Making Fertilisers (OCR GCSE Chemistry A (Gateway)) : Revision Note

Essential Elements

Higher Tier Only

• 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

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

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