a roast should do for it to turn white?




greenrusters.JPG - 43kB



[Edited on 19-3-2013 by platedish29]
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[*] posted on 19-3-2013 at 06:24


Quote: Originally posted by platedish29  
EDIT 2: yea ammonium iron phosphate is really grey-green. But when it was first prepared starting through iron hydroxide, thats green in color, and phosphoric acid, it first dissolved completely and turn red-brown? Then uppon addition of ammonium hydroxide it turned to the final color.

[Edited on 19-3-2013 by platedish29]


Take us through what you did precisely?




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[*] posted on 19-3-2013 at 07:29


Iron(II) hydroxide is green in color... until it meets atmospheric oxygen. Then it turns to rust-brown Fe(OH)3.



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[*] posted on 19-3-2013 at 09:17


Ok, when i scrapped off that stuff from my watchglass, i saw that it was the same ol' bluish color (seen in the first image) . I stored it in a amber coloured glass vessel. I don't think i will need it so i'm gonna discard it shortly :)

Well, i wanna make some FeCl3, can anyone tell me a fast method of oxidizing FeCl2, without the use of H2O2 or Cl2 ?




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[*] posted on 19-3-2013 at 10:05


Quote: Originally posted by rstar  

Well, i wanna make some FeCl3, can anyone tell me a fast method of oxidizing FeCl2, without the use of H2O2 or Cl2 ?


There are plenty of oxidisers that will oxidise Fe(II) to Fe(III) but few leave no trace. Then you need to separate the oxidiser by-product (its reduced form) from the desired FeCl3. Not easy. That's why Cl2 and H2O2 are the most desirable. H2O2 is also cheap as chips and readily available.

Air will do it too but it's a bit of a slow boat to China for practical, 100 % conversions.

HNO3 also does it but NO/NO2 (and water) is released, so be careful. And for most, HNO3 is not OTC or easily available...



[Edited on 19-3-2013 by blogfast25]




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