User:Brutulf/Ferric Ammonium Sulfate

Ferric Ammonium Sulfate Doceahydrate, NH4Fe(SO4)2 · 12 H2O ('FAS'), is a double salt in the class of aluns, which consists of compunds with the general formula AB(SO4)2 · 12 H2O. It has the appearance of weakly violet, octahedrical crystals. There has been some discussion regarding the origin of the crystals' colour, with some ascribing it to impurities in the compound, and others claiming it to be a property of the crystal itself. FAS is paramagnetic, acidic and toxic towards microorganisms.

Areas of use for FAS include waste water treatment, tanning , production of dyestuffs , and as an etching agent in the production of electronic components. It has been used in a wide area of applications, including adiabatic refrigeration equipment, biochemical analysis and organic synthesis.

Preparation
FAS can be prepared by crystallization from a solution of ferric sulfate and ammonium sulfate. Iron(II) in ferrous sulfate is oxidised to Iron(III) in ferric sulfate by addition of sulfuric and nitric acid. Upon addition of ammonium sulfate to the solution and damping in of the solution, ferric ammonium sulfate crystals will preticipate.

Oxidation: 6 FeSO4 + 2 HNO3 + 3 H2SO4 = 3 Fe2(SO4)3 + 2 NO + 4 H2O

Synthesis: Fe2(SO4)3 + (NH4)2SO4 = 2 NH4Fe(SO4)2

Procedure :

Add 75 mL diluted sulfuric acid to a 250 mL Erlenmeyer flask. Dissolve 25 g iron(II) ammonium sulfate (Mohr's Salt) in the flask while heating. Add water if neccesary. Dissolve the neccesary amount of iron(II)sulfate heptahydrate. Add 5 mL nitric acid to oxidize the iron. Caution: The solution will boil violently if the acid is added too quickly. Boil the solution until no more red fumes rise from the solution.

Test the solution for Iron(II) ion by transferring a drop to a test tube and adding potassium iron(III)cyanide. If the contents of the test tube turns blue, Iron(II) ion is present, and more nitric acid needs to be added to complete the oxidation.

After oxidation, dampen in the solution to about half volume. If the solution becomes sluggish, slowly add water until it regains the consistency of water. Let the solution cool and stand. Crystallization might take several hours to days to start, but may in some cases be started by scratching the inside of the glass with a glass rod or adding a seed crystal.