Ferrous Sulphate

Ferrous Sulphate

This is the most important ferrous salt. It is also called green vitriol. We also call it as copperas because it is a by product of the hydrometallurgy of copper. In the laboratory it is prepared: (i)By dissolving iron in dil. H₂SO₄filtering, evaporating and crystallizing.
Fe + H₂SO₄ → FeSO₄+ H₂
(ii)From Kipp’s waste: Kipp’s waste (or residue) contains FeSO₄along with  free H₂SO₄. The waste is heated with a small quantity of scrap iron which reacts with excess of H₂S to give FeSO₄and nascent hydrogen. This nascent hydrogen reduces any ferric iron, if present, to ferrous state. On allowing the hot solution to stand, crystals of FeSO₄.7H₂O separate out. On a large scale it is obtained by the slow oxidation of iron pyrites in the presence of air and moisture. The pyrites are exposed to air in big heaps and the following reaction takes place.
2FeS₂ + 2H₂O + 7O₂ →  2FeSO₄+ 2H₂SO₄
The free H₂SO₄is removed by the addition of scrap iron, which also reduces any ferric sulphate formed. On crystallization we get green coloured crystals containing seven molecules of water of crystallization. These crystals are monoclinic and are isomorphous with MgSO₄.7H₂O. Ferrous sulphate containing 6, 5, 3, 2, 1 molecules and no water of crystallization is also known.

Properties

• Ferrous sulphate forms pale green monoclinic crystals of heptahydrate, FeSO₄.7H₂O which is isomorphous with the corresponding heptahydrates of cobalt, nickel, chromium, manganese, magnesium, zinc, etc i.e. with MSO₄.7H₂O where M = Co, Ni, Cr, Mn, Mg, Zn etc. Crystalline hydrates having 6, 5, 4, 3 and 2 water molecules are also known. FeSO₄.5H₂O is isomorphous with blue vitriol, CuSO₄.5H₂O.Anhydrous salt (i.e. FeSO) is white.
• Action of heat: When heated to redness, FeSO₄ and FeSO₄.7HO both decompose giving a residue of Fewhich is used as a pigment under the name of rogue or venetian red.
  2FeSO₄Fe₂O₃+ SO₂ + SO₃
  2[FeSO₄.7H₂O]Fe₂O₃ + H₂ SO₄ + SO₂ + 13H₂O
• Reducing Properties: It is an active reducing agent, e.g.
  It decolorizes acidified KMnO₄ solution
  2KMnO₄+ 10FeSO₄ + 8H₂SO₄ → K₂SO₄+ 2MnSO₄+ 5Fe₂(SO₄)₃ + 8H₂O
  It turns acidified K₂Cr₂O₇ green
   → K₂SO₄ + Cr₂(SO₄)₃ + 3Fe(SO₄)₃ + 7H₂O
  It reduces NO₂ to NO which then gives nitrosoferrous sulphate, Fe(NO)SO₄
  2FeSO₄ + H₂SO₄ + → Fe₂(SO₄)₃ + H₂O + NO
  FeSO₄ + NO Fe(NO)SO₄
  Nitrosoferrous sulphate
  It reduces silver and gold salts to metal.
  Ag₊ + Fe₂₊ → Ag↓ + Fe₃₊
  Au₃₊ + 3Fe₂₊ → Au↓ + 3Fe₃₊
  It reduces HgCl₂ + 2Fe₂₊ → Hg₂Cl₂ + 2Fe₃₊ + 2Cl₋
• Formation of addition compound with NO: A cold solution of ferrous sulphate absorbs NO forming dark brown nitroso ferrous sulphate. Fe(NO)SO₄ which is destroyed by the action of heat.
  FeSO₄NO Fe(NO)SO₄
  Nitroso ferrous sulphate


• Action of air: When exposed to air, the green crystals of FeSO₄effloresce and turn brown due to formation of basic ferric sulphate, Fe(OH)SO₄. Its formation is due to the surface oxidation of FeSO₄ to Fe(OH)SO₄.
  4FeSO₄ + 2H₂O + O ₂ → 4Fe(OH)SO₄
  GreenBrown
• Formation of double salts: With sulphates of alkali metals it readily forms double salts with the composition, R₂SO₄.FeSO₄.6H₂O where R = an alkali metal or radical. Out of these double salts, ferrous ammonium sulphate (called Mohr’s salt), (NH₄)₂SO₄.FeSO₄.6H₂O is of great use.


• Acid character: It is readily soluble in H₂O, the solution is acidic in nature due to its hydrolysis
  FeSO₄ + 2H₂O Fe(OH)₂ + H₂SO₄
  The solution readily passes into ferric state by atmospheric oxidation.
  4FeSO₄2H₂SO ₄ + O₂ → 2Fe(SO₄)₃ + 2H₂O
• Action of KCN: When excess of KCN is added to ferrous sulphate solution, potassium ferrocyanide, K₄[Fe(CN)₆] is formed.
  FeSO₄ + 2KCN  →  Fe(CN)₂+ K₂SO₄
  Brown ppt.
  Fe(CN)₂ + 4KCN  → K₄[Fe(CN)₆]
  ——————————————————
  FeSO₄+ 6KCN → K₄[Fe(CN)₆] + K₂SO₄

Uses

• In the manufacture of writing ink, ferrous ammonium sulphate (Mohr’s salt), iron alum. ferric oxide (rouge) etc.
• As a mordant in dyeing
• As a weed−killer in agriculture.
• As a reducing agent.
• As a reagent in the laboratory.
• For clarifying water.