Making Fertilisers (OCR Gateway GCSE Chemistry)

• Plants need some key elements, such as nitrogen, potassium, and phosphorus, in order to grow well
• If there are limited amounts of these in the soil, the yield of crop will be reduced 
• Farmers use NPK fertilisers, formulations containing appropriate ratios of all three elements
• From these three essential elements:
  • Nitrogen promotes healthy leaves
  • Potassium promotes growth, healthy fruit and flowers
  • Phosphorus promotes healthy roots
• Plant roots only absorb elements in water soluble form, so cannot use nitrogen from the air
• The table below gives the names of some fertilisers and the essential elements they contain:

Table of Fertilisers and their Essential Elements

• Fertilisers can be made both in a lab on a smaller scale, and industrially on a larger scale
• This is a neutralisation reaction involving an acid and an alkali forming a salt
• One example of this is the preparation of ammonium sulfate 

Preparation of Ammonium Sulfate in the Laboratory

• Aim: To prepare ammonium sulfate by titration:

ammonia + sulfuric acid → ammonium sulfate

2NH₃ + H₂SO₄ → (NH₄)₂SO₄

• Materials:
  • Dilute ammonia solution
  • Dilute sulfuric acid
  • Methyl orange indicator
  • Retort stand, boss and clamp
  • Burette
  • Volumetric pipette
  • Conical flask
  • White tile
  • Funnel

Steps in the laboratory preparation of ammonium sulfate

Method:

• Add an exact volume of ammonia to the conical flask and place on the white tile
• Add a few drops of indicator and swirl, it should turn yellow
• Add the acid to the flask solution drop by drop from the burette, swirling the flask in between additions and rinsing down the sides with distilled water
• Continue until the colour turns red sharply and record the titre by reading the volume from the bottom of the meniscus
• Repeat by adding exactly the same amount of acid but this time without the indicator which is an impurity
• Pour the reaction mixture in an evaporating dish and gently heat in a water bath to remove some of the water
• Stop heating when the volume has been reduced to roughly one third of its volume
• Leave in a warm, dry place so the remaining water evaporates, allowing crystallisation to occur
• This may take a few days depending on ambient conditions

Analysis of results:

• After a few days ammonium sulfate crystals should appear
• Filter to remove any remaining water

Preparation of Ammonium Sulfate Industrially 

• The industrial preparation of ammonium sulfate is a large scale operation consisting of several stages
• Ammonia is prepared by the Haber process and sulfuric acid by the Contact process
• Both processes require their own supplies of raw materials, energy and equipment
• The most common industrial process of manufacturing ammonium sulfate involves filling a large reactor chamber with ammonia gas.
• Sulfuric acid is sprayed into the chamber from above and ammonium sulfate powder is produced
• Another method involves pumping a mixture of ammonia gas and steam in a reactor which contains some sulfuric acid and a concentrated solution of ammonium sulfate
• The reaction is carried out at 60 ºC and concentrated sulfuric is added gradually

• Making a fertiliser industrially is a continuous process- the product is made continuously all of the time as long as there are raw materials available. Large quantities can be made.
• Making fertilisers within a lab is a batch process- a small amount is made at any one time, with the equipment cleaned before the next batch can be made.
• At an industrial scale, fertiliser factories have many integrated processes happening at any one time to make multiple fertilisers
• A variety of raw materials are provided such as the sulfur to make sulfuric acid and phosphate rock to make phosphoric acid so that the fertilisers are made simultaneously