What Is the Connection between Ammonia and Sulfuric Acid?

Ammonia and sulfuric acid are two of the most widely used chemicals and are manufactured in huge quantities throughout the world. Both have a variety of uses, but the major connection between ammonia and sulfuric acid is due to their use in the fertilizer industry. Ammonia is used for the production of a number of fertilizers, such as urea, ammonium sulfate, ammonium nitrate and, indirectly, other nitrate fertilizers. Sulfuric acid is used in the manufacture of “superphosphate” fertilizer and ammonium sulfate, made by combining it with ammonia.

The industrial production of ammonia (NH₃) is carried out mostly by the Haber-Bosch process, whereby hydrogen and nitrogen are combined directly by heating under pressure with a catalyst — usually a mixture of iron oxides. Most ammonia is used in the production of fertilizers. The catalytic oxidation of ammonia to nitrogen dioxide (NO₂) is nowadays the main industrial process for the manufacture of nitric acid, which is used in the production of nitrate fertilizers such as ammonium, sodium and potassium nitrate. The term "ammonia," strictly speaking, refers to the pure compound, which is a gas at room temperature; however, the term is often used to refer to a solution of ammonia gas in water.

There are two main processes for the industrial production of sulfuric acid. The first to be introduced involves the oxidation of sulfur dioxide — produced by the burning of sulfur or the heating of sulfur-containing minerals — by nitrogen dioxide (NO₂) through the reaction: SO₂ + NO₂ -> SO₃ + NO. The nitric oxide (NO) produced by the reaction is oxidized back to NO₂ on contact with air, and so can be recycled.

Ammonia is the source of NO₂ for this reaction, being produced by catalytic oxidation as per the nitric acid production process. The sulfur trioxide is then combined with water to produce sulfuric acid. The greater part of the sulfuric acid manufactured today, however, comes from the oxidation of sulfur dioxide in air using a vanadium pentoxide catalyst.

Like ammonia, one of the major uses of sulfuric acid is in the production of fertilizers. Phosphate rock and bone meal from animals can be used to produce “superphosphate” — monocalcium phosphate — by the reaction: Ca₃(PO₄)₂ + 2H₂SO₄ -> 2CaSOsub>4 + Ca(H₂PO₄)₂. Sulfuric acid and ammonia can also be combined to give the salt ammonium sulfate: 2NH₃ + H₂SO₄ -> (NH₄)2SO₄. This salt is an important fertilizer, acting as a source of both nitrogen and sulfur, and helping to lower the pH of alkaline soils.

Another connection between ammonia and sulfuric acid is in the explosives industry. Almost all commercial and military explosives are nitrated organic compounds, such as trinitrotoluene (TNT). The nitration process involves the replacement of hydroxyl (OH) groups in the organic compound with nitro (NO₂) groups and is usually carried out by treating the compound with a mixture of nitric and sulfuric acids. This mixture provides the nitronium (NO₂⁺) ions for the nitration process. Ammonia is the main raw material used in the production of nitric acid.

Ammonia and sulfuric acid are also found as pollutants in the atmosphere. Sulfuric acid comes from the natural oxidation of sulfur dioxide produced by the burning of fossil fuels and by volcanic activity. Ammonia comes from the decay of nitrogen-containing organic materials. The two can combine in the atmosphere to produce particles of ammonium sulfate by the same reaction as in the industrial production of this compound, or to produce ammonium hydrogen sulfate — also known as ammonium bisulfate — by the reaction NH₃ + H₂SO₄ -> (NH₄)HSO₄. These compounds are very soluble in water and so tend to absorb atmospheric water vapor, causing them to act as condensation nuclei; along with other particulate compounds in the atmosphere, natural and man-made, they are thought to be an important factor in cloud formation.