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6.1.4 Making Fertilisers

Essential Elements

  • 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

6-1-4-fertilisers--their-essential-elements

Making Fertilisers

  • 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

Using-titration-to-prepare-a-salt1, IGCSE & GCSE Chemistry revision notes

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

Industrial Production of Fertiliser

  • 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 

industrial-production-of-fertiliser-corrected

 

Exam Tip

You might have to identify errors within a method for making a fertiliser within a lab, so make sure you know the steps involved and why they are done. 

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