Ammonium Nitrate

Ammonium nitrate is a chemical compound that has many uses and is made up of atoms of nitrogen, hydrogen, oxygen. Ammonium nitrate has distinct chemical and physical properties which are used for many purposes. 

Ammonium nitrate is a chemical compound that is made up when an ammonium ion unites with a nitrate ion. The molecule of ammonium nitrate is formed by the union of 2 nitrogen atoms, 4 hydrogen atoms and 3 oxygen atoms. Ammonium has a chemical formula NH⁴⁺ and nitrate has the chemical formula NO^3-. When these two unite they form Ammonium nitrate with a formula NH₄NO₃.  It is a white solid crystal that is readily soluble in water. It is largely used in the agricultural industry as a fertilizer. It is also used as an explosive. However, due to concern over its misuse, many countries are debating about stopping using ammonium nitrate.

What are the Physical Properties of Ammonium Nitrate?

Ammonium nitrate is a solid colourless crystalline substance. It is crystalline. It has no odour of its own i.e. it is odourless. Ammonium nitrate is either white or grey coloured in its pure form. The density of the substance is noted as 1.725 gm per cm³, which makes it easily soluble in different substances. Ammonium nitrate acts stable under normal conditions during its usage and storage but emits nitrous oxide, commonly known as the laughing gas when heated to decomposition. It tends to absorb moisture from the air, making it hygroscopic. 

What is the Molar Mass of Ammonium Nitrate?

The molar mass of Ammonium nitrate is 80.043 gm per mole.

How to Calculate the Molecular Weight of the Ammonium Nitrate?

Let us look at how to calculate the molecular weight of the given compound:

The molar mass of ammonium nitrate= the sum of the molar mass of all its constituents in the correct proportion.

Molecular weight of nitrogen = 14.0067

The molecular weight of hydrogen= 1.00784

The molecular weight of oxygen= 15.9999

The molecular weight of ammonium nitrate= 2 * weight of nitrogen atom + 4 * weight of hydrogen atom + 3 * weight of an oxygen atom

= 28.0134 + 4.03136 + 47.9997

= 80.043.

Where Does Ammonium Nitrate Occur Naturally? 

In nature, ammonium nitrate is found as a natural mineral. It is found as a gigabyte which is called the Ammonium analogue of saltpetre. It is commonly found in the Atacama Desert in Chile. Only the driest regions of the desert are suitable for finding ammonium nitrate. In these regions, it is found as a crust on the ground. It is often found in conjunction with other compounds namely nitrates, iodates and other halide minerals.

Earlier, Ammonium nitrate used to be mined as a mineral from these regions. However, now most, if not all, of the ammonium nitrate is produced chemically.

How is Ammonium Nitrate Produced Industrially?

Most of the Ammonium nitrate that is produced is synthesized chemically. Industrial production of Ammonium nitrate is based on an acid-base reaction between ammonia and nitric acid. A molecule of Nitric acid (acidic) joins together with a molecule of ammonia (basic) to form a single molecule of ammonium nitrate. The reaction is summarized as:

HNO₃ + NH₃ → NH₄NO₃

For this process, ammonia is used in its gas form or anhydrous form. This gaseous ammonia is then added to concentrated nitric acid to form Ammonium nitrate. This reaction is highly exothermic. This means that it produces a large amount of heat energy during the process.

The resultant solution contains as much as 83% of Ammonium nitrate. The solution is then evaporated to remove excess water and obtain an Ammonium nitrate content of 95-99% (depending upon the grade of the product).

This highly concentrated Ammonium nitrate is called Ammonium nitrate melt or AN melt. This AN is then converted into small beads in a spray tower. These beads are called prills. The AN melt is also converted to granules by spraying as well as tumbling inside a rotating drum. These granules or beads are then dried further and cooled. To prevent caking these prills or granules are coated. They are then prepared and sold off as commercial AND products.

This process requires ammonia which is produced by the Haber process of combining hydrogen and nitrogen. The Ammonia produced by the Haber process is then oxidised to nitric acid through the Ostwald process.

Another method of Ammonium nitrate production that is in use at an industrial scale is the Odda process.

In this process, one molecule of calcium nitrate reacts with two molecules of Ammonia, one molecule of carbon dioxide and one molecule of water to produce calcium carbonate and Ammonium nitrate. The reaction is summarized as: 

Ca(NO₃)₂ + 2 NH₃ + CO₂ + H₂O → 2 NH₄NO₃ + CaCO₃

 The resultant products of this reaction are calcium carbonate and ammonium nitrate. These compounds can be separated and purified before being sold. Or they may be sold together as a mixture.

Another method used for Ammonium nitrate synthesis is the metathesis reactions.

The first type of reaction involves one molecule of Ammonium sulphate reacting with one molecule of barium nitrate. The reaction produces two molecules of ammonium nitrate and one molecule of barium sulphate. 

(NH₄)2SO₄ + Ba(NO₃)₂ → 2 NH₄NO₃ + BaSO₄

In the second reaction, one molecule of ammonium chloride reacts with one molecule of silver nitrate. The result is one molecule of Ammonium nitrate and one molecule of silver chloride.

NH₄Cl + AgNO₃ → NH₄NO₃ + AgCl

What are Some Important Reactions for Ammonium Nitrate?

Ammonium nitrate has many important reactions. Some of these are listed below:

1. Reactions with Metal Hydroxides.

When metal hydroxides react with ammonium nitrate alkali metal nitrates are formed. Along with alkali metal nitrates, Ammonia and water are also released. The reaction is summarized below:

NH₄NO₃ + MOH → NH₃ + H₂O + MNO₃ (M = Na, K)

2. Ammonium Nitrate Releasing Nitrous Oxide

When Ammonium nitrate is subjected to decomposition it releases nitrous oxide and water. Both these results are in gaseous form. So on heating Ammonium nitrate decomposes but leaves no residues behind. The reaction is summarized below:

NH₄NO₃ → N₂O + 2H₂O

3. Ammonium Nitrate Releasing Nitrogen

As mentioned earlier, heating Ammonium nitrate releases water vapour and nitrous oxide. However in case, the compound is subjected to rapid heating, the decomposition of the compound yields nitrogen, oxygen and water. This reaction also occurs when ammonium nitrate undergoes explosion. The reaction is summarized as:

2NH₄NO₃ → 2N₂ + O₂ + 4H₂O.

What is the Chemical Structure of Ammonium Nitrate?

This compound is formed through an ionic bond uniting an ammonium ion and a nitrate ion.

The nitrate ions are resonance stabilized and their pi electrons are delocalized. The net charge on the ion is calculated below:

Charge on nitrogen atom = +1

Charge on oxygen = - ⅔

Thus net charge on Ammonium nitrate is -1.

This compound is acidic. This salt readily dissolves in water forming HNO₃ which is a strong acid and NH₃ which is a weak base.

What conditions cause Ammonium Nitrate to explode?

Ammonium Nitrate itself doesn’t burn. It causes explosion by providing oxygen and making suitable conditions for other substances to burn i.e. combustion to occur. It acts as an oxidiser. Ammonium Nitrate beads emit highly concentrated oxygen; the concentration of oxygen is much more than the concentration already present in the air, which makes the combustion process of different substances rapid and immense.

Although, at high temperatures, ammonium nitrate is compatible with decomposing rapidly and releasing various gases- mainly nitrogen oxide and water vapour- that leads to dangerous explosions. Therefore, it is highly risky to store inflammable materials and ammonium nitrate together, especially in areas of warm temperature and with detonators( compounds that trigger the combustion).

Various incidents have shown the destructive property of ammonium nitrate when not handled with excessive caution. One such accident occurred in a factory in eastern China in 2015 and the other explosion occurred recently in Beirut, the capital of Lebanon.

Due to its dangerous, inflammable nature ammonium nitrate has been banned from usage in various countries including Pakistan, Australia, Ireland, United Kingdoms and Germany.

In India, however, its use has not been banned but restricted to only licensed companies. A compound or mixture with more than 45% ammonium nitrate by weight is labelled as a highly inflammable substance limiting its use.

Innovations that have resulted in safer use of Ammonium Nitrate?

There is no doubt that ammonium nitrate is one of the most important and exclusively used fertilizers in the world because of its property to provide significant nitrogen for the production of crops at a cheap price. However, due to its inflammable property, which has caused explosive incidents in various countries, scientists across the world have come up with innovations that ensure more efficient and safer use of this chemical. Such innovations include different forms of coating, mixings, formulations and hygroscopicity (moisture absorbing) techniques- all of which limit the hazardous effects of ammonium nitrate. For example, the ammonium nitrate fertilisers that are now produced come with a safe coating that blocks the infiltration of fuel oils into the pores of the fertiliser, making it safer to use in agriculture. Similarly, different hygroscopic techniques have been invented that involves mixing a substance such as urea double salt which makes the fertilisers in use more secure.

Potassium compounds have also been proven to reduce the inflammable nature of ammonium nitrate in fertilisers and other materials and have ensured its effective use across the world. Alternatively, aluminium sulphate acts as a drying chemical when added to ammonium nitrate-reducing its explosive probability significantly.

How is Ammonium Nitrate stored cautiously?

It is an absolute requirement to store ammonium nitrate under suitable conditions to reduce the destructive consequences of its inflammable properties.

Following are the ways and conditions in which ammonium nitrate is stored, minimising its hazardous effects:

• In ventilated storage units: The storage units where ammonium nitrate is kept need to be adequately ventilated in case of fire. Since, this substance is highly combustible and there is always a probability for it to catch fire, ventilation is the primary necessity in storage units of ammonium nitrate.

• Non-combustible materials: Non-combustible substances are required to be kept along with ammonium nitrate substances to act as resistant compounds during an explosive outbreak.

• Ideal temperature: The temperature of the storage units should be 130 degrees Fahrenheit or less as ammonium nitrate is more prone to catching fire in warm temperatures.

• Ideal volume: Only 2270 tonnes of ammonium nitrate should be stored together. It is necessary to store only a limited amount as the more the quantity the higher the risk of inflammation.

• Complete fire protection and prevention equipment: It is not only advisable but also essential for companies storing ammonium nitrate (in any form) to keep in hand fire protection tools and equipment to control and limit the damage in case of explosions.