Nitric acid is a highly corrosive mineral acid and a common industrial chemical. It has been utilized for centuries, historically known as “aqua fortis.” This compound plays a foundational role in modern manufacturing, serving as a precursor for countless other products. Its importance stems from its unique chemical properties and the efficiency of its large-scale production.
Chemical Identity and Fundamental Properties
Nitric acid is chemically defined by the formula HNO3. A single molecule consists of one atom each of Hydrogen and Nitrogen, and three atoms of Oxygen. This arrangement makes it a strong acid, meaning it fully ionizes in water and readily gives up its hydrogen ion. When pure, the substance is a colorless liquid with a suffocating, pungent odor.
The pure acid is chemically unstable and tends to decompose over time, especially when exposed to light or heat. This decomposition produces nitrogen dioxide gas, which dissolves in the liquid. The presence of dissolved nitrogen dioxide is responsible for the characteristic yellow or brown tint often observed in commercial samples. Because it is a powerful oxidizing agent, it reacts vigorously with many materials and is highly corrosive.
The Industrial Manufacturing Process
The majority of nitric acid is manufactured through the Ostwald process, a multi-step chemical synthesis developed in the late 19th century. This industrial method starts with ammonia (NH3) and atmospheric oxygen. The entire synthesis is divided into three main reaction stages, all occurring sequentially within specialized reaction vessels.
The first stage is the catalytic oxidation of ammonia, where ammonia gas and air are passed over a platinum-rhodium catalyst at high temperatures. This highly exothermic reaction converts the ammonia and oxygen into nitric oxide (NO) and water vapor. The catalyst promotes the reaction without being consumed itself.
The hot gases are then cooled rapidly, and the nitric oxide reacts further with residual oxygen in the air. This second step is a non-catalytic oxidation, which converts the nitric oxide into nitrogen dioxide (NO2). Cooling is important because this reaction proceeds more efficiently at lower temperatures.
The final stage involves absorbing the nitrogen dioxide gas in water, often within a large absorption tower. This reaction yields the final product, nitric acid (HNO3). The process also produces some nitric oxide, which is recycled back to minimize waste and increase efficiency. The initial product is an aqueous solution, which is typically concentrated to the industry standard of 68% for commercial use.
Primary Applications and Importance
The largest single application for nitric acid is in the production of fertilizers, consuming an estimated 75% to 80% of the global supply. It is neutralized with ammonia to create ammonium nitrate, a compound with a high nitrogen content. This compound is necessary for promoting healthy plant growth and is foundational to modern agriculture.
Beyond agriculture, the compound is a foundational component in the synthesis of high-energy materials. It is used in the nitration of organic compounds to produce various explosives, including nitroglycerin and trinitrotoluene (TNT). The ability of nitric acid to introduce a nitro group (NO2) into organic molecules makes it indispensable for this industry.
In the field of metallurgy, nitric acid is utilized for etching, cleaning, and treating metals. It is a powerful oxidizer that can convert many metals into their soluble nitrate salts. When mixed with hydrochloric acid, it forms a highly corrosive mixture called aqua regia. Aqua regia is capable of dissolving noble metals like gold and platinum, making it useful in metal refining and purification processes.