nitric acid, chemical compound, HNO₃, colorless, highly corrosive, poisonous liquid that gives off choking red or yellow fumes in moist air. It is miscible with water in all proportions. It forms an azeotrope (constant-boiling mixture) that has the composition 68% nitric acid and 32% water and that boils at 120.5°C;. The nitric acid of commerce is typically a solution of 52% to 68% nitric acid in water. Solutions containing over 86% nitric acid are commonly called fuming nitric acid. White fuming nitric acid (WFNA) is similar to the anhydrous variety, and red fuming nitric acid (RFNA) has a reddish brown color from dissolved nitrogen oxides. When treated with hydrogen fluoride, both varieties form inhibited fuming nitric acid, which has increased corrosion resistance in metal tanks, e.g., when used as an oxidizer in liquid fuel rockets.

Nitric acid is a strong oxidizing agent. It ionizes readily in solution, forming a good conductor of electricity. It reacts with metals, oxides, and hydroxides, forming nitrate salts. Chief uses of nitric acid are in the preparation of fertilizers, e.g., ammonium nitrate, and explosives, e.g., nitroglycerin and trinitrotoluene (TNT). It is also used in the manufacture of chemicals, e.g., in making dyes, and in metallurgy, ore flotation, etching steel, photoengraving, and reprocessing of spent nuclear fuel. It is produced chiefly by oxidation of ammonia (the Ostwald process). Small amounts are produced by the treatment of sodium nitrate with sulfuric acid. Nitric acid was known to the alchemists as aqua fortis; the name is used in commerce for impure grades of it. Aqua regia is a mixture of nitric and hydrochloric acids. Niric acid is a component of acid rain.

Inorganic compound, colourless, fuming, highly corrosive liquid, chemical formula HNO₃. A common laboratory reagent, it is important in the manufacture of fertilizers and explosives (including nitroglycerin), as well as in organic syntheses, metallurgy, ore flotation, and reprocessing of spent nuclear fuel. A strong acid, it is toxic and can cause severe burns. It attacks most metals and is used for etching steel and photoengraving.

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Nitric acid (HNO₃), also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid that can cause severe burns. The synthesis of nitric acid was first recorded circa 800 AD by the alchemist Jabir ibn Hayyan (Geber).

Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming nitric acid. Fuming nitric acid is characterized as white fuming nitric acid and red fuming nitric acid, depending on the amount of nitrogen dioxide present.

Properties

Pure anhydrous nitric acid (100%) is a colorless liquid with a density of 1522 kg/m³ which solidifies at -42 °C to form white crystals and boils at 83 °C. When boiling in light, even at room temperature, there is a partial decomposition with the formation of nitrogen dioxide following the reaction:

4HNO₃ → 2H₂O + 4NO₂ + O₂ (72°C)

which means that anhydrous nitric acid should be stored below 0 °C to avoid decomposition. The nitrogen dioxide (NO₂) remains dissolved in the nitric acid coloring it yellow, or red at higher temperatures. While the pure acid tends to give off white fumes when exposed to air, acid with dissolved nitrogen dioxide gives off reddish-brown vapours, leading to the common name "red fuming acid" or "fuming nitric acid". Fuming nitric acid is also referred to as 16-molar nitric acid –– as the most concentrated form of nitric acid at Standard Temperature & Pressure (STP).

Nitric acid is miscible with water in all proportions and distillation gives an azeotrope with a concentration of 68% HNO₃ and a boiling temperature of 120.5 °C at 1 atm. Two solid hydrates are known; the monohydrate (HNO₃·H₂O) and the trihydrate (HNO₃·3H₂O).

Nitrogen oxides (NO_x) are soluble in nitric acid and this property influences more or less, all the physical characteristics depending on the concentration of the oxides. These mainly include the vapor pressure above the liquid and the boiling temperature, as well as the color mentioned above.

Nitric acid is subject to thermal or light decomposition with increasing concentration and this may give rise to some non-negligible variations in the vapour pressure above the liquid because the nitrogen oxides produced dissolve partly or completely in the acid.

Acidic properties

Being a typical acid, nitric acid reacts with alkalis, basic oxides, and carbonates to form salts, such as ammonium nitrate. Due to its oxidizing nature, nitric acid generally does not donate its proton (that is, it does not liberate hydrogen) on reaction with metals and the resulting salts are usually in the higher oxidized states. For this reason, heavy corrosion can be expected and should be guarded against by the appropriate use of corrosion resistant metals or alloys.

Nitric acid has an acid dissociation constant (pK_a) of −1.4: in aqueous solution, it almost completely (93% at 0.1 mol/L) ionizes into the nitrate ion NO₃⁻ and a hydrated proton, known as a hydronium ion, H₃O⁺.

HNO₃ + H₂O → H₃O⁺ + NO₃^-

Oxidizing properties

Reactions with metals

Being a powerful oxidizing agent, nitric acid reacts violently with many organic materials and the reactions may be explosive. Depending on the acid concentration, temperature and the reducing agent involved, the end products can be variable. Reaction then takes place with all metals except the precious metal series and certain alloys. This characteristic has made it a common agent to be used in acid tests. As a general rule, oxidizing reactions occur primarily with the concentrated acid, favouring the formation of nitrogen dioxide (NO₂).

Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O

The acidic properties tend to dominate with dilute acid, coupled with the preferential formation of nitrogen oxide (NO).

3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 4H₂O

Since nitric acid is an oxidizing agent, hydrogen (H₂) is rarely formed. Only magnesium (Mg), Manganese (Mn) and calcium (Ca) react with cold, dilute nitric acid to give hydrogen:

Mg_(s) + 2HNO_{3 (aq)} → Mg(NO₃)_{2 (aq)} + H_{2 (g)}

Nitric acid has the highest distinction (amongst all acids) of attacking and dissolving all metals on the periodic table except Gold and Platinum.

Passivation

Although chromium (Cr), iron (Fe) and aluminium (Al) readily dissolve in dilute nitric acid, the concentrated acid forms a metal oxide layer that protects the metal from further oxidation, which is called passivation. Typical passivation concentrations range from 18% to 22% by weight.

Reactions with non-metals

Reaction with non-metallic elements, with the exception of silicon and halogens, usually oxidizes them to their highest oxidation states as acids with the formation of nitrogen dioxide for concentrated acid and nitric oxide for dilute acid.

C + 4HNO₃ → CO₂ + 4NO₂ + 2H₂O

or

3C + 4HNO₃ → 3CO₂ + 4NO + 2H₂O

Grades

White fuming nitric acid, also called 100% nitric acid or WFNA, is very close to the anhydrous nitric acid product. One specification for white fuming nitric acid is that it has a maximum of 2% water and a maximum of 0.5% dissolved NO₂.

Red fuming nitric acid, or RFNA, contains substantial quantities of dissolved nitrogen dioxide (NO₂) leaving the solution with a reddish-brown color. One formulation of RFNA specifies a minimum of 17% NO₂, another specifies 13% NO₂.

An inhibited fuming nitric acid (either IWFNA, or IRFNA) can be made by the addition of 0.6 to 0.7% hydrogen fluoride, HF. This fluoride is added for corrosion resistance in metal tanks (the fluoride creates a metal fluoride layer that protects the metal).

Industrial production

Nitric acid is made by mixing nitrogen dioxide (NO₂) with water in the presence of oxygen or air to oxidize the nitrous acid also produced by the reaction.

Dilute nitric acid may be concentrated by distillation up to 68% acid, which is an azeotropic mixture with 32% water. Further concentration involves distillation with sulphuric acid which acts as a dehydrating agent. In the laboratory, such distillations must be done with all-glass apparatus at reduced pressure, to prevent decomposition of the acid.

Commercial grade nitric acid solutions are usually between 52% and 68% nitric acid. Commercial production of nitric acid is via the Ostwald process, named after Wilhelm Ostwald.

The acid can also be synthesized by oxidizing ammonia, but the product is diluted by the water also formed as part of the reaction. However, this method is important in producing ammonium nitrate from ammonia derived from the Haber process, because the final product can be produced from nitrogen, hydrogen, and oxygen as the sole feedstocks.

Laboratory synthesis

In laboratory, nitric acid can be made from copper(II) nitrate or by reacting approximately equal masses of potassium nitrate (KNO₃) with 96% sulfuric acid (H₂SO₄), and distilling this mixture at nitric acid's boiling point of 83 °C until only a white crystalline mass, potassium hydrogen sulfate (KHSO₄), remains in the reaction vessel. The obtained red fuming nitric acid may be converted to the white nitric acid.

H₂SO₄ + KNO₃ → KHSO₄ + HNO₃

The dissolved NO_x are readily removed using reduced pressure at room temperature (10-30 min at 200 mmHg or 27 kPa) to give white fuming nitric acid. This procedure can also be performed under reduced pressure and temperature in one step in order to produce less nitrogen dioxide gas.

Uses

IWFNA may be used as the oxidizer in liquid fuel rockets. IRFNA was one of 3 liquid fuel components for the BOMARC missile

A solution of nitric acid and alcohol, Nital, is used for etching of metals to reveal the microstructure.

Commercially available aqueous blends of 5-30% nitric acid and 15-40% phosphoric acid are commonly used for cleaning food and dairy equipment primarily to remove precipitated calcium and magnesium compounds (either deposited from the process stream or resulting from the use of hard water during production and cleaning).

Nitric acid is also used in explosives, and is one of the key components of Nitroglycerin and RDX.

Digestion

In elemental analysis by ICP-MS, ICP-AES, GFAA, and Flame AA, dilute nitric acid (0.5 to 5.0 %) is used as a matrix compound for determining metal traces in solutions. Ultrapure acid is required for such determination, because small amounts of metal ions could affect the result of the analysis.

Woodworking

In a low concentration (approximately 10%), nitric acid is often used to artificially age pine and maple. The color produced is a grey-gold very much like very old wax or oil finished wood (wood finishing).

Other uses

Alone, it is useful in metallurgy and refining as it reacts with most metals, and in organic syntheses. When mixed with hydrochloric acid, nitric acid forms Aqua Regia, one of the few reagents capable of dissolving gold and platinum.

Aqua Regia is technically a reagent –– not an acid. Aqua Regia is made by (carefully) mixing three parts of hydrochloric acid with one part nitric acid. That mixture produces nitrosyl chloride and free chlorine gas (in the aqueous solution). These two agents are individually exceptionally powerful oxidizing agents. Combined, each acid performs a different function. This is what gives Aqua Regia the ability to dissolve gold and platinum –– where neither acid by itself was able to dissolve the metals by themselves.

Safety

Nitric acid is a powerful oxidizing agent, and the reactions of nitric acid with compounds such as cyanides, carbides, and metallic powders can be explosive. Reactions of nitric acid with many organic compounds, such as turpentine, are violent and hypergolic (i.e., self-igniting).

Concentrated nitric acid dyes human skin yellow due to a reaction with the keratin. These yellow stains turn orange when neutralized.

References

External links

• International Chemical Safety Card 0183
• NIOSH Pocket Guide to Chemical Hazards
• National Pollutant Inventory - Nitric Acid Fact Sheet
• Properties and classification of nitric acid