Nitric acid - Wikipedia, the free encyclopedia

IUPAC name Nitric acid
Other namesAqua fortis, Spirit of niter, Eau forte, Hydrogen nitrate, Acidum nitricum
	7697-37-2 Y
3DMet	B00068
ChEBI	CHEBI:48107 Y
ChEMBL	ChEMBL1352 Y
ChemSpider	919 Y
EC Number	231-714-2
	1576
Jmol 3D model	Interactive imageInteractive image
KEGG	D02313 Y
MeSH	Nitric+acid
PubChem	944
RTECS number	QU5775000
UNII	411VRN1TV4 Y
UN number	2031
InChI=1/HNO3/c2-1(3)4/h(H,2,3,4) Key: GRYLNZFGIOXLOG-UHFFFAOYAO
	HNO3
Molar mass	63.01 g·mol−1
Appearance	Colorless, yellow or red fuming liquid[1]
Odor	acrid, suffocating[1]
Density	1.5129 g cm−3
Melting point	−42 °C (−44 °F; 231 K)
Boiling point	83 °C (181 °F; 356 K) 68% solution boils at 121 °C (250 °F; 394 K)
	Completely miscible
Vapor pressure	48 mmHg (20 °C)[1]
Acidity (pKa)	-1.4[2]
	1.397 (16.5 °C)
	2.17 ± 0.02 D
	146 J·mol−1·K−1[3]
	−207 kJ·mol−1[3]
Safety data sheet	ICSC 0183PCTL Safety Website
	COT+
R-phrases	R8R35
S-phrases	(S1/2)S23S26S36S45
NFPA 704
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
	138 ppm (rat, 30 min)[1]
US health exposure limits (NIOSH):
	TWA 2 ppm (5 mg/m3)[1]
	TWA 2 ppm (5 mg/m3) ST 4 ppm (10 mg/m3)[1]
	25 ppm[1]
	Nitrous acid
	Sodium nitratePotassium nitrateAmmonium nitrate
Related compounds	Dinitrogen pentoxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Nitric acid (HNO3), also known as aqua fortis and spirit of niter, is a highly corrosivemineral acid.

The pure compound is colorless, but older samples tend to acquire a yellow cast due to decomposition into oxides of nitrogen and water. Most commercially available nitric acid has a concentration of 68% in water. 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 the primary reagent used for nitration – the addition of a nitro group, typically to an organic molecule. While some resulting nitro compounds are shock- and thermally-sensitive explosives, a few are stable enough to be used in munitions and demolition, while others are still more stable and used as pigments in inks and dyes. Nitric acid is also commonly used as a strong oxidizing agent.

Contents

Physical and chemical propertiesEdit

Commercially available nitric acid is 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).

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). 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[inconsistent]. An older density scale is occasionally seen, with concentrated nitric acid specified as 42° Baumé.[4]

Contamination with nitrogen dioxideEdit

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.

The nitrogen dioxide (NO2) remains dissolved in the nitric acid coloring it yellow or even 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" – the most concentrated form of nitric acid at Standard Temperature and Pressure (STP). Nitrogen oxides (NOx) are soluble in nitric acid.

Fuming nitric acidEdit

A commercial grade of fuming 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. Due to 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.

Anhydrous nitric acidEdit

White fuming nitric acid, pure nitric acid or WFNA, is very close to anhydrous nitric acid. It is available as 99.9% nitric acid by assay. 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. Anhydrous nitric acid is a colorless mobile liquid with a density of 1.512 g/cm3, which solidifies at −42 °C to form white crystals. As it decomposes to NO2 and water, it obtains a yellow tint. It boils at 83 °C. It is usually stored in a glass shatterproof amber bottle with twice the volume of head space to allow for pressure build up. When received, the pressure must be released and repeated monthly until finished.

Structure and bondingEdit

The molecule is planar. Two of the N-O bonds are equivalent and relatively short (this can be explained by theories of resonance. The canonical forms show double bond character in these two bonds, causing them to be shorter than typical N-O bonds.), and the third N-O bond is elongated because the O is also attached to a proton.[5][6]

Acid-base propertiesEdit

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 pKa value is usually reported as less than −1. This means that the nitric acid in diluted solution is fully dissociated except in extremely acidic solutions. The pKa value rises to 1 at a temperature of 250 °C.[7]

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

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:

Reactions with metalsEdit

Nitric acid reacts with most metals but the details depend on the concentration of the acid and the nature of the metal. Dilute nitric acid behaves as a typical acid in its reaction with most metals. Magnesium, manganese and zinc liberate H2. Others give the nitrogen oxides.[8]

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:

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.

Upon reaction with nitric acid, most metals give the corresponding nitrates. Some metalloids and metals give the oxides; for instance, Sn, As, Sb, and Ti are oxidized into SnO2, As2O5, Sb2O5, and TiO2, respectively.[8]

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 (