Crystal growing: Mohr's salt

Some fresh crystals of the common laboratory preparate of iron (II): Mohr's salt, or ammonium iron sulfate hexahydrate (NH₄)₂Fe(SO₄)₂·6H₂O. (Whole gallery)

Some crystals of Mohr's salt. Cell size is 5mm.

Among other iron (II) compounds it is notable for its stability when exposed to air. Most other salts are easily oxidized by air, but Mohr's salt can be stored for years without notable changes (by some reports). This (together with non-toxicity and availability) makes it a good material for hobby crystal growing.

Obtaining

Surely, the easiest way to get Mohr's salt is to buy it in the chemicals store. It is the most boring way too.

I prepared my salt from more common compounds:

• Iron (II) sulfate Fe₂SO₄ from the gardener's store,
• Sulfuric acid H₂SO₄, used as electrolyte for acid accumulators,
• And ammonia NH₃ from the drug store, in a 10% aqueous solution.

First step was to prepare ammonia sulfate from the NH₃ solution and sulfuric acid, by equation:
$$ 2NH_3 + H_2SO_4 \rightarrow (NH_4)_2SO4 $$
The reaction is very exothermic, and must be done carefully. With 10% NH₃ and 43% H₂SO₄, temperature reached 80-90°. Moreover, NH₃ is evaporating quickly from the hot solution, while the reaction is not finished. So wear goggle, do it outside and ensure that the reaction vessel will not crack of sudden temperature change. I added acid with a small excess.

Then I added Fe₂SO₄·7H₂O to the hot solution. After stirring, everything dissolved, giving brown solution (pure compound is green, brown color is caused by Fe(III) contamination). After evaporating it partially and cooling, light-green crystals of Mohr's salt precipitated.
$$(NH_4)_2SO_4 + FeSO_4 \cdot 7H_2O \rightarrow (NH_4)_2Fe(SO_4)_2 \cdot 6H_2O \downarrow + H_2O $$
To grow bigger crystals, I used them for traditional growing procedure with a glass and a thread.

Three biggest crystals

Growing

The compound easily forms large, pale-green crystals. I used slow evaporation technique, with a seed crystal suspended on a thin nylon thread. Crystalline Mohr's salt has good resistance to oxidation by air, but its solution is more prone to it. Initially light green solution gradually becomes brown in a couple of days. To reduce oxidation, I added some excess of the sulfuric acid to it. There are also reports, that additional acid improves crystal clarity too.

Crystals are monoclinic, their form depends on impurities and additions. When solution is fresh, flat tabular crystals grow.

Crystals grown from the fresh solution has flat tabular form.

From the older solution, contaminated with Fe³⁺ ions, more elongated crystals are growing.

Crystal grown from the older solution is elongated.

Despite all my efforts, I failed to grow crystals with even perfect faces. In all cases, faces had irregularities crystals were not entirely transparent. I tried to add glycerol (which gives good results with copper sulfate), which caused flattening of crystals and slightly improved transparency.

Safety notes

The compound itself is generally safe and low toxic, its pure form is used as iron supplement for humans. Acidic solution is corrosive and irritant. The preparation procedure I used, involving hot acid and ammonia could be dangerous. Also, spilled solution leaves permanent stains on everything, handle it with caution.