Ferrous sulfate heptahydrate is an iron compound used as a mineral supplement and to alleviate iron deficiency in soil. It has a pale green crystalline solid and is known as “green vitriol”. It is not toxic but can cause gastrointestinal irritation if taken in excess. It is also used in wastewater treatment and to prevent moss growth on lawns.
Ferrous sulfate heptahydrate (FeSO4 7H2O) is an iron compound formed by the action of sulfuric acid on iron. It is also found in nature as a fairly common mineral melanterite, which is thought to arise from the oxidation of iron sulfides such as pyrite. The compound is a pale green crystalline solid and was called copperas or “green vitriol”. In the early days it was used in the production of sulfuric acid, but this method was superseded in the early 18th century. Ferrous sulfate heptahydrate is now used as a mineral supplement, providing a source of iron, and is applied to soils to alleviate iron deficiency.
It is an ionic compound that has iron in its +2 oxidation state, which means that it forms a cation that supplies two electrons to an anion, in this case the sulfate anion, which accepts the two electrons. Iron can exist in two oxidation states, +2 and +3. In modern chemical nomenclature, these are known as iron II and iron III, but were previously known as “ferrous” and “ferric” respectively. The modern name for ferrous sulfate heptahydrate is therefore iron II sulfate heptahydrate, but it is still often referred to by its old name.
The “heptahydrate” part of the name indicates the presence of seven water molecules in the crystalline form of the compound. This is sometimes known as “water of crystallization” and is found in many metallic salts, such as copper sulphate and cobalt chloride. Heating the ferrous sulfate heptahydrate drives the water away, leaving anhydrous ferrous sulfate, a white, non-crystalline powder. It can be restored to its hydrated crystalline form by adding water. In most contexts, the term ferrous sulfate is used to refer to the heptahydrate.
Stronger heating of the anhydrous form releases sulfur dioxide (SO2) and sulfur dioxide (SO3), leaving iron III oxide (Fe2O3): 2FeSO4 → Fe2O3 + SO2 + SO3. This was the basis of a long outdated method of making sulfuric acid; the sulfur dioxide was combined with water to produce the acid. The severe heating required made the process uneconomical and it was soon halted as cheaper means of production became established.
The human body needs iron in significant quantities for the production of hemoglobin, the protein in red blood cells that carries oxygen. Because ferrous sulfate provides this element in an easily absorbed form, it is used for the treatment of iron deficiency anemia. Women are often prescribed ferrous sulfate tablets during pregnancy to maintain iron levels. Ferrous sulfate heptahydrate is not considered particularly toxic, but exceeding the recommended dose can cause irritation of the gastrointestinal tract, resulting in abdominal pain, nausea and vomiting. Ingestion of excessive amounts can be life-threatening.
Iron deficiency in soils can cause a form of chlorosis in plants which can be remedied by applying ferrous sulphate to the soil. Another horticultural application is in the prevention of moss growth on lawns. Ferrous sulfate is also used in wastewater treatment, often in combination with lime, to remove unwanted substances by precipitation.
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