Inorganic Chemicals

Formula : NH4NO3; MW 80.043

Uses

The ammonium salt produced or consumed in largest amounts is ammonium nitrate. It is used widely as a fertilizer, and is the leading nitrogen fertilizer in the world. An advantage of this compound over other ammonium fertilizers is that it provides both nitrate and ammonia to soil without changing the pH. Also, it is used as a mixed fertilizer with other compounds, such as calcium phosphate, or calcium carbonate. It also is used as an explosive for blasting, or as an ingredient of various mines, or in highway construction. The salt itself, or in combination with fuel oil, powdered aluminum, or carbonaceous matter, is a high explosive. Its blend with trinitrotoluene, known as Amatol, is a military explosive. Other uses of ammonium nitrate are in the manufacture of nitrous oxide, an anesthetic, and as a component of freezing mixtures.

Physical Properties

White crystalline solid; occurs in five different crystallographic modifications as follows:

(i) tetragonal form below –18°C
(ii) rhombic form between –18 to 32.1°C
(iii) rhombic form between 32.1 to 84.2°C
(iv) tetragonal form between 84.2 to 125.2° C
(v) cubic form between 125.2 to 169.6°C;

density 1.725 g/m³ at 20°C; highly hygroscopic; the finely divided powder cakes to a hard mass on storage; melts at 169.6°C; extremely soluble in water; its solubility in 100 g water is as follows:

Dissolution is endothermic, solution becomes cold (and hence its application in freezing bath); elevates the boiling point of water by 1°, 7.5°, 28.5° and 70°C at 10, 40, 80 and 95% (w/w) concentrations, respectively; vapor pressure of saturated solution, 11.2 torr at 20°C.

Thermochemical Properties

Manufacture

is made by the neutralization reaction of ammonia with nitric acid:

The reaction is carried out in aqueous phase using a slight excess of nitric acid. The heat of reaction is utilized to evaporate the water. Also, evaporation may be carried out under vacuum. Alternatively, solid ammonium nitrate is obtained by crystallization from a concentrated solution. The particle size of the dry product may be controlled by vacuum crystallization, granulation or other processes. (Young, R.D. 1978. Ammonium Compounds. In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed., Vol. 2, pp. 525–532. New York: Wiley Interscience.) The solid powder should be protected from moisture to
minimize caking.

Ammonium nitrate alternatively may be prepared by double decomposition reactions of ammonium salt with a nitrate salt; e.g., ammonium sulfate and sodium nitrate:

Reactions

Ammonium nitrate volatilizes reversibly with dissociation at moderate temperatures:

Thermal decomposition occurs at 170°C producing nitrous oxide and water:

evolves smoothly; however, above 250°C or if the solid is strongly shocked, violent decomposition occurs:

Aqueous solutions of ammonium nitrate undergo a double decomposition reaction with metal salts. acts as an oxidizing agent in aqueous solutions and is reduced by various metals at ambient temperatures.

Spongy copper slowly reduces it to ammonium nitrite in the absence of air; however, no reaction occurs with copper wire or gauge. (Basset, H. and R. G. Durrant. 1922. J. Chem. Soc., 121, 2631):

Chemical Analysis

Ammonium nitrate dissociates in aqueous solution to and ions. The former may be measured by ammonium ion-selective electrode and the latter by nitrate ion-selective electrode. The solid may be heated carefully at low temperature (around 90°C) and the evolved ammonia and nitric acid vapors are absorbed in water and measured by selective ion electrodes, respectively.

Hazard

Heating ammonium nitrate can present a severe explosion hazard. When heated above 210°C, its decomposition is exothermic, producing nitrous oxide and water vapor. In closed confinement, heating the molten mass can cause a pressure build-up. Above 300°C, there is rapid evolution of nitrogen, water vapor and oxygen—two mols solid producing seven mols of gaseous products. This can cause a dangerous explosion. In the presence of readily oxidizable substances, such as fuel oil soaked into the pores of the solid or finely divided metal, the ignition is self-sustained—occurring at lower temperatures, and the explosivity is enhanced. Also, it can explode dangerously in a fire. At ordinary temperatures, the compound is stable and safe to handle. Calcium carbonate, phosphate or other substances are mixed with fertilizer grade ammonium nitrate to reduce its explosivity. There are many cases of loss of human lives from ammonium nitrate fire or explosion.

Tags: AMMONIUM NITRATE, NH4NO3; MW 80.043, NITRATE

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