Nitric acid
IUPAC name Nitric acid
Identifiers
CAS number	7697-37-2 Y
PubChem	944
ChemSpider	919 Y
UNII	411VRN1TV4 Y
EC number	231-714-2
UN number	2031
KEGG	D02313 Y
MeSH	Nitric+acid
ChEBI	CHEBI:48107 Y
ChEMBL	CHEMBL1352 Y
RTECS number	QU5775000
Gmelin Reference	1576
3DMet	B00068
Jmol-3D images	Image 1 Image 2
SMILES [N+](=O)(O)[O-] ON(=O)=O
InChI InChI=1S/HNO3/c2-1(3)4/h(H,2,3,4) Y Key: GRYLNZFGIOXLOG-UHFFFAOYSA-N Y InChI=1/HNO3/c2-1(3)4/h(H,2,3,4) Key: GRYLNZFGIOXLOG-UHFFFAOYAO
Properties
Molecular formula	HNO3
Molar mass	63.01 g mol−1
Appearance	Colorless liquid
Density	1.5129 g cm-3
Melting point	-42 °C, 231 K, -44 °F
Boiling point	83 °C, 356 K, 181 °F (68% solution boils at 121 °C)
Solubility in water	Completely miscible
Acidity (pKa)	-1.4
Refractive index (nD)	1.397 (16.5 °C)
Dipole moment	2.17 ± 0.02 D
Thermochemistry
Std enthalpy of formation ΔfHo298	−207 kJ·mol−1[1]
Standard molar entropy So298	146 J·mol−1·K−1[1]
Hazards
MSDS	ICSC 0183 PCTL Safety Website
EU Index	007-004-00-1
EU classification	C O
R-phrases	R8 R35
S-phrases	(S1/2) S23 S26 S36 S45
NFPA 704	0 4 0 OX
Flash point	Non-flammable
Related compounds
Other anions	Nitrous acid
Other cations	Sodium nitrate Potassium nitrate Ammonium nitrate
Related compounds	Dinitrogen pentoxide
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Nitric acid (HNO₃), also known as aqua fortis and spirit of niter, is a highly corrosive and toxic strong mineral acid which is normally colorless but tends to acquire a yellow cast due to the accumulation of oxides of nitrogen if long-stored. Ordinary nitric acid has a concentration of 68%.^[2] When the solution contains more than 86% HNO₃, it is referred to as fuming nitric acid. Depending on the amount of nitrogen dioxide present, fuming nitric acid is further characterized as white fuming nitric acid or red fuming nitric acid, at concentrations above 95%. Nitric acid is also commonly used as a strong oxidizing agent.

Physical and chemical properties[link]

Nitric acid (nearly 100%) turned yellow.

Pure anhydrous nitric acid (100%) is a colorless mobile liquid with a density of 1.512 g/cm³ which solidifies at −42 °C to form white crystals and boils at 83 °C. 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 vapors, leading to the common name "red fuming acid" or "fuming nitric acid". Fuming nitric acid is also referred to as 16 molar nitric acid. It is the most concentrated form of nitric acid at Standard Temperature and Pressure (STP).

Nitric acid forms an azeotrope with water at a concentration of 68% HNO₃, which is the ordinary concentrated nitric acid of commerce. This solution has 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: 4 HNO₃ → 2 H₂O + 4 NO₂ + O₂. This reaction may give rise to some non-negligible variations in the vapor pressure above the liquid because the nitrogen oxides produced dissolve partly or completely in the acid.

Acid-base properties[link]

Nitric acid is normally considered to be a strong acid at ambient temperatures. There is some disagreement over the value of the acid dissociation constant, though the pK_a value is usually reported as less than –1. This means that the nitric acid in solution is fully dissociated except in extremely acidic solutions. The pK_a value rises to 1 at a temperature of 250 °C.^[3]

Nitric acid can act as a base with respect to an acid such as sulfuric acid.

HNO₃ + 2H₂SO₄ NO₂⁺ + H₃O⁺ + 2HSO₄^–; K ~ 22

The nitronium ion, NO₂⁺, is the active reagent in aromatic nitration reactions. Since nitric acid has both acidic and basic properties it can undergo an autoprotolysis reaction, similar to the self-ionization of water

2HNO₃ NO₂⁺ + NO₃^– + H₂O

Reactions with metals and strong oxidizing properties[link]

Nitric acid reacts with most metals. This characteristic has made it a common agent to be used in acid tests.

Very dilute nitric acid acts as a typical acid to react with active metals. For example, strongly electropositive metals, such as magnesium react with nitric acid as with other acids, reducing hydrogen.

Mg + 2 H⁺ → Mg²⁺ + H₂

Note that only Magnesium, Manganese and Zinc can liberate H₂ gas from very dilute nitric acid. Others give the nitrogen oxides.^[4]

However, both dilute and concentrated nitric acid is a strong oxidizing agent as shown by its large positive reduction potential (E₀^r) and does not react with metals in the same way as most other acids. Nitrogen monoxide is reduced from the dilute acid reacting with metals while nitrogen dioxide is reduced from the concentrated one. Hydrogen is not evolved.

NO₃^- + 4 H⁺ + 3 e^- → NO  + 2 H₂O,     E₀^r = 0.96 V

NO₃^- + 2 H⁺ +    e^- → NO₂ +   H₂O,     E₀^r = 0.79 V

Nitric acid can oxidize non-active metals such as copper and silver. With these non-active or less electropositive metals the products depend on temperature and the acid concentration. For example, copper reacts with dilute nitric acid at ambient temperatures with a 3:8 stoichiometry.

3 Cu + 8 HNO₃ → 3 Cu²⁺ + 2 NO + 4 H₂O + 6 NO₃^-

The nitric oxide produced may react with atmospheric oxygen to give nitrogen dioxide. With more concentrated nitric acid, nitrogen dioxide is produced directly in a reaction with 1:4 stoichiometry.

Cu + 4 H⁺ + 2 NO₃⁻ → Cu²⁺ + 2 NO₂ + 2 H₂O

Most metals react with nitric acid to give the corresponding nitrates.^[4] Some other metalloids and metals give the oxides, for instance, Sn, As, Sb, Ti are oxidized into SnO₂, As₂O₅, Sb₂O₅ and TiO₂ respectively.^[4]

Some precious metals, such as pure gold and platinum group metals do not react with nitric acid, though pure gold does react with aqua regia, a mixture of concentrated nitric acid and hydrochloric acid. However, some less noble metals (Ag, Cu, ...) present in some gold alloys relatively poor in gold such as colored gold can be easily oxidized and dissolved by nitric acid, leading to colour changes of the gold-alloy surface. Nitric acid is used as a cheap means in jewelry shops to quickly spot low-gold alloys (< 14 carats) and to rapidly assess the gold purity.

Being a powerful oxidizing agent, nitric acid reacts violently with many non-metallic compounds and the reactions may be explosive. Depending on the acid concentration, temperature and the reducing agent involved, the end products can be variable. Reaction takes place with all metals except the noble metals series and certain alloys. As a general rule, oxidizing reactions occur primarily with the concentrated acid, favoring the formation of nitrogen dioxide (NO₂). However, the powerful oxidizing properties of nitric acid is thermodynamic in nature, but sometimes its oxidation reactions are rather kinetically non-favored. The presence of small amounts of nitrous acid (HNO₂) greatly enhance the rate of reaction.^[4]

Reactions with non-metals[link]

Being a powerful oxidizing acid, nitric acid reacts violently with many organic materials and the reactions may be explosive.

Reaction with non-metallic elements, with the exceptions of nitrogen, oxygen, noble gases, 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 + 4 HNO₃ → CO₂ + 4 NO₂ + 2 H₂O

or

3 C + 4 HNO₃ → 3 CO₂ + 4 NO + 2 H₂O

Concentrated nitric acid oxidizes I₂, P₄ and S₈ into HIO₃, H₃PO₄ and H₂SO₄ respectively.^[4]

Passivation[link]

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 20-50% by volume. (See ASTM A967-05) The metals which are passivated by concentrated nitric acid are Iron, Cobalt, Chromium, Nickel, and Aluminium.^[4]

Xanthoproteic test[link]

Nitric acid reacts with proteins to form yellow nitrated products. This reaction is known as the xanthoproteic reaction. This test is carried out by adding concentrated nitric acid to the substance being tested, and then heating the mixture. If proteins that contain amino acids with aromatic rings are present, the mixture turns yellow. Upon adding a strong base such as liquid ammonia, the color turns orange. These color changes are caused by nitrated aromatic rings in the protein.^[5][6] Xanthoproteic acid is formed when the acid contacts epithelial cells and is indicative of inadequate safety precautions when handling nitric acid.

Grades[link]

Nitric acid 70%

Nitric acid of commercial interest usually consists of the maximum boiling azeotrope of nitric acid and water, which is approximately 68% HNO₃, (approx. 15 molar)^[2]. This is considered concentrated or technical grade, while reagent grades are specified at 70% HNO₃. The density of concentrated nitric acid is 1.42 g/mL. An older density scale is occasionally seen, with concentrated nitric acid specified as 42° Baumé.^[7]

White fuming nitric acid, also called 100% nitric acid or WFNA, is very close to anhydrous nitric acid. One specification for white fuming nitric acid is that it has a maximum of 2% water and a maximum of 0.5% dissolved NO₂. Anhydrous nitric acid has a density of 1.513 g/mL and has the approximate concentration of 24 molar.

A commercial grade of fuming nitric acid, referred to in the trade as "strong nitric acid" contains 90% HNO₃ and has a density of 1.50 g/mL. This grade is much used in the explosives industry. It is not as volatile nor as corrosive as the anhydrous acid and has the approximate concentration of 21.4 molar.

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. Because of the dissolved nitrogen dioxide, the density of red fuming nitric acid is lower at 1.490 g/mL.

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[link]

Nitric acid is made by reaction of nitrogen dioxide (NO₂) with water.

3 NO₂ + H₂O → 2 HNO₃ + NO

Normally, the nitric oxide produced by the reaction is reoxidized by the oxygen in air to produce additional nitrogen dioxide.

Bubbling nitrogen dioxide through hydrogen peroxide can help to improve acid yield.

2 NO₂ + H₂O₂ → 2 HNO₃

Almost pure nitric acid can be made by adding sulfuric acid to a nitrate salt, and heating the mixture with an oil bath. A condenser is used to condense the nitric acid fumes that bubble out of the solution.

2 NaNO₃ + H₂SO₄ → 2 HNO₃ + Na₂SO₄

Dilute nitric acid may be concentrated by distillation up to 68% acid, which is a maximum boiling azeotrope containing 32% water. In the laboratory, further concentration involves distillation with either sulfuric acid or magnesium nitrate which act as dehydrating agents. Such distillations must be done with all-glass apparatus at reduced pressure, to prevent decomposition of the acid. Industrially, strong nitric acid is produced by dissolving additional nitrogen dioxide in 68% nitric acid in an absorption tower.^[8] Dissolved nitrogen oxides are either stripped in the case of white fuming nitric acid, or remain in solution to form red fuming nitric acid. More recently, electrochemical means have been developed to produce anhydrous acid from concentrated nitric acid feedstock.^[9]

Commercial grade nitric acid solutions are usually between 52% and 68% nitric acid. Production of nitric acid is via the Ostwald process, named after German chemist Wilhelm Ostwald. In this process, anhydrous ammonia is oxidized to nitric oxide, in the presence of platinum or rhodium gauge catalyst at a high temperature of about 500K and a pressure of 9 bar.

4 NH₃ (g) + 5 O₂ (g) → 4 NO (g) + 6 H₂O (g) (ΔH = −905.2 kJ)

Nitric oxide is then reacted with oxygen in air to form nitrogen dioxide.

2 NO (g) + O₂ (g) → 2 NO₂ (g) (ΔH = −114 kJ/mol)

This is subsequently absorbed in water to form nitric acid and nitric oxide.

3 NO₂ (g) + H₂O (l) → 2 HNO₃ (aq) + NO (g) (ΔH = −117 kJ/mol)

The nitric oxide is cycled back for reoxidation. Alternatively, if the last step is carried out in air:

4 NO₂ (g) + O₂ (g) + 2 H₂O (l) → 4 HNO₃ (aq)

The aqueous HNO₃ obtained can be concentrated by distillation up to about 68% by mass. Further concentration to 98% can be achieved by dehydration with concentrated H₂SO₄. By using ammonia derived from the Haber process, the final product can be produced from nitrogen, hydrogen, and oxygen which are derived from air and natural gas as the sole feedstocks.^[10]

Prior to the introduction of the Haber process for the production of ammonia in 1913, nitric acid was produced using the Birkeland–Eyde process, also known as the arc process. This process is based upon the oxidation of atmospheric nitrogen by atmospheric oxygen to nitric oxide at very high temperatures. An electric arc was used to provide the high temperatures, and yields of up to 4% nitric oxide were obtained. The nitric oxide was cooled and oxidized by the remaining atmospheric oxygen to nitrogen dioxide, and this was subsequently absorbed in dilute nitric acid. The process was very energy intensive and was rapidly displaced by the Ostwald process once cheap ammonia became available.

Laboratory synthesis[link]

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

H₂SO₄ + NO−
3 → HSO−
4(s) + HNO₃(g)

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.^{[citation needed]}

Uses[link]

Nitric acid in a laboratory.

The main use of nitric acid is for the production of fertilizers; other important uses include the production of explosives, etching and dissolution of metals, especially as a component of aqua regia for the purification and extraction of gold, and in chemical synthesis.

Rocket fuel[link]

Nitric acid has been used in various forms as the oxidizer in liquid-fueled rockets. These forms include red fuming nitric acid, white fuming nitric acid, mixtures with sulfuric acid, and these forms with HF inhibitor.^[11] IRFNA (inhibited red fuming nitric acid) was one of 3 liquid fuel components for the BOMARC missile.^[12]

Chemical reagent[link]

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.^[13] Ultrapure trace metal grade acid is required for such determination, because small amounts of metal ions could affect the result of the analysis.

It is also typically used in the digestion process of turbid water samples, sludge samples, solid samples as well as other types of unique samples which require elemental analysis via ICP-MS, ICP-OES, ICP-AES, GFAA and flame atomic absorption spectroscopy. Typically these digestions use a 50% solution of the purchased HNO₃ mixed with Type 1 DI Water.^[14]

In organic synthesis, nitric acid may be used to introduce the nitro group. When used with sulfuric acid, it generates the nitronium ion, which electrophilically reacts with aromatic compounds such as benzene.

In electrochemistry, nitric acid is used as a chemical doping agent for organic semiconductors, and in purification processes for raw carbon nanotubes.

Woodworking[link]

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).^[15]

Other uses[link]

A solution of nitric acid, water and alcohol, Nital, is used for etching of metals to reveal the microstructure. ISO 14104 is one of the standards detailing this well known procedure.

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). The phosphoric acid content helps to passivate ferrous alloys against corrosion by the dilute nitric acid.^{[citation needed]}

Nitric acid can be used as a spot test for alkaloids like LSD, giving a variety of colours depending on the alkaloid.^[16]

Safety[link]

Nitric acid is a strong acid and a powerful oxidizing agent. The major hazard posed by it is chemical burn as it carries out acid hydrolysis with proteins (amide) and fats (ester) and decomposes animals' muscles. Concentrated nitric acid attacks tissues readily and stains human skin yellow due to its reaction with the keratin. These yellow stains turn orange when neutralized. ^[17] Systemic effects are unlikely, however, and the substance is not considered a carcinogen or mutagen.^[18]

The standard first aid treatment for acid spills on the skin is, as for other corrosive agents, irrigation with large quantities of water. Washing is continued for at least ten to fifteen minutes to cool the tissue surrounding the acid burn and to prevent secondary damage. Contaminated clothing is removed immediately and the underlying skin washed thoroughly.

Being a strong oxidizing agent, reactions of nitric acid with compounds such as cyanides, carbides, metallic powders can be explosive and those with many organic compounds, such as turpentine, are violent and hypergolic (i.e. self-igniting). Hence, it should be stored away from bases and organics.

History[link]

The first mention of nitric acid is in Pseudo-Geber´s De Inventione Veritatis, wherein it is obtained by calcining a mixture of niter, alum and blue vitriol. It was again described by Albert the Great in the 13th century and by Ramon Lull, who prepared it by heating niter and clay and called it "eau forte" (aqua fortis).^[19]

Glauber devised the process still used today to obtain it, namely by heating niter with strong sulfuric acid. In 1776 Lavoisier showed that it contained oxygen, and in 1785 Henry Cavendish determined its precise composition and showed that it could be synthesized by passing a stream of electric sparks through moist air.^[19]

References[link]

External links[link]

• NIOSH Pocket Guide to Chemical Hazards
• National Pollutant Inventory - Nitric Acid Fact Sheet