While anhydrous ferrous sulfate (FeSO₄) and its most common hydrate, ferrous sulfate heptahydrate (FeSO₄·7H₂O), are generally classified as simple salts, specific hydrated forms, such as FeSO₄·6H₂O, are considered a double salt and are not a complex compound or coordination compound. This distinction highlights the unique crystallographic and chemical behavior of certain ferrous sulfate hydrates.
Understanding Double Salts
A double salt is a type of salt that contains two different simple salts but crystallizes as a single substance. When dissolved in water, a double salt dissociates completely into its constituent ions, which is a key characteristic distinguishing it from complex salts or coordination compounds.
Key Characteristics of Double Salts:
- Formation: Formed by the combination of two different simple salts, usually in equimolar proportions.
- Crystal Structure: Possess a distinct crystal lattice different from either of their constituent simple salts.
- Dissociation in Solution: In aqueous solution, they dissociate into all the ions that comprise the original simple salts. This means they lose their individual identity as a double salt in solution.
- Stability: Tend to be stable only in the solid state.
- Examples: Common examples include potash alum (K₂SO₄·Al₂(SO₄)₃·24H₂O) and Mohr's salt ((NH₄)₂SO₄·FeSO₄·6H₂O).
Ferrous Sulfate and Its Role in Double Salts
Ferrous sulfate (FeSO₄) is a versatile inorganic compound. While FeSO₄·7H₂O (often called green vitriol) is a simple hydrated salt, it often serves as a crucial component in the formation of double salts.
Mohr's Salt: A Prominent Example
The most well-known double salt involving ferrous sulfate is Mohr's salt, chemically known as ferrous ammonium sulfate hexahydrate ((NH₄)₂Fe(SO₄)₂·6H₂O).
- Composition: Mohr's salt is formed by the crystallization of ammonium sulfate ((NH₄)₂SO₄) and ferrous sulfate (FeSO₄) in a 1:1 molar ratio, along with six molecules of water of crystallization.
- Formula: (NH₄)₂SO₄·FeSO₄·6H₂O or (NH₄)₂Fe(SO₄)₂·6H₂O
- Properties: When dissolved in water, Mohr's salt dissociates into Fe²⁺, NH₄⁺, and SO₄²⁻ ions. It is preferred over simple ferrous sulfate for titrations because it is less susceptible to oxidation by air.
- Applications: Widely used in analytical chemistry, particularly in redox titrations as a primary standard.
Double Salts vs. Complex Compounds
It's crucial to differentiate double salts from complex (or coordination) compounds, as they behave very differently in solution.
| Feature | Double Salt (e.g., Mohr's Salt) | Complex Compound (e.g., K₄[Fe(CN)₆]) |
|---|---|---|
| Formation | Two simple salts crystallize together. | Central metal ion bonded to ligands (molecules/ions). |
| Dissociation | Dissociates completely into all constituent ions in water. | Dissociates into complex ion and counter-ions; complex ion retains its identity. |
| Identity in Solution | Loses its individual identity. | Maintains its identity as a complex ion. |
| Bonding | Primarily ionic. | Both ionic (between complex and counter-ions) and covalent (within complex ion). |
| Examples | (NH₄)₂Fe(SO₄)₂·6H₂O, KAl(SO₄)₂·12H₂O | K₄[Fe(CN)₆], [Cu(NH₃)₄]SO₄ |
Practical Insights and Applications
- Stability: Double salts like Mohr's salt offer enhanced stability compared to their constituent simple salts. Ferrous sulfate, for instance, readily oxidizes to ferric sulfate in air. However, in Mohr's salt, the presence of ammonium sulfate helps stabilize the Fe²⁺ ion, making it a more reliable reagent.
- Analytical Chemistry: The precise and stable nature of double salts makes them ideal for use as primary standards in volumetric analysis.
- Crystallography: The study of double salts contributes significantly to understanding crystal packing, hydration, and intermolecular forces.
In conclusion, while "FeSO₄" broadly refers to ferrous sulfate, its classification as a double salt depends on its specific hydrated form. While FeSO₄·7H₂O is a simple salt, specific forms like FeSO₄·6H₂O are considered double salts. More broadly, ferrous sulfate is a key constituent of well-known double salts like Mohr's salt, which exhibit distinct chemical properties and important applications.
