Sciencemadness Discussion Board

Purple Iron Compound?

Jackson - 27-1-2019 at 11:48

Hi, I recently prepared some iron (ii) sulfate and decided to mix a small amount with some ammonia solution. It resulted in a grey precipitate. the solution produced a brownish precipitate on contact with the air. I observed a light purple film on top. It was of a similar color to iodine. It slowly turned into the same brown stuff (probably iron oxides/hydroxides) after sitting over night.

What could it be?

fusso - 27-1-2019 at 12:24

Probably be thin (hydr)oxide film, like rainbowy thin oxide layer on metals tend to look like.

Abromination - 27-1-2019 at 14:52

Quote: Originally posted by fusso  
Probably be thin (hydr)oxide film, like rainbowy thin oxide layer on metals tend to look like.

I have noticed this phenomenon before and it it is probably a hydroxide film reflecting light.
If you are interested in purple iron compounds, try preparing iron salicylate. Its yellow as a solid but purple in solution, rather like permanganate.

phlogiston - 28-1-2019 at 05:14

No answer for your question, but if we are talking about purple iron compounds:

(2-hydroxyethyl) ammonium thioacetate also forms a beautiful purple iron complex.

I've come across a solution of this being sold as a cleaning product for removing rust stains (it dissolves iron oxides and hydroxides).

woelen - 29-1-2019 at 00:46

When iron(II) is added to a base, then you get a nearly white precipitate of Fe(OH)2. This, however, is very easily oxidized by oxygen from air and then it turns grey. On further oxidation it becomes dark blue, almost black. This is a mixed iron(II)/iron(III) hydroxide. When it is even further oxidized, then it becomes orange/brown, the end product, which is iron(III) hydroxide. The purplish color you observed may be due to formation of a thin layer of a mixed iron(II)/iron(III) hydroxide, at the surface of the solution, where it is in good contact with oxygen from air. This color quickly disappears again on further oxidation, leading to a brown layer of iron(III) hydroxide.

markx - 29-1-2019 at 09:49

The ferrate ion (FeO4-2) has a purple coloration much alike permanganate.
It will form under anodic conditions in alkaline solutions and is rather shortlived.

Jackson - 29-1-2019 at 11:30

Thanks for the responses.

Woolen, the process that you said is exactly what I have observed. Grey precipitate, shiny blue-black or purple-black film, and then when the film is broken, like by sloshing around, a brown residue is formed.

AJKOER - 1-2-2019 at 09:48

This is a link (https://pdfs.semanticscholar.org/f83d/d855e637c2cd9ad1f9e0b2... ) to a pdf explaining the pH dependent chemistry, title 'The effect of pH on the kinetics of spontaneous Fe(II) oxidation by O2 in aqueous solution – basic principles and a simple heuristic description'.

Per page 2082, the reactions include the possible presence of the radicals .OH and .O2-. In the further presence of NH3 (producing also alkaline conditions as noted in the opening thread) more stuff like .NH2 radical:

.OH + NH3= H2O + .NH2

And, from there the oxidation of .NH2 from O2 in a chain leading to NO2-,..

The first thing I would suggest is to repeat the experiment substituting a very small amount of NaOH for aqueous NH3 and see if the purple compound is stilled created, which could limit the possible paths to the purple compound involving nitrogen compounds.
-------------------------------------------------------

As the superoxide radical presence is cited on page 2082, I have yet to verify the possibility of the following radical reaction in the presence of the amino radical:

.NH2 + .O2- = NH2- + O2

which in aqueous conditions could reform NH3 and elevate pH:

NH2- + H+ (from water) --> NH3 ( + OH-, raising pH, see https://www.google.com/search?source=hp&ei=rI9VXPWQDcqK5... )

[Edited on 2-2-2019 by AJKOER]

Jackson - 1-2-2019 at 10:06

Wait, so iron (II) can oxidize ammonia to NO2?

fusso - 1-2-2019 at 10:16

Quote: Originally posted by Jackson  
Wait, so iron (II) can oxidize ammonia to NO2?
Apparently AJKOER is speaking bollocks again:P

AJKOER - 1-2-2019 at 17:41

As some doubt my comments, an extract citing sources:
Quote: Originally posted by AJKOER  
Some theory suggests, to create nitrite/nitrate from ammonia, start by employing a hydroxyl radical generating mechanism including photolysis (see http://pubs.acs.org/doi/abs/10.1021/jp0349132 ), fenton and fenton-type reactions involving transition metals (discussed below), radiation (microwave pulse), electrolysis, sonochemistry,...

Then, the action of the •OH on ammonia in the presence of dissolved oxygen (or employ H2O2) is claimed by one reference to proceed as follows:

NH3 + •OH → H2O + •NH2 (see Laszlo extract and link below)
•NH2 + O2 → NH2O2• (aminylperoxyl radical unstable) → NO + H2O (See , "On the aqueous reactions of the aminyl radical with molecular oxygen and the superoxide anion", Table 2.1 at https://www.google.com/url?sa=t&source=web&rct=j&... )

Having formed some NO, in the further presence of a hydroxyl radical source generator and hydrogen peroxide the following reactions, for example, could introduce the formation of NO2:

H2O2 + •OH → H2O + •HO2
•HO2 + NO → •OH + NO2 (see https://www.google.com/url?url=http://scholar.google.com/sch... )

or, as the net of last two reactions equals:

H2O2 + NO ---UV or Fe(++), Cu(+), Co(++),..→ H2O + NO2

And further:

NO2 + H2O → HNO2 + HNO3

which introduces aqueous nitrite that is a better light induced promoter of hydroxyl radicals then nitrate, which is superior to H2O2. In the presence of uv, strong solar light or select transition metals, this postulated reaction chain can likely accelerates the conversion of ammonia to nitrite/nitrate. Interestingly, there are researchers reporting tissue cell damage from the seeming direct action of NO on H2O2 in biological systems (see, for example, "Hydroxyl radical formation resulting from the interaction of nitric oxide and hydrogen peroxide.",by Nappi AJ, Vass E. ) although, in my opinion, such a direct radical reaction need not take place given the enabling effects of light or transition metals. Note, my simple reaction chain consumes and regenerates the very short lived hydroxyl radical which, in essence, could increase its apparent reactivity life span (as measured by random cell collisions leading to cellular damage) having been, in effect, resurrected.
...........
[Edited on 16-9-2016 by AJKOER]


Link to extract: http://www.sciencemadness.org/talk/viewthread.php?tid=30473 .

Note, I make no mention of the time frame involved in the conversion of ammonia to nitrite or the yield, so no claim on the practical nature of the science. However, in the current context, the purple compound, while visible, is also likely at a very low concentration, assuming a possible nitrogen based path (which I actually doubt).

[Edited on 2-2-2019 by AJKOER]

Rhodanide - 10-4-2019 at 13:33

Iron compounds with colors in the blue spectrum aren't entirely unheard of. Ferric Rhodizonate has a strikingly blue color in solution, however what you've got is likely a mix of a bunch of things as opposed to one singular compound.