Making fertilisers

Chemistry only

Many fertilisers contain ammonium ions (NH4+) e.g. ammonium nitrate, ammonium sulfate and ammonium phosphate. The starting point for making many of these fertilisers is ammonia. Ammonia is an excellent base. A base is a substance that will neutralise an acid. Recall that all acids contain hydrogen ions (H+) in their formula e.g.

acid molecular formula
hydrochloric HCl
sulfuric H2SO4
nitric HNO3
phosphoric H3PO4

Ammonia being a base will "grab" hydrogen ions (H+) from substances e.g. Ammonia dissolves readily in water to form an alkaline solution called ammonium hydroxide. The ammonia here acts as a base and removes a hydrogen ion from a water molecule (H2O), leaving behind a hydroxide ion OH-

ammonia(g) + water(l) ⇌ ammonium hydroxide(aq)
NH3(g) + H2O(l) ⇌ NH4OH(aq)
Ammonia and water reacting to make ammonium hydroxide, a weak alkali.

We can use the alkali ammonium hydroxide to neutralise acids to produce the compounds found in fertilisers e.g.

1. Using hydrochloric acid to neutralise ammonium hydroxide

ammonium hydroxide(aq) + hydrochloric acid(aq) → ammonium chloride(aq) + water(l)
NH4OH(aq) + HCl(aq) → NH4Cl(aq) + H2O(aq)

2. Using sulfuric acid to neutralise ammonium hydroxide

ammonium hydroxide(aq) + sulfuric acid(aq) → ammonium sulfate(aq) + water(l)
2NH4OH(aq) + H2SO4(aq) → (NH4)2SO4(aq) + 2H2O(aq)

3. Using nitric acid to neutralise ammonium hydroxide

ammonium hydroxide(aq) + nitric acid(aq) → ammonium nitrate(aq) + water(l)
NH4OH(aq) + HNO3(aq) → NH4NO3(aq) + H2O(aq)

4. Using phosphoric acid to neutralise ammonium hydroxide

ammonium hydroxide(aq) + phosphoric acid(aq) → ammonium phosphate(aq) + water(l)
3NH4OH(aq) + H3PO4(aq) → (NH4)3PO4(aq) + 3H2O(aq)
Other reactions are possible if less ammonium hydroxide is present. In the above example 3 moles of ammonium hydroxide react with 1 mole of phosphoric acid. Since the reaction is a neutralisation reaction if we remove the spectator ions then essentially all that is reacting is the OH- ions in the alkali ammonium hydroxide with the H+ ions in the phosphoric acid:
3OH-(aq) + 3H+(aq) → 3H2O(l)
All the other ions remain unchanged in the reaction so are simply there to make up the numbers! They take no part in any reaction - they are spectators! Since the phosphoric acid has 3 hydrogen ions it will require 3 hydroxide ions to neutralise them. However if only 1 or 2 moles of ammonium hydroxide are added then the acid will only be partially neutralised. In the example below only 1 mole of ammonium hydroxide is added, this means it can only react with one of the three hydrogen ions in the acid:
NH4OH(aq) + H3PO4(aq) → (NH4)H2PO4(aq) + H2O(aq)
Or if we add 2 moles of ammonium hydroxide, then it will react with 2 of the H+ ions from the phosphoric acid:
2NH4OH(aq) + H3PO4(aq) → (NH4)HPO4(aq) + 2H2O(aq)

Key points

  • Fertilisers are made in acid-base neutralisation reactions.
  • Hydrochloric acid will gives salts called chloride. Hydrochloric acid in a monoprotic acid, it contains only one acidic hydrogen ion, (H+).
  • Nitric acid will gives salts called nitrate. Nitric acid in a monoprotic acid, it contains only one acidic hydrogen ion, (H+).
  • sulfuric acid will gives salts called sulfate. Sulfuric acid is a diprotic acid, this means it has two acidic hydrogen ions (H+).
  • Phosphoric acid will gives salts called phosphates or hydrogen phosphates. Phosphoric acid is a triprotic acid, this means it has three acidic hydrogen ions, (H+).

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