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Author: Subject: Unexpected green crystals from a failed attempt on making ferric alum
Science Nerd
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shocked.gif posted on 7-7-2019 at 21:01
Unexpected green crystals from a failed attempt on making ferric alum


I tried making ferric alum by mixing ferric sulfate and ammonium sulfate, but got green crystals after left the solution to evaporate for several weeks :o
I suspect they are crystals of Mohr's salt, but that means something must have reduced the Fe3+ into Fe2+, how did that happened?
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[*] posted on 7-7-2019 at 21:03


Also, the ferric sulfate didn't all dissolve, and the solution turned yellow green.
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[*] posted on 7-7-2019 at 23:53


Where did you get your ferric sulphate from? I sounds as though its rather impure. Ferric sulphate when pure occurs as pale lilac coloured crystals or as a slightly buff coloured powder if it is the lower hydrate type. Either way ferric sulphate hydrolyses to produce insoluble brown-yellow basic compounds in contact with water until enough sulphuric acid has been liberated to prevent further hydrolysis. You can prevent this by adding sulphuric acid but this does not result in reduction of the Fe3+ and the initial formation of a green solution from the "ferric sulphate" suggests that you have much Fe2+ in your original material. It may be that after several week of exposure to direct sunlight that NH4+ could act as a reducing agent but I have never heard of this effect in the abscence of organic acids.

On balance it sounds like your starting material is impure. You could try recrystallising it from dilute (1M) sulphuric acid but if there is a lot of Fe2+ present then many different mixed Fe2+/Fe3+ crystals can form. Alternatively consult one of the older inorganic chemistry books re the oxidation of ferrous sulphate to ferric sulphate.
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[*] posted on 8-7-2019 at 02:07


I got the ferric sulfate from a local chemical supplier, it's a light brownish yellow powder.
The solution is more of a olive yellow.


[Edited on 8-7-2019 by Science Nerd]
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[*] posted on 8-7-2019 at 02:12


Also, I don't have sulfuric acid.
Would this be pure enough for making potassium ferrioxalate?

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[*] posted on 8-7-2019 at 12:05


Maybe your ferric sulphate have lot of ferrous sulphate impurities. Pipete small amount of your solution in to the test tube and add few drops of H2O2 and see what happens. It should turn colour in to brown-red (maybe it will be more brown and cloudy due to hydrolysis).

Sulfuric acid is easy available in stores with paints and varnishes in concentration of 30-40% (car battery acid). This concentration is high enough for your purposes. This acid have little bit of iron contamination but that's ok - you make iron salt. I use this acid for many synthesis.

[Edited on 8-7-2019 by Bedlasky]
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[*] posted on 8-7-2019 at 12:24


Quote: Originally posted by Science Nerd  
Also, I don't have sulfuric acid.
Would this be pure enough for making potassium ferrioxalate?



?? how should I know! I don't know what experimental details you are following and we don't know what the composition of your material is.
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[*] posted on 8-7-2019 at 16:02


Ok,I will test this with ferricyanide and ferrocyanide.
I want to make potassium ferrioxalate by mix ferric sulfate with potassium oxalate in hot water and recrystallize.
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