Pseudomonia
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Purification of plant foods
Yesterday I bought a 1 Kg tub of 'sulphate of iron' plant food tonic and a 1kg tub of 'sulphate of ammonia' green vegetable plant food for a
reasonable price. So as you can expect I wasn't expecting Lab pure stuff.
I intend to purify both these substances.
First with the 'sulphate of Iron' I looked at the MSDS. As I expected it's 95% Iron (II) Sulphate. Visually it looks a mess, it is a regular green
colour but with red to brown specs.
Secondly; 'Sulphate of ammonia' visually looks as I would expect, colourless crystals of Ammonium Sulphate. Then I checked the MSDS and to my surprise
it contains 5% sand!
My Question is this, would you recommend any special purification procedures or do you think that simple filtration and recrystallisation would do?
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Lambda-Eyde
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The ammonium sulfate should be OK by just crystallising.
The iron(II) sulfate, however, might be troublesome as it easily oxidizes in aqueous solution.
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blogfast25
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Is it ferrous or ferric sulphate you're wanting?
Your garden grade is probably best dissolved then filtered, then oxidised completely to ferric (commercial bleach should do it). Then carefully
alkalise to precipitate Fe(OH)3. n H2O. Wash copiously to obtain relatively pure Fe(OH)3.n H2O.
This will dissolve in strong HCl, HNO3 or H2SO4 giving you a relatively pure solution of the corresponding ferric salt (filter again). As Lamda Eye
claims, Fe2+ in solution oxidises in the presence of air, so reducing the solutions back to ferrous and keeping them in the +II oxidation state isn't
so easy...
Your ammonium sulphate garden grade is certainly better than mine, which looks brown and contains both sand and organics! But mere filtering and
recrystalising already improved it a lot. Filtering over activated carbon (Norit) would probably brighten it a lot more... A few successive
recrystallisations will also improve it a lot. Don't forget to allow some mother liquor above the crystallite, that where most impurities will gather.
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Pseudomonia
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I would much prefer to keep ferrous sulphate rather then converting it to ferric sulphate. I've realised that the red/brown specs i've found in the
powder are probably Iron oxides and ferric sulphates. Apart from that it's a pretty nice powder and I think i'll just keep it as a bog standard
technical grade reagent.
As for the the Ammonium sulphate, one of the other boxes I looked at (which looked alot older and was alot cheaper) was like yours, very brown. What I
can see here however is a good grade of Ammonium Sulphate so i'll have fun with the recrystalisation.
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blogfast25
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Yep. My 'plant food' FeSO4 dissolves more or less completely to a slightly turbid solution. I'm guessing after quick filtering the Fe is
overwhelmingly in the +II state. But don't leave it to stand in open air or you can practically see the rust being formed under your very eyes!
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ScienceSquirrel
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The purification of iron II sulphate is quite straightforward.
The purification has to be carried out out in acidic solution with some iron metal present to maintain a reducing environment.
I have done it using some nice clean steel wool and some clean technical grade sulphuric acid.
Add a few ml of acid to 100ml of water and then add a piece of clean rust free steel wool degreased with acetone, a few grams is plenty. Stopper the
flask with a piece of cotton wool to minimise gas exchange and acid spray and stir magnetically while warming to around 55C.
You can see the reason why this temperature is used from the solubility table here.
http://en.wikipedia.org/wiki/Solubility_table#I
Start adding the technical grade iron sulphate by the spatula load until you have a slight excess, good temperature controi is important as the
solubility curve is hump shaped, too hot or cold and you will not get the best yield or you will get another hydrate coming out of solution.
Filter the solution into a pre warmed flask and then stopper with cotton wool and cool in the fridge.
A lovely crop of blue green crystrals of hydrated iron II sulphate will be your reward, filter quickly and rinse with ice cold water acidulated with a
few drops of concentrated sulphuric acid, press dry and finish in a dessicator or in a sealed jar over calcium chloride.
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blogfast25
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Nice one...
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Pseudomonia
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That seems like a nice simple method I will have a go at that
Thank you 
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densest
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@ScienceSquirrel - thanks for the solubility table reference!!!
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woelen
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Another interesting option is to make Mohr's salt (ferrous ammonium sulfate) from the impure ferrous sulfate. Ferrous ammonium sulfate more easily
forms good crystals and this compound also keeps much better. It is not easily oxidized to basic ferric sulfate like normal ferrous sulfate.
Making Mohr's salt can be done by dissolving the impure ferrous sulfate in a slightly acidic solution (a few drops of sulphuric acid added to keep it
somewhat acidic) and filtering this such that a clear solution is obtained. To this, a calculated amount (slight excess) of solution of ammonium
sulfate is added and then the liquid is allowed to crystallize slowly (e.g. put it in a dry warm place, free of dust). You will obtain pale mint-green
crystals of (NH4)2Fe(SO4)2.6H2O.
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DJF90
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Yes Mohr's salt is stable to aerial oxidation compared to ferrous sulfate. I recall a teacher at school having some in a petri dish on the window
sill. It had collected alot of dust but no speck of brown. Apparently it had sat there (in the open petri dish) for three or four years!
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blogfast25
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Is there real proof that Mohr's salt is significantly more resistant to oxidation than FeSO4 neat? I've seen a Mohr crystal about the same size of a
fist in an old (and somewhat delapidated) science museum: it had lost quite a bit of water around the edges and oxidation was clearly visible. But it
was, in my estimation about 10 years old...
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Pseudomonia
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Quote: Originally posted by woelen  | Another interesting option is to make Mohr's salt (ferrous ammonium sulfate) from the impure ferrous sulfate. Ferrous ammonium sulfate more easily
forms good crystals and this compound also keeps much better. It is not easily oxidized to basic ferric sulfate like normal ferrous sulfate.
Making Mohr's salt can be done by dissolving the impure ferrous sulfate in a slightly acidic solution (a few drops of sulphuric acid added to keep it
somewhat acidic) and filtering this such that a clear solution is obtained. To this, a calculated amount (slight excess) of solution of ammonium
sulfate is added and then the liquid is allowed to crystallize slowly (e.g. put it in a dry warm place, free of dust). You will obtain pale mint-green
crystals of (NH4)2Fe(SO4)2.6H2O. |
I like the sound of that and at least i've now got a use for both my plant foods! Thanks
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