1 Properties
Nitric acid is a strong mineral acid with notable oxidizing power and broad chemical reactivity. Its behavior in solution, its tendency to decompose, and its interactions with metals and organic matter make it one of the most important inorganic acids in chemistry and industry.
1.1 Chemical formula and structure
Nitric acid has the molecular formula HNO3. Its structure consists of one hydrogen atom bonded to an oxygen atom, with the remaining atoms forming a trigonal planar nitrate framework around nitrogen. The molecule is commonly described by resonance forms, reflecting delocalization of charge among the oxygen atoms.
In pure form, nitric acid exists as discrete molecules, while in aqueous solution it dissociates to produce hydronium and nitrate ions. This ionic behavior underlies its strong acidity and many of its reactions.
1.2 Physical properties
Nitric acid is a volatile, corrosive liquid whose physical characteristics depend somewhat on concentration and temperature. Commercial samples are usually aqueous solutions rather than the anhydrous compound.
1.2.1 Appearance and odor
Pure nitric acid is colorless, but stored samples often develop a yellow to brown tint from dissolved nitrogen oxides produced during slow decomposition. It has a sharp, pungent odor that can be readily noticed at low concentrations and is irritating to the eyes and respiratory system.
1.2.2 Density, boiling point, and melting point
Nitric acid is relatively dense compared with water, and its density increases with concentration. It boils at a moderate temperature for a strong acid, but exact values vary with purity and the proportion of water present because nitric acid forms mixtures that can distill differently from the pure compound. Its melting point is also concentration dependent.
1.3 Chemical properties
Nitric acid is valued in chemistry for combining strong acidity with strong oxidizing character. This dual nature distinguishes it from many other mineral acids.
1.3.1 Acidity and ionization
In water, nitric acid is essentially fully ionized, making it a strong acid. It readily donates a proton and forms nitrate, a highly stable conjugate base. Because of this near-complete dissociation, dilute nitric acid conducts electricity well and neutralizes bases efficiently.
1.3.2 Oxidizing behavior
Nitric acid acts as a powerful oxidizing agent, especially in concentrated form. It can oxidize a wide range of substances, including many metals and nonmetals, often producing nitrogen oxides rather than hydrogen gas. The exact reduction products depend on concentration, temperature, and the reducing agent involved.
1.3.3 Decomposition and instability
Nitric acid is less stable than many other acids and slowly decomposes, particularly when exposed to light, heat, or prolonged storage. Decomposition generates nitrogen dioxide and related oxides, which account for the characteristic yellow or brown coloration seen in aged samples. This tendency also limits the long-term purity of concentrated nitric acid.
2 Production
Nitric acid is produced on a large industrial scale and also prepared in smaller quantities for laboratory use. Modern manufacture is based mainly on catalytic oxidation of ammonia, a process that is efficient and compatible with mass fertilizer production.
2.1 Industrial manufacture
Industrial nitric acid production is closely linked to ammonia chemistry. The process converts inexpensive feedstocks into a versatile acid used across agriculture and manufacturing.
2.1.1 Ostwald process
The Ostwald process is the standard industrial route to nitric acid. In this method, ammonia is first oxidized to nitric oxide, then converted to nitrogen dioxide, and finally absorbed in water to form nitric acid. The process is continuous and typically integrated with systems for heat recovery and gas purification.
2.1.2 Catalytic oxidation of ammonia
Catalytic oxidation of ammonia is the key initial step in industrial nitric acid manufacture. A platinum-based catalyst, often in alloy form, promotes the rapid conversion of ammonia and oxygen at high temperature. The reaction conditions are chosen to maximize yield while minimizing unwanted side products.
2.2 Laboratory preparation
In the laboratory, nitric acid may be prepared by heating a nitrate salt with a strong nonvolatile acid, such as sulfuric acid, to release nitric acid by distillation. This method is simple and historically important, though it is less practical for large-scale production. The product may require careful handling because it is corrosive and can fume strongly.
2.3 Purification and concentration
Purification of nitric acid commonly involves distillation and control of water content. Because the acid forms mixtures with water that can show special boiling behavior, concentration requires attention to composition and temperature. Industrial and laboratory grades are produced in different strengths depending on intended use.
2.3.1 Fuming nitric acid
Fuming nitric acid is highly concentrated nitric acid containing dissolved nitrogen dioxide. It emits dense fumes in air and is more reactive and more hazardous than ordinary concentrated acid. Fuming grades are used in specialized chemical syntheses, especially where strong nitrating conditions are desired.
3 Reactions
Nitric acid reacts with a wide variety of substances, often with vigorous heat evolution. Its products depend strongly on concentration and on whether the other reactant is a metal, base, organic material, or reducing agent.
3.1 With metals
Many metals react with nitric acid, but the products differ from those formed with nonoxidizing acids. Hydrogen gas is usually not liberated because the acid oxidizes the metal and itself undergoes reduction.
3.1.1 Passive films and metal reactivity
Some metals, such as iron, chromium, and aluminum, can become passive in concentrated nitric acid. A thin protective oxide layer forms on the surface and reduces further attack. Other metals, including copper and silver, dissolve more readily, producing metal nitrates and nitrogen oxides.
3.2 With bases and oxides
Nitric acid neutralizes bases and reacts with metal oxides to form nitrate salts and water. These reactions are typical acid-base processes and are widely used in the preparation of nitrates. The resulting salts are often highly soluble in water.
3.3 With organic compounds
Nitric acid can react strongly with organic substances, sometimes by substitution and sometimes by oxidation. Because these reactions may be rapid and exothermic, they require strict control in practical chemistry.
3.3.1 Nitration reactions
In the presence of sulfuric acid or under suitable conditions, nitric acid introduces nitro groups into aromatic compounds. This nitration is central to the production of many dyes, pharmaceuticals, and explosives. The nitronium ion is the effective nitrating species in many of these reactions.
3.3.2 Oxidation of organics
Concentrated nitric acid can oxidize alcohols, aldehydes, sugars, and many other organic materials. In severe cases, the reaction may be violent and can cause charring or ignition. This strong oxidizing tendency is one reason the acid must be kept away from combustible materials.
3.4 With nonmetals and reducing agents
Nitric acid also reacts with many nonmetals and reducing agents, often forming oxides of nitrogen. Sulfur, phosphorus, carbon, and hydrogen sulfide are among the substances that may be oxidized under appropriate conditions. The reduction products of nitric acid can vary, ranging from nitric oxide to nitrogen dioxide and, in stronger reductions, lower nitrogen species.
4 Uses
Nitric acid is a foundational industrial reagent with major roles in agriculture, materials processing, and chemical synthesis. Its value comes from both its acidity and its ability to introduce nitrate or nitro groups into compounds.
4.1 Fertilizer production
A major use of nitric acid is the manufacture of nitrogen fertilizers, especially ammonium nitrate and related nitrate salts. These materials supply plants with accessible nitrogen and are central to modern agricultural chemistry.
4.2 Explosives and propellants
Nitric acid is used in producing explosives and propellants because it nitrates organic molecules and helps create energy-rich compounds. Nitrated aromatic compounds and nitrate esters have long been important in industrial and military chemistry. Control of reaction conditions is essential because these products can be highly sensitive.
4.3 Dyes and intermediates
The acid is widely used in making dye intermediates, including nitrated aromatic compounds that serve as precursors for colored materials. It also participates in the synthesis of numerous organic intermediates used across the chemical industry.
4.4 Metal processing and etching
Nitric acid is used to clean, etch, and passivate metals. In metallurgy and fabrication, it helps remove oxides, reveals surface details, and prepare metal surfaces for further treatment. Its ability to dissolve some metals while leaving others protected is especially useful in alloy work.
4.5 Laboratory applications
In analytical and preparative chemistry, nitric acid is used for digestion of samples, preparation of nitrate solutions, and pH adjustment in highly acidic media. It is also a common reagent in wet analysis because it can dissolve many materials that resist weaker acids.
5 Safety and handling
Nitric acid requires careful handling because it is corrosive, reactive, and capable of releasing hazardous vapors. Safe use depends on good ventilation, compatible materials, and protective equipment.
5.1 Corrosive hazards
Nitric acid can cause severe burns to skin, eyes, and mucous membranes. Contact may produce immediate pain and tissue damage, especially with concentrated solutions. It also attacks many materials, including some plastics, metals, and organic fabrics.
5.2 Toxic fumes and inhalation risks
Nitrogen dioxide and related fumes may be released from nitric acid, particularly when the liquid is warm or decomposing. Inhalation can irritate the respiratory tract and may cause coughing, chest discomfort, and delayed lung injury. Work with the acid is therefore usually performed in a fume hood.
5.3 Storage and compatibility
Nitric acid should be stored in tightly closed containers made of compatible materials and kept away from bases, reducing agents, combustibles, and many metals. Cool, well-ventilated storage helps limit decomposition and fume buildup. Separation from incompatible chemicals is essential to prevent hazardous reactions.
5.4 Protective equipment and spill response
Appropriate protective equipment includes chemical-resistant gloves, eye protection, a face shield when necessary, and protective clothing. Small spills are typically handled by trained personnel using neutralization or absorption procedures suited to the laboratory or industrial setting. Because the acid can react violently with organic absorbents, cleanup materials must be chosen carefully.
6 Occurrence and environmental aspects
Nitric acid occurs naturally in the atmosphere in small amounts and also forms through human activity. Its environmental significance is tied to rainfall acidity, nitrogen chemistry, and the movement of reactive nitrogen through ecosystems.
6.1 Formation in the atmosphere
Atmospheric nitric acid forms when nitrogen oxides react with water vapor and oxidants in air. These nitrogen oxides may arise from lightning, microbial activity, combustion, and other sources. The acid is an important component of atmospheric chemistry and is commonly found in aerosol or vapor form.
6.2 Role in acid rain and nitrogen cycling
Nitric acid contributes to acid deposition, where acidic compounds return to Earth in rain, snow, or dry particles. It also plays a role in nitrogen cycling by converting reactive nitrogen gases into nitrate, which can then be deposited on soils and waters. This nitrate may be taken up by plants or washed into aquatic systems.
6.3 Environmental impact of nitric acid emissions
Emissions associated with nitric acid production and nitrogen oxide release can influence air quality and ecosystem chemistry. These impacts include acidification of soils and waters, nutrient imbalance, and the formation of fine particulate matter when nitrates accumulate in the atmosphere. Modern controls on industrial emissions aim to reduce these effects.
7 History
Nitric acid has a long history in alchemy, laboratory chemistry, and industry. Its discovery and use helped shape the development of mineral acid chemistry and many branches of chemical manufacturing.
7.1 Early preparation and discovery
Early forms of nitric acid were produced by distilling nitrate salts with vitriols or related acidic materials. Medieval and early modern alchemists recognized its strong corrosive power and used it in metal dissolution and refining. Over time, its distinct properties led to broader chemical investigation.
7.2 Development of industrial production
The rise of industrial chemistry made nitric acid an essential bulk chemical. Large-scale methods were developed to meet demand for fertilizers, dyes, and explosives, and the catalytic oxidation of ammonia became the dominant route in the twentieth century. This shift greatly expanded production capacity and lowered costs.
7.3 Modern applications and significance
Today nitric acid remains a core reagent in chemical manufacturing and laboratory practice. Its role in fertilizer production is especially important because of global demand for fixed nitrogen. At the same time, its oxidizing and nitrating properties continue to make it indispensable in synthetic chemistry and materials processing.