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chemoleo
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Preparation of Ammonium iron (III) sulphate (ferric alum)
Ferric alum (NH4Fe(SO4)2), of similar structure to ordinary alum, (potassium aluminium sulphate, KAl(SO4)2), is of interest because it forms
(sometimes?) violet to clear crystals that are supposed to be suitable for crystallisation trials.
One established method for making it is via boiling of ammonium sulphate and ferrous sulphate (II) togther with nitric acid, which functions here as
the oxidiser to convert Fe2+ to Fe3+.
A preparation is detailed on Lambdasyn (german). However, this prep uses precious HNO3, and it evolves NOx during the oxidation - neither which I want to waste.
Thinking, I am cleverer than this, I tried the same thing with H2O2, with interesting, but unexpected results:
Reaction:
K2SO4 (or (NH4)2SO4) + 2 FeSO4 + H2SO4 ----H2O2----> 2 KFe(SO4)2
I used 0.01 moles initially, all in stoichiometric amounts, including the H2SO4. I even checked the pH before and after the reaction, it changed from
0.3 to 0.6, so very little, and still damn acidic. For the H2O2, I used 30%, in a total volume of ~100 ml for each reaction.
I first dissolved all the salts, and the H2SO4. The solution was nice and clear, with a smaragd green tint (1). Adding a total of 13 ml of H2O2 (which
works out as a slight stoichiometric excess of H2O2 per oxidation to be done). Already after the addition of 1 ml, the solution immediately turned
yellow. After adding the whole 13 ml, the solution was bubbly, and quite intensely yellow (2), it also became handwarm. - suggesting that a reaction
was certainly taking place. Thinking, let's really drive this reaction to the right, by heat, and also to remove the left-over H2O2, I heated this in
the microwave until the foaming stopped, and true boiling began. By this point the solution was dark red/brown (3). In the case of the potassium salt,
small yellow crystals could be seen, which covered the bottom of the beaker alongside with a brownish sludge, which was thankfully very little. When
this cooled down, the solution turned yellow again, but the precipitate remained (4). Interestingly, the complex displayed heat-sensitive properties,
in that the hot solution was dark-red while the cold solution was a clean yellow - an entirely reversible process. I discovered, however, that
overheating this for a long time (I tried to boil down the whole solution) in the microwave yielded a brown preciptiate, which would not redissolve -
suggesting it is some FeIII oxide/hydroxide. The ammonium salt seemed more stable, although I didnt push it by harshly boiling it in the MW.
Anyway, with the K-solution, I precipitated the soluble leftover (not the yellow crystals/brown sludge (4)) with ethanol, an immediate yellow
precipitate formed, which turned the solution thick. Upon centrifugation, I got a nice cream-yellow voluminous pellet (5) that happily redissolved in
water, forming a dark yellow solution again.
Even more interestingly, after the oxidation of the K-salt with H2O2, the yellow crystal leftover, as well as the brownish sludge, happily redissolved
upon adding more water, no residue left. So clearly, This is not an insoluble hydroxide/oxide.
Right now I have this sitting in crystallisation trays, in the hope of getting some crystals from it. I don't, however, think that this is ferric
alum, because the colour is wrong, and the crystals were clearly yellow, but not violet or clear (apparently super-clean ferro-alum crystals are
clear, the colour derives from manganese impurities), according to the lambdasyn description. Neither were they octaedral, but rather very small and
needle-like, although this is hard to judge.
The question is, what did I get? There aren't that many variables to this, but it clearly is a new salt, which is not some crappy non-dissolvable
oxy-hydroxide.
Ideas?
I will be trying the same thing with HNO3, for reference and comparison, in a few days. I'll be darned if that solution turns violet!
Alternatively, I'll try and turn the FeSO4 (II) into Fe2(SO4)3 (III) by direct oxidation with H2O2 itself, and see what I get. Iron III sulphate
should yield the ferro-alum once ammonium sulfate or potassium sulfate are added.
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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guy
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Shouldn't the hexaaquoiron(III) ion be almost colorless? If its a different color, does that means some kind of complex is formed?
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BromicAcid
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A little unsure as to what you formed, but although it's obvious it should be noted that Fe(II)/H<sub>2</sub>O<sub>2</sub> is
a Fenton's Reagent type of situation and it is able to oxidize aqueous NH<sub>4</sub><sup>+</sup> to nitrogen and water. Here
is a few pages on alums that I found in my chem book 'Descriptive Inorganic Chemistry' Third edition, Rayner-Canham Overtone (you already have this
though).
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woelen
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What you created is an iron (IV) species, or even an iron (V) species. There still is quite some doubt on the precise nature of the material you made,
but as BromicAcid already pointed out, Fenton's reagent is the key to understanding what is going on.
On my website I have a description of what is going on, together with a link to a paper which covers this subject. Look at the following page on iron
salt solutions and especially the part on iron (IV).
Also, iron (III) ions are almost colorless at such a low pH, without complexing agent. With chloride present, the color is intense yellow. This also
is explained on the webpage.
http://woelen.homescience.net/science/chem/solutions/fe.html
EDIT: Changed link, so that it works again.
[Edited on 27-9-12 by woelen]
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ScienceSquirrel
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Preparation of Ammonium Iron (III) Sulphate (Ferric Alum)
Experimental
Ferrous sulphate heptahydrate (11.1g, 40mM) was weighed into a 50ml flask. Deionised water (10mL) was added and the mixture was stirred magnetically
with gentle warming until the salt had dissolved. Sulphuric acid (90%, 1.8mL, 30mM) was added dropwise and the mixture was placed in a water bath.
Hydrogen peroxide (35%, 2.9mL, 30mM) was added dropwise to the stirred mixture. The reaction is vigorous and the reaction mixture goes a muddy brown,
clearing to a bright orange as it nears completion.
After the addition the water bath was removed and the mixture was heated for 5 - 10 minutes until effervescence ceased. A solution of ammonium
sulphate (2.6g, 20mM) in deionised water (5mL) was added in one portion and the mixture was filtered. The filter paper was washed with deionised water
(5mL) and the solution was cooled in an ice salt bath. Pale lilac crystals gradually formed from the deep brown solution. The crystals were filtered
off, pressed dry and then allowed to dry in the air. The solid is a pale amethyst in the mass but colourless as small. single crystals. Yield 12.7g,
66%. mp 39-40.5C, Sigma Aldrich catalogue 37-41C.
Discussion
The reaction gives good yields of the alum and could easily be scaled up to produce sufficient material to grow a large single crystal.
It is an advantage to keep the mixture slightly acid to avoid the precipitation of ferric hydroxide.
[Edited on 7-9-2008 by ScienceSquirrel]
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kmno4
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Yes, I confirm this procedure. FeSO4 + H2SO4 + H2O2 + (NH4 )2SO4 really works fine.
I used to make larger amount ( ~ 200g) of this alun.
Indeed, from deep brown solution small white crystals are formed.
On the picture - mentioned alun.
It is almost coloreless, with a little amethyst (~violet) shade.
ps. my fellow made (in similar way) beautiful, large (> 1 cm), bright violet crystals of Fe(III) sulfate ( x 9 H2O).
ps2. with aid of ~5% acidic solution of K3[Fe(CN)6] you can test presence of Fe(II) in your oxidated solution. When Fe(II) ions are present, deep blue
colour/precipitate appears when you mix ferricyanide sol. with tested sol. It is very sensitive test. When only Fe(III) ions are present, mixing gives
clean brown solution.
[Edited on 8-9-2008 by kmno4]
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blogfast25
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I’ve made ferric alum many times, usually with useful product to show for but rarely without problems. Very variable yield, unexpected precipitates,
incomplete oxidation, excessive foaming (with HNO3) are all problems I’ve encountered. And once I attempted one madcap scheme to try and synthesize
this alum which involved fusing together ferric oxide, ammonium sulphate and conc. sulphuric acid, which actually gave some alum (after leaching) but
at low yield. Googling for ‘ferric alum synthesis’ I came across this thread and now finally found the time to test the method cited by
ScienceSquirrel, a few posts up. I followed the recipe closely with only a couple of minor modifications.
I applied it to try and make 0.25 mol of the alum (NH4Fe(SO4)2.12H2O, molar mass about 482 g/mol) and firstly combined 69.5 g FeSO4.7H2O, 63 ml of DIW
and 11.3 ml of H2SO4 95 % and heated the mixture to about 50 C while stirring vigorously. The sulphate dissolved easily.
Then I put the beaker on ice bath to cool as much as possible but at about 40 C, turbidity (crystallites?) set in, so I started adding the 18 ml of
prescribed H2O2 35 % (NOTE: that’s a 45 % excess with respect to stoichiometry). It was ice cold (from the fridge) and added drop by drop (about
one drop per second, continuously) from a mounted separation funnel, whilst constantly stirring the reacting mix. Relatively little effervescence was
noted and temperature of the liquid only increased by about 10 C, while the mixture darkened to about the colour of not-too-strong coffee. This colour
is a good sign because it’s caused by quite concentrated ferric sulphate. No precipitate or turbidity developed.
Effervescence during oxidation seems to point that not only the reduction:
H2O2 + 2 H+ + e- → 2 H2O takes place but presumably also:
H2O2 → H2O + ½ O2 and this is basically lost for the desired oxidation reaction:
Fe2+ → Fe2+ + e-
The mixture was then simmered until all effervescence stopped, which only took a few minutes.
Then 16.3 g of ammonium sulphate was added as a solid (not a solution) and the mix simmered further until the amm. sulphate had dissolved completely
(a few minutes). The final volume of solution was about 175 ml.
The coffee brown liquid was cooled to room temperature, a seed crystal was then added and it was refrigerated (about 5 C) overnight. A nice mass of
the usual light lilac crystals developed which after recovery, washing with a bit of iced DIW and drying amounted to 108 g of NH4Fe(SO4)2.12H2O or an
actual yield of 90 %, the highest I’ve ever achieved and higher than ScienceSquirrel’s yield too. Supernatant liquid solution weighed in at 70 g.
This confirms the validity of the method described by ScienceSquirrel and confirms also what I suspected based on previous experiences: with H2O2 as
the oxidant care must be taken that the oxidation of FeSO4 to Fe2(SO4)3 is complete to ensure maximum and consistent yields. Slow addition in
relatively cold conditions to avoid breaking down the peroxide to oxygen is essential to achieve this.
[Edited on 27-9-2012 by blogfast25]
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Poppy
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So...
You performed oxydation with H2O2 at low temperatures. This is what I was all worried about.
True is that Fe III is colorless if in the presence of an appropriate non complexating agent.
blogfast,
Your method must have achieved yields over 99% for sure.
The proof can be attained by the undiscriminate addition of hot H2O2 35% to a conc. solution of FeSO4 with a very stabilizing excess of H2SO4
equimolar to that of SO4(2-) already present in the dissolved ferrous salt (that is, enough to react forming Fe2(SO4)3 with a lot of H2SO4 left over
to grant proper acidic conditions for Fe3+ to do not hydrolyse).
Add how much H2O2 as you want. A vigorous bubbling is observed. Looking at the solution against light should still show hues of green. Add more H2O2
as you please.
Then, put this to react with slight excess Ca(NO3)2 as to neutralise all sulfate ions in the solution. As reaction slowly takes place (yes, calcium
sulfate will not precipitate that fast here).
Fe2(SO4)3 + H2SO4 + 4 Ca(NO3)2 --> Fe(NO3)3 + 3 CaSO4
After precipitation of calcium sulphate, the supernatant liquid should be greenish: the proof that hot addition of H2O2 does not fully oxydise Fe2+ to
Fe3+.
All the Fe3+ resulting from the double exchange reaction is now colorless, but Fe2+ is still green. I think its a fairly sensitive test for a pair of
good looking eyes.
Another problem between is the stability of Fe3+ in solution I believe it can oxydise water and go back to +2 state. I would point out the best way to
prepare Fe3+ is to carry with blogfast's method with very little or just NO excess sulfuric acid, such as upon formation Fe3+ will settle as
precipitate, the reason, IMHO is that solid Fe3+ is not as reactive as the aqueous counterpart.
blogfast,
Do you have means to perform this test?
Edit:
Nvm, for low concentration a "Fe2+ meter" would be best employed! It consists of an elongated acrylic parallelepiped which serves as to provide a
thicker section of the liquid to be tested. The longer it better. Use a white LED to shiny upon the solution, or any other pure white light source
which suits best for you, at one end, and at the other a piece of very bright white board.
I would also suggest making standards with known drops of FeSO4 in H2SO4 solution.
[Edited on 9-27-2012 by Poppy]
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blogfast25
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Poppy:
A few points.
Hot oxidation of Fe2+ with H2O2 is inefficient, as previous runs showed (low yield, I even got some Mohr’s Salt to sit on top of the alum in one
case). Also, a yellow precipitate (an insoluble basic ferric sulphate, I suspect) has been an unwelcome visitor on occasion.
The best and easiest test for Fe2+ in these conditions is K3Fe(CN)6 solution: added to a mix of Fe2+/Fe3+ it forms deep blue insoluble Prussian Blue
(aka Berlin Blue, Turnbull’s Blue), due to the Fe2+ present (with only Fe3+ present the solution only darkens somewhat).
I doubt very much that Fe3+ is capable of oxidising water in any conditions: it’s in fact a very weak oxidiser. I’ve used ferric alum as a titrant
for Ti(III) determinations but then Ti(III) is extremely easy to oxidise. But if you have literature (or your own empirical) evidence to that effect,
I’d be very interested to see it.
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White Yeti
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Double salts are a pain to make. I recently attempted the synthesis of ferric ammonium citrate and it didn't quite work out. I should have gotten a
yellow-brown powder upon evaporation but I got a substance with the consistency and colour of molasses even though I followed the instructions to the
letter. I guess the synthesis of these salts is never an exact science.
"Ja, Kalzium, das ist alles!" -Otto Loewi
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blogfast25
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Quote: Originally posted by White Yeti  | | Double salts are a pain to make. I recently attempted the synthesis of ferric ammonium citrate and it didn't quite work out. I should have gotten a
yellow-brown powder upon evaporation but I got a substance with the consistency and colour of molasses even though I followed the instructions to the
letter. I guess the synthesis of these salts is never an exact science. |
Ferric ammonium citrate is arguably far harder to synth than ferric ammonium alum. FAC doesn't even have a fixed composition. Many double salts I've
made presented no problem at all: potassium alum, ammonium alum, Mohr's Salt and others are really straightforward...
[Edited on 28-9-2012 by blogfast25]
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ScienceSquirrel
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Mohr's Salt is ridiculously easy. Garden centre grade ferrous sulphate and ammonium sulphate, a small amount of drain cleaner sulphuric acid in hot
water. Cool and watch it snow out in 99+% purity.
It is easily dried, very stable.
There is a preparation of Mohr's Salt here;
<del>http://www.chem.mun.ca/courseinfo/c2210/lab/Experiment%204%20Ammonium%20Iron%28II%29%20Sulfate%20Hexahydrate%20W2011.pdf</del>
Attachment: W2013 Experiment 4.pdf (74kB)
This file has been downloaded 9519 times
Most of the alums are very easy as well.
Potassium alum can be made as white crystals from used cans or aluminium foil, potassium hydroxide and sulphuric acid. Loads of recipes on the net.
Chemistry does not get much easier than this 
[Edited on 5-11-2012 by ScienceSquirrel]
<!-- bfesser_edit_tag -->[<a href="u2u.php?action=send&username=bfesser">bfesser</a>: replaced
broken link with attachment]
[Edited on 16.10.13 by bfesser]
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Poppy
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Yea it works, I just can't understand why they proposed an erlenmeyer flask. Because of that, crystals are going to be creep to remove. I had to use a
drill bit last time, otherwise I would miss the glassware. That gotta be ajoke..
[Edited on 11-5-2012 by Poppy]
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blogfast25
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I’ve tried yet another route to ferric ammonium alum, this one to try and further reduce the effervescence (O2 evolution) observed during the
oxidation step with H2O2, and seemingly with success.
70 g FeSO4.7H2O (0.25 mole) were dissolved in 63 g of water in a 500 ml beaker, by stirring and heating on an electrical plate. 15 ml of 33 w% (d =
0.88) NH3 solution (that is approx. 0.25 mole NH3) was then added, causing a dark blue black precipitate (a basic ferrous sulphate I think) to form.
The slurry was fairly viscous but still easily stirred and was cooled to about RT on an ice bath. It did not smell of ammonia at all.
As in the previous run, 18 ml of ice cold 35 % H2O2 was placed in dropper funnel and added drop by drop (about 1 drop / s) to the slurry, still on ice
bath and while stirring intensely (but manually). The oxidation took place immediately and with very little effervescence with the blackish slurry
changing colour to reddish/brown. After a while, with some of the peroxide left in the dropper funnel, I concluded the oxidation was complete. 6.5 ml
of 35 % H2O2 was left in the dropper funnel.
To this slurry (Fe(OH)3.nH2O + ammonium sulphate) was then added 15 ml H2SO4 (96 %), a slight excess. Most of the ferric hydroxide dissolved
immediately, and after about 5 minutes of simmering the solution was completely clear (but very dark in colour). The final volume was just over 100
ml. The hot solution was transferred to a PP beaker for cooling and refrigeration. Yield and colour of the product will be determined tomorrow.
Theoretical yield should be 120.5 g of NH4Fe(SO4)2.12H2O.
Another variant could be to avoid the peroxide altogether and use air oxygen as oxidant but that would require the right conditions of blowing air
through the slurry, heating and time.
[Edited on 12-10-2013 by blogfast25]
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MrHomeScientist
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I actually have some ferric ammonium sulfate at work, purchased from sigma something like 15 years ago. It came as beautiful lavender chunks. I wanted
to try recrystallizing it to shoot for a big, transparent single crystal, but it didn't behave like I thought it would.
According to the wiki, its solubility is 1240 g/L. Accordingly, I tried dissolving 62g in 50mL of room temperature water. This is neat to watch as the
purple solid dissolves into a very dark brown solution. After stirring overnight, I was only able to actually dissolve about 39g(!).
I took the remaining 23g and was able to dissolve that in only 20mL of hot water.
I left both solutions out to cool/evaporate and hopefully precipitate crystals, but after about 4 days there is zero crystallization.
It seems like this compound is indeed very soluble in water, but needs a bit of heat before it will dissolve. Once in solution, it seems reluctant to
come back out.
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blogfast25
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Mr HS:
Solubility of this alum is extremely high. So high that at about 40 C it melts in its own crystal water! I recrystallize this material by taking about
2.5 part of the alum, then add 1 part of water, then heat gently until everything is dissolved, there's no need to boil. On cooling overnight the
crystals then appear as one large mass. The double pyramid structure can clearly be seen in the larger crystals.
How one would go about making these massive mono-crystals, I'm not sure though. I think without knowledge of the entire solubility-temperature curve
it would be hard to do. They are in any case grown from multiple, repeated deposits onto a seed crystal.
[Edited on 16-10-2013 by blogfast25]
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woelen
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Are your solutions perfectly clear? Ferric ion is very prone to hydrolysis and in neutral solution, the free aquated Fe(3+) ion cannot exist. Aqueous
Fe(3+) is nearly colorless (and as you know, solid ferric ammonium sulfate or solid ferric nitrate has a very pale color), the brown color of
solutions of iron(III) are due to hydrolysis:
Simplified: Fe(3+) + H2O --> [Fe(OH)](2+) + H(+)
If you add a few drops of dilute sulphuric acid, then the liquid should become much lighter and then you should be capable of producing nice crystals.
If you do not have a slight amount of acid in the solution, then you may get a brown contamination of your solid, these are small hydroxide particles
encapsulated in the crystalline solid.
Do not add hydrochloric acid, because that forms the intensely yellow colored complex FeCl4(-).
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blogfast25
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Yes. During synth I always have a slight excess H2SO4 present..
But for recrystallization purposes, pure water is good enough. I think the acidity of the ammonium ions is enough to prevent ferric hydroxide to form.
The other remaining mystery with ferric alum is the colour. I've obtained both lavender and off white versions in seemingly identical conditions. Wiki
shows the off white version. 'kmno4' seems also to have obtained an off white product. A commercial sample I bought years ago was lavender. And Mr
HS's Sigma Aldrich product was also lavender.
And my latest batch (post) above was also lavender. So what causes this colour difference which is quite striking?
[Edited on 16-10-2013 by blogfast25]
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MrHomeScientist
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Yep my solutions are clear as well, no precipitate at all. They are just very dark reddish brown like picture 3 in the OP. They come up as pH of 1
with pH paper, with no addition of acid. My initial crystals are very lavender, very much like cloudy amethyst.
[Edited on 10-16-2013 by MrHomeScientist]
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ScienceSquirrel
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The account of the reaction is not quoted but my own.
The experiment is written up in my best JACS's style :-)
The solution is a very dark brown but it is not cloudy.
The crystals are a light lavender and they have a melting point within the range reported by Sigma Aldrich et al.
I reckon the product is pretty pure within the limits of home science.
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blogfast25
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Not sure what you mean by this. Higher up I gave you full credit for the acid/peroxide recipe.
The yield with the ammonia/peroxide recipe was just over 70 %. Not spectacular either.
[Edited on 19-10-2013 by blogfast25]
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ScienceSquirrel
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Sorry, my mistake.
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Poppy
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A very large (1 inch) single crystal of the alum was prepared by adding aluminium foil to ferrous ammonium sulphate with a little sulfuric until the
aluminium foil shifted into solution finally forming a massive sample of the so told Fe (III) alum. The solution color was not that red but rather
supposedly a 50/50 mixture of Fe3+and Fe2+ at best. I did not repeat this, but the foil was rolled as to fit about 7 - 10 full spires in a 250mL
beaker.
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blogfast25
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Poppy: was it an actual monocrystal or just one large mass of crystals, clumped together?
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Poppy
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It was really a single crystal, a picture is located somewhere along the subject of iron III!
I just cant believe that, the conditions were harsh, and I tought there had been put a stone in the flask at first sight, then woohoops! A big purple
single crystal!
Careful control of this might yield the fastest greatest crystal one can control AT RT!!!!
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