Nitric acid, historically known by the Latin term aqua fortis or “strong water,” is a colorless to slightly yellow liquid that serves as one of the most important mineral acids in modern industry. It is a highly corrosive substance and a powerful oxidizing agent. This dual nature as a strong acid and a potent oxidizer makes it versatile and widely used across numerous sectors, from agriculture to high-tech manufacturing.
Primary Role in Fertilizer Production
The single largest application for nitric acid globally is in the production of nitrogen-based fertilizers, consuming approximately 80% of the world’s supply. This demand stems from the acid’s ability to efficiently provide nitrogen, a fundamental nutrient required for plant growth. The industrial synthesis of nitric acid, primarily through the Ostwald process, begins with ammonia, which is reacted with oxygen and water.
The reaction of nitric acid with ammonia is the foundational step for creating ammonium nitrate, the most common nitrogen fertilizer derived from the acid. This acid-base neutralization yields a compound that supplies nitrogen in two forms: the immediately available nitrate ion and the slower-release ammonium ion. The nitrate form is easily absorbed by plant roots, while the ammonium form is gradually converted to nitrate by soil microorganisms.
Nitric acid is also used to produce calcium nitrate, another highly soluble nitrogen fertilizer. The high solubility of these fertilizers ensures that plants quickly absorb the necessary nitrogen, which helps boost crop yields.
Manufacturing Explosives and Rocket Propellants
Nitric acid is a foundational component in the manufacturing of high-energy compounds, including explosives and rocket propellants. Its utility comes from its ability to act as a nitrating agent, a process where a nitro group is introduced into an organic molecule. This nitration reaction, often performed with a mixture of nitric and sulfuric acids, is fundamental to creating energetic materials.
Examples include the nitration of toluene to produce trinitrotoluene (TNT) and the nitration of glycerol to form nitroglycerin. The introduction of multiple nitro groups into these structures makes the resulting compounds highly unstable and energy-rich. These compounds are extensively used in mining, construction for demolition, and military applications.
In the aerospace industry, nitric acid functions as a powerful oxidizer in certain liquid rocket propellants. One notable formulation is Red Fuming Nitric Acid (RFNA), which is concentrated nitric acid containing dissolved nitrogen dioxide. RFNA is often used in hypergolic systems, meaning the fuel and oxidizer ignite spontaneously upon contact without an external ignition source.
Essential Functions in Metal Processing and Chemical Synthesis
Nitric acid’s dual nature as a strong acid and a strong oxidizer makes it indispensable in various industrial processes involving metals and complex chemical synthesis. In metal processing, it is used for passivation, a technique applied to stainless steel and other metals to enhance their corrosion resistance. This involves treating the metal surface to remove free iron and promote the formation of a protective, stable oxide layer.
The acid is also employed in metal etching and cleaning, leveraging its ability to selectively dissolve certain metals. In the electronics industry, this property is used to precisely etch patterns onto copper-clad circuit boards. Furthermore, a mixture of nitric acid and hydrochloric acid, known as aqua regia, dissolves noble metals like gold and platinum, making it useful in precious metal refining.
In organic chemical synthesis, nitric acid serves as a fundamental chemical feedstock for creating consumer products. For instance, it is used in the production of precursors for nylon 6,6, a widely used polymer in textiles and plastics, by oxidizing materials to form adipic acid. It is also an intermediate in the production of chemicals used for creating polyurethanes, foams, and coatings.
Critical Safety and Handling Requirements
Handling and storage of nitric acid require strict adherence to safety protocols. Concentrated nitric acid is a severe hazard because it causes immediate, deep burns upon contact with skin or eyes, and its fumes are toxic. Inhaling the vapors can cause irritation to the respiratory tract and lead to delayed effects like pulmonary edema.
Personal protective equipment (PPE) is mandatory when working with the acid, including chemical-resistant gloves, a face shield, and safety goggles. Operations must be conducted in well-ventilated areas, such as a chemical fume hood, to prevent the buildup of hazardous vapors.
Proper storage is necessary to mitigate the risk of violent reactions. Nitric acid must be kept away from incompatible materials, including organic compounds, powdered metals, and other reducing agents, as contact can trigger a fire or explosion due to its strong oxidizing power. It should be stored in designated, cool, and dry areas, typically in containers made of compatible materials like high-density polyethylene (HDPE).