- Ammonium sulfate is another fertiliser which is commonly used in alkaline soils as it forms slightly acidic solutions in soil water, caused by the ammonium ion, NH4+.
- It is prepared on large scale industrial proportions but can also be prepared in the laboratory.
- In the laboratory it is prepared by titrating ammonia with sulfuric acid:
2NH3 + H2SO4 → (NH4)2SO4
- The titration method is used since both reactants are soluble.
Preparation of Ammonium Sulfate in the Laboratory
- To prepare ammonium sulfate by titration:
2NH3 + H2SO4 → (NH4)2SO4
- Dilute ammonia solution, dilute solution of sulfuric acid, methyl orange indicator
- Clamp and stand, burette and volumetric pipette, conical flask, white tile
Experimental set-up for a titration
- 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, swirling the flask in between.
- Continue until the colour turns red sharply and record the titre.
- 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 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.
- The industrial preparation of ammonium sulfate is a large scale operation consisting of several stages.
- The raw materials are ammonia and sulfuric acid which must first be either produced or purchased.
- Often they are produced on-site which is more economical than buying them.
- 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 60ºC anc concentrated sulfuric is added gradually.
Diagram showing the multiple steps in the industrial production of ammonium sulfate