IRON SULFATE (FESO4): STRUCTURE, PROPERTIES, SYNTHESIS - CHEMISTRY - 2025

The iron sulfate is an inorganic salt having the chemical formula FeSO ₄. It consists of a crystalline solid of a variable color, obtained industrially as a by-product of steel processing.

It is found in nature in different forms, the most common being the ferrous sulfate heptahydrate, FeSO ₄ · 7H ₂ O ("green vitriol", present in the mineral melenterite). This hydrate is easily distinguished by the bluish-green color of its crystals (lower image). Other hydrates have the general formula FeSO ₄ · xH ₂ O, where x varies from 1 to 7.

Iron sulfate crystals heptahydrate. Source: Leiem

Ferrous sulfate heptahydrate loses water molecules on heating and can be transformed into other forms of ferrous sulfate; thus, when heated to 57 ºC, it loses three molecules of water and is transformed into ferrous sulfate tetrahydrate. How many in total can you lose? Seven molecules of water, that is, too much water.

Ferrous sulfate is used in the treatment and prevention of iron deficiency anemia. However, it can have toxic effects, so you have to be careful in its dosage.

On the other hand, this iron salt has numerous uses and applications that include the coloring of textile material and leather; chemical reducing agent; radiation dosimeter; wood preservative agent. It is also used in the prevention of chlorosis in plants, and in engraving and lithography processes.

FeSO ₄ can be oxidized in air to iron (III) sulfate, Fe ₂ (SO ₄) ₃ at a rate that can be increased by temperature, light, or a rise in pH.

Many of the physical and chemical properties of ferrous sulfate, such as solubility in water, melting point, the type of crystals it forms, and density, depend on the number of water molecules incorporated in the crystals; that is to say, of its hydrates.

Structure of iron sulfate

Structure of FeSO4 · 7H2O. Source: Smokefoot

The chemical formula FeSO ₄ highlights that this salt is composed of Fe ²⁺ and SO ₄ ^2- ions in a 1: 1 ratio. Both ions interact through electrostatic forces in such a way that they are arranged in an orthorhombic crystal system; which, logically, corresponds to the anhydrous salt.

In the upper image, on the other hand, the structure of FeSO ₄ · 7H ₂ O is shown. The orange sphere represents the Fe ²⁺ cation, which, as can be seen, coordinates with six water molecules to form an octahedron. The Fe ^{2+ charge} attracts the SO ₄ ^2- anion, and this in turn, if observed, forms a hydrogen bond with the seventh water molecule.

The seventh water molecule (the one that is distant from the octahedron), also forms another hydrogen bond with another water molecule belonging to a neighboring octahedron. The result of these interactions is that the crystal changes from being orthorhombic to monoclinic.

As the crystals of anhydrous FeSO ₄ hydrate, the SO ₄ ^2- anions around Fe ²⁺ are replaced by H ₂ O molecules. These substitutions disturb the d electrons of iron, forcing them to pass through different levels of energies; which are responsible for the color changes from white to bluish green.

Acidity

Some SO ₄ ^2- anions can be protonated from the acidic medium. As a consequence, inside the FeSO ₄ · 7H ₂ O crystals there may be H ₂ SO ₄ molecules if the pH is very acidic; and therefore, touching these beautiful crystals in such conditions can cause serious burns.

Physical and chemical properties

Names

Ferrous sulfate or iron (II) sulfate

Molecular formula

-Anhydrous ferrous sulfate (FeSO ₄)

-Ferrous sulfate heptahydrate (FeSO ₄.7H ₂ O)

Molecular weight

It varies with the degree of hydration of the sulfate. For example, iron sulfate heptahydrate has a molecular weight of 278.02 g / mol; while the anhydrous one has a molecular weight of 151.91 g / mol.

Physical appearance

It also varies with the degree of hydration. For example, the anhydrous form has white orthorhombic crystals; whereas in the heptahydrous form, the crystals are monoclinic blue-greenish.

Odor

Toilet

Density

Anhydrous ferrous sulfate is the densest salt form (3.65 g / cm ³). The heptahydrate form, on the other hand, is the least dense (1.895 g / cm ³).

Melting point

Likewise, this varies depending on the degree of hydration. The anhydrous form has a melting point of 680 ° C (1,856 ° F, 973 K) and the heptahydrate form, 60-64 ° C (140-147 ° F, 333-337 K).

Water solubility

-Monohydrate form: 44.69 g / 100 ml of water (77 ºC)

-Heptahydrate form 51.35 g / 100 ml of water (54 ºC).

Solubility in alcohol

Insoluble.

Vapor pressure

1.95 kPa (heptahydrate form)

Refractive index

1,591 (monohydrate) and 1,471 (heptahydrate).

Stability

In air it can oxidize rapidly and becomes covered with a yellow-brown color, which indicates the presence of the Fe ³⁺ cation. The rate of oxidation is increased by the addition of alkali or by exposure to light.

Decomposition

When heated to decomposition it emits toxic fumes of sulfur dioxide and sulfur trioxide, leaving a reddish iron oxide as a residue.

Reactions

It is a reducing agent that acts on nitric acid reducing it to nitrogen monoxide. Likewise, it reduces chlorine to chloride, and the toxic forms of chromium present in cement to chromium (III), of less toxicity.

Synthesis

From steel wool

Ferrous sulfate is produced by reacting steel (Fe) with sulfuric acid. In the method described, the following procedure is followed: the steel is used in the form of steel wool, which is previously degreased with acetone.

Next, the steel wool is placed in a glass beaker and completely covered with 30-40% sulfuric acid, allowing acid digestion to occur for several hours; until the steel wool disappears. More steel wool can be added and the procedure repeated several times.

Green crystals that may have formed are redissolved using water acidified to pH 1-2 with sulfuric acid. This solution is filtered on filter paper, and the pH is adjusted by adding sodium carbonate. The solution is stored, to avoid its contact with oxygen, and thus discourage the oxidation of Fe ²⁺ to Fe ³⁺

Subsequently, the filtrate is subjected to evaporation at a temperature between 80-90 ºC. The procedure is carried out in Pietri capsules placed on a heating plate. Then, the green crystals formed are collected, which can be taken to a desiccator to complete their dehydration.

From pyrite

Ferrous sulfate is also produced by oxidation of pyrite (FeS ₂).

2 FeS ₂ + 7 O ₂ + 2 H ₂ O => 2 FeSO ₄ + 2 H ₂ SO ₄

Risks

Inhalation of FeSO ₄ causes irritation to the nose, throat and lungs. If you have physical contact with this salt, it can cause skin and eye irritation; in addition, prolonged contact with the latter can cause a brownish stain and eye damage.

Repeated ingestion can cause nausea, vomiting, stomach pain, constipation, and irregular bowel movements.

Signs of ferrous sulfate poisoning include: black or bloody stools; bluish skin and nails; changes in the volume of urine excreted; fainting; dry mouth or eyes; chest pain; eat; respiratory distress

In addition, rapid and irregular heartbeat, increased thirst and hunger, unusual paleness, and shortness of breath may occur.

Altered coagulation is an indication of ferrous sulfate poisoning, with prolongation of thrombin, prothrombin, and partial thromboplastin times being observed.

The studies carried out on the effect of ferrous sulfate on isolated muscles of the heart of rabbits, allowed to observe that it produced a reduction in the maximum tension developed by the studied cardiac muscles, as well as the maximum speed of tension development.

Applications

In agriculture

-It is used as a pesticide to control the pinch of wheat and the decomposition of fruit trees.

-It is used in the treatment of chlorosis, a disease characterized by the yellowish color of the leaves, caused by the alkalinity of the soils.

-Ferrous sulfate controls alkalinity, lowering the pH of soils.

- Eliminates moss and conditions the lawn.

As a reagent and in industry

Among the uses of FeSO ₄ as a reagent and in industry there are the following:

-Analytical reagent

-Raw material for obtaining ferrite and magnetic iron oxide

-Ingredient for the elaboration of the inorganic blue pigment

-Reagent reducing nitric acid, chlorine and chromium

-In the manufacture of other sulfates

-It is used in electroplating baths with iron

-Wood preservative

-In aluminum etchings

-Qualitative analysis of nitrates (brown yellow test by oxidation of Fe ²⁺)

-Colymerization catalyst

-It is used as a precursor to the synthesis of other irons

-It is used industrially as a stain fixer

-In the manufacture of iron dye

-Mordient in wool staining

-To give maple wood a silver color

-Iron catalyst in the Fenton reaction

In medicine and for food fortification

It is used in the treatment of iron deficiency anemia, using a dose of 150-300 mg of ferrous sulfate, three times a day, which produces a perceptible increase in the concentration of hemoglobin in a week of treatment.

Its use has also been recommended in pregnant women as a supplement to their diet. Ferrous sulfate has been used as an astringent in wound healing in cattle.

Others

It is used in the treatment of wastewater by flocculation and also for the elimination of phosphate from these waters. Ferrous sulfate heptahydrate is used in the identification of the types of fungi.

References

1. CR Scientific. (sf). Lab preparation of ferrous sulfate. Recovered from: crscientific.com
2. Werner H. Baur. (1964). On the crystal chemistry of salt hydrates. III. The determination of the crystal structure of FeSO ₄.7H ₂ O (melanterite). Acta Cryst. doi.org/10.1107/S0365110X64003000
3. PubChem. (2019). Ferrous sulfate heptahydrate. Recovered from: pubchem.ncbi.nlm.nih.gov
4. Marks Lynn. (December 19, 2014). What Is Ferrous Sulfate (Feosol)? Every Health. Recovered from: everydayhealth.com
5. Wikipedia. (2019). Iron (II) sulfate. Recovered from: en.wikipedia.org