What happens to nitric acid in water

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nitric acid


CAS 7697-37-2

nitric acid

Nitric acid

molar mass (pure ENT3)


Melting point
boiling point
Water solubility

63.013 g / mol

1 ml / m3 (TRGS 900)
1.513 g / cm3 
−41.6 ° C
+83 ° C
mixable in any ratio

Colorless or yellowish, air-smoking liquid (100%)

Brown bottles and teflon caps (concentrated solutions)

Hazard classes + category
Oxidizing liquids 2 (> = 99%)
Ox. Liquids 3 (65 to 99%)
Etching on the skin 1A (> = 20%)
Etching on the skin 1B (<20%)
Serious eye damage 1
Acute Tox. 3 (inhalation) (<= 70%)
Acute Tox. 1 (inhalation) (> 70%)
Corrosive to metals 1
Corrosive effects on the respiratory tract (EUH)

HP rates (See note)
H 272, 314, 331, 330 (> 70%), 290, EUH071
P 260, 280.1-4 + 7, 303 + 361 + 353, 304 + 340, 305 + 351 + 338, 310

Pre-treatment acids / bases

GHS 03
GHS 05
GHS 06





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Special notes for schools

Expectant or nursing mothers should not work with concentrated nitric acid. It is recommended to use only dilute solutions up to a maximum of 12% for student experiments in lower secondary level. Dilute solutions are also recommended for student experiments in higher grades. Only concentrations up to a maximum of 65% are stored in the school laboratory. Smoking or concentrated nitric acid is not an explosive, but it can be used to manufacture such substances (> Notes). It is very corrosive to the skin and eyes. Inhaling the vapors can lead to life-threatening pulmonary edema. Safety glasses, protective gowns and protective gloves are mandatory for all concentrations. All work with nitric acid in the fume cupboard should be used. Bottles with Teflon caps or the original container are recommended for storing the concentrated acid. Screw caps made of PBT coated with PTFE are also suitable. Nitric acid can release toxic nitrogen dioxide when heated or when reacting with metals. Experiments with nitrogen dioxide are not suitable for schools. The private possession of concentrated nitric acid is prohibited. The diluted acid is free up to 3%, up to 10% it is subject to approval.

Risk assessment Germany
(also EU)
GBU Working with nitric acid

Security assessment of Switzerland
SB Working with nitric acid

Effect on the human body

The solution and the vapors are extremely corrosive to the skin, eyes and mucous membranes. Inhalation leads to bronchial catarrh, pneumonia and burns of the alveoli. If pulmonary edema develops, there is a danger to life. On contact with the skin, it turns yellow in a xanthoprotein reaction. A yellow nitro dye is formed:

Chemical-physical properties

“Concentrated nitric acid” refers to the commercially available 65 to 69% solution of nitric acid in water. The 100%, red, fuming nitric acid is colorless in its pure state if it is kept refrigerated in the absence of air. The smoking nitric acid decomposes in the air or when boiling, forming red-brown and highly toxic nitrogen dioxide, which causes the orange color:

4 ENT3  4 NO2 + 2 H.2O + O2    

Smoking nitric acid releases nitrogen dioxide.

Smoking nitric acid is a very strong oxidizing agent and can spontaneously ignite wood or straw. The density varies with the different concentrations. This must be taken into account when diluting the acid. "Diluted" and "concentrated" nitric acid is used for teaching in schools, which usually means 12% and 65% nitric acid. The “smoking” nitric acid has a high risk potential, it is not recommended for teaching.

Concentration, mass percentage
Concentration, amount of substance
Density at 20 ° C
100 %24.01 mol / l"smoking"1.513 g / cm³
69 %15.42 mol / l
"concentrated"1.409 g / cm³
65 %14.35 mol / l
"concentrated"1.391 g / cm³
12 %2.03 mol / l
"Diluted"1.066 g / cm³

A 69.2% nitric acid forms an azeotropic mixture that boils at 121.8 ° C. Nitric acid is a relatively strong acid, it is almost completely dissociated in aqueous solution:

Nitric acid + water Nitrate ion + hydronium ion
ENT3 + H2O NO3+ H3O+  

The reaction with the metals produces the salts of nitric acid, the nitrates. With zinc you get zinc nitrate, with copper you get copper (II) nitrate. The nitrogen monoxide NO formed in this reaction immediately reacts further with air to form red-brown nitrogen dioxide NO2

3 Zn + 8 ENT3  3 Zn (NO3)2 + 4 H.2O + 2 NO
3 Cu + 8 HNO3  3 Cu (NO3)2 + 4 H.2O + 2 NO

When the concentrated nitric acid reacts with metals, magnesium, zinc, copper and silver dissolve. Gold, on the other hand, is resistant, only with the addition of concentrated nitric acid and concentrated hydrochloric acid does this also dissolve:

These experiments are not suitable for schools. Film

Separation water and aqua regia

Along with hydrochloric acid and sulfuric acid, nitric acid is one of the strongest acids and even decomposes precious metals such as silver. Only gold and platinum are permanent. Since one can dissolve silver from gold with a 50% nitric acid, this solution was formerly also known as separating water. If one volume of concentrated nitric acid is mixed with three volumes of concentrated hydrochloric acid, aqua regia is formed. When the two acids are mixed together, aqua regia produces nascent chlorine and nitrosyl chloride NOCl:

ENT3 + 3 HCl 2 clnasc.+ NOCl + 2 H2O

These two reaction products can oxidize gold or platinum. Gold becomes tetrachloroauric acid H [AuCl4] and platinum to form hexachloroplatinic acid H2[PtCl6] oxidized. These substances can be allowed to crystallize out of the solution, producing yellow needles that dissolve in the air and dissolve very well in water and alcohol. If a strong reducing agent such as tin (II) chloride is added to the tetrachloroauric acid solution, colloidal gold is formed, which is purple-red or purple in color. The pigment is also known as Cassius' gold purple. It is named after the doctor Cassius who discovered it in 1663. The tetrachloroauric (III) acid is used to produce other gold compounds. Gold (III) chloride and gold-organic compounds are suitable as catalysts for organic syntheses. Glasses can be colored with gold (III) oxide.

Colloidal gold
The production of nitric acid is already used by medieval alchemists such as Geber. In the plant De inventione veritatis("From the discovery of truth", probably published in the 14th century) is described as the heating of a mixture of vitriol (copper sulphate), alum (potassium aluminum sulphate) and saltpeter (potassium nitrate) to red heat. The resulting nitrous gases (according to current knowledge) could be converted into nitric acid with water. The production of aqua regia from nitric acid and ammonium chloride probably goes back to the time before the Middle Ages, to the Arab alchemists in the 7th or 8th century. In the Raimundus Lullus subordinated pseudo-Lullian writings around the year 1332, the effect of nitric acid on metals, the production of septic water (“aqua fortis acuta”) and aqua regia is described. The production from sulfuric acid and potassium nitrate, which is still used in the laboratory today, goes back to Johann Rudolph Glauber (1604–1670):

Potassium nitrate + sulfuric acid Nitric acid + potassium hydrogen sulfate
KNO3 + H2SO4  ENT3 + KHSO4  

Glauber was also the first to develop a method for representing aqua regia ("Aqua regis") from table salt and nitric acid.
In 1903 the Norwegian physicist Kristian Olaf Bernhard Birkeland (1867-1917) and the Norwegian engineer Sam Eyde (1866-1940) developed a process in which they made atmospheric nitrogen react with atmospheric oxygen in an arc. If a high voltage is applied between two electrodes in a reaction sphere, an arc is created. The nitrogen in the air reacts with the oxygen in the air to form nitrogen monoxide NO. This reacts with any oxygen that is still present to form nitrogen dioxide NO2that is orange-brown in color:

N2 + O2  2 NO
2 NO + O2   2 NO2 

Production of nitrogen oxides in a reaction sphere on the arc


If the resulting nitrogen dioxide is passed into water, then nitric acid and nitric oxide again are formed:

3 NO2 + H2O 2 ENT3 + NO

This process for the production of nitric acid is rarely used. For some time it was only able to hold its own in Scandinavia, in other words in countries where electricity from hydropower is available at low cost. Today nitric acid is mainly produced in the chemical industry by the Ostwald process by the oxidation of ammonia.

Ostwald process for nitric acid production
History of the development of the Ostwald process
Nitric acid is an important intermediate in the production of fertilizers and other chemical compounds, for example phosphoric acid, oxalic acid, collodion, amines, dyes or medicines. It is also a starting material for the production of explosives such as nitroglycerine: when you soak up diatomaceous earth, you get dynamite with this. Nitric acid is also required for the production of trinitrotoluene (TNT) or hexogen (RDX). Potassium nitrate, a salt of nitric acid, is a component of black powder. Jewelers use nitric acid in various concentrations and in combination with hydrochloric acid as test acid to determine the gold content in jewelry. In doing so, they check whether any abrasion from the piece of jewelery dissolves in the test acid. The concentration is constantly increased, if the abrasion loosens, then the gold content is lower.

Test acid box with nitric acid
for testing different concentrations


Drizzle with the tasting acid on abrasions
of gold jewelry on a slate


Further information and media
The household and chemical term acid, nitrates
Films: platinum and gold in nitric acid
Manufacture of nitric acid in the Ostwald process
Experiments on colloidal gold and silver
Digital foils: acids, bases, salts

Create your own book: Basic text nitric acid


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