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nitric acid

Product name nitric acid
GOST 701-89
CAS 7697-37-2

Nitric acid HNO3 is one of most significant chemical products. It is produced commercially worldwide. It is clear, colorless, highly toxic and corrosive liquid with a pungent odor. Nitric acid becomes brown-reddish being heated or upon exposure of light due to partial reversible decomposition according with equation:

4HNO3 ⇄ 4NO2 + 2H2O + O2

Nitrogen dioxide NO2 admixture embrowns the acid. Nitric acid is fully miscible in water, boundedly soluble in diethyl ether. It is strong monobasic acid, fully dissociated in water solutions to give nitrate anion NO3- and hydroxonium cation H3O+. Nitric acid forms an azeotrope with water (68.4% wt HNO3, b.p. 120.7°C, d420 1.41). Also it forms mono- and trihydrates, which can be isolated in solid state.

Molecule of nitric acid has a planar geometry. Bond lengths (Å) and valent angles are shown on the figure. Nitric acid can exist in monoclinic or rhombic crystal modification in solid state.

Like almost acids, nitric acid reacts with basics, basic oxides to form salts. It also displace weaker acids from their salts. Nitric acid salts are called nitrates.

Oxidative properties.

Nitric acid is a strong oxidizing agent. It dissolves all metal excluding gold, rhodium, platinum, niobium, tantalum, iridium and zirconium, to produce nitrates or oxides. It also readily oxidizes elementary sulfur to give sulfuric acid H2SO4 and phosphorus up to phosphoric anhydride P2O5.

The products of the reaction with metals are determined by the concentration of nitric acid, the metal involved (i.e., its reactivity), and the temperature. In general diluted acid reduces in these reactions to release nitrogen monooxide NO, but sometimes nitrous oxide N2O or dinitrogen N2 formation is observed. Interaction with copper results in copper nitrate and releases NO gas:

3Cu + 8HNO3(diluted) → 3Cu(NO3)2 + 2NO + 4H2O

Concentrated nitric acid do not dissolve some metals such as chromium, iron or aluminium. These metals readily dissolve in diluted acid, but concentrated acid forms a metal oxide film at the surface that protects the metal from further oxidation. This phenomenon is called passivation and it allows to transport concentrated nitric acid in steel containers.

Generally, concentrated acid reacts with metals to release nitrogen dioxide:

Cu + 4HNO3(concentrated) → 3Cu(NO3)2 + 2NO2 + 2H2O

Because of nitric acid is a strong oxidizing agent, dihydrogen gas H2 in reaction with metals is extremely rarely formed. It is known that only magnesium and calcium interact with dilute nitric acid at low temperature to release dihydrogen:

Ca + 2HNO3(diluted) → Ca(NO3)2 + H2

Many organic compounds, especially consisting a labile functional groups, interact with nitric acid with destruction of their structure and to form carbon dioxide CO2 and water. These reactions can be highly exothermic and can cause severe burns or explosion. Some organic substances are nitrated by action of the mixture of concentrated sulfuric and nitric acids, e.g. phenol is nitrated step by step to produce picric acid (2,4,6-trinitrophenol) finally.

Nitration of phenol to form picric acid


Interaction of nitric acid with strong reducing agents like hydrazine or its substituted derivatives occurs extremely violently and accompanied with liberation of large amount of heat.

Commercial forms of nitric acid.

Modern chemical industry produces HNO3 in several forms. Most commonly used are 52-68% water solutions. Some applications requires 98-100% acid. White fuming nitric acid (WFNA) is very closed to anhydrous nitric acid. Inhibited white fuming nitric acid (IWFNA) is stabilized by the addition of 0.6-0.7% of hydrogen fluoride HF to prevent corrosion of metal containers upon shipment. The mixture of concentrated nitric and hydrochloric acids with a volume ratio 1:3 is called aqua regia. This mixture dissolves gold, platinum and other noble metals. The mixture of 100% HNO3 and 96% H2SO4 with volume ratio 9:1 is called melange acid and used for nitration of organic compounds.

Nitric acid is extremely toxic and corrosive compound. Exposure to vapours leads to immediate and severe irritation of the eyes, respiratory tract and mucous membranes. Its contact with skin or mucous layer results in serious injury. The contacts of nitric acid with flammable liquids and gases or organic materials (paper, oil, timber, coal and others) may cause burns.

Production.

  • Almost of nitric acid is manufactured industrially by ammonia oxidation. This process includes three stages:

    1. Catalytic oxidation of ammonia with oxygen of air. (reaction is conducted on grid that made of alloy of platinum with metals of platinum group):

    4NH3 + 5O2 → 4NO + 6H2O + 907.3KJ

    2. Oxidation of nitrogen monooxide NO into nitrogen dioxide NO2 in gaseous phase at 160-250°C:

    2NO + O2 → 2NO2 + 124KJ

    3. Absorption of nitrogen dioxide by water in plate-type column with a large interplate volumes:

    3NO2 + H2O → 2HNO3 + NO + 136.2KJ

    Nitrogen oxides in effluent gas are reduced either with ammonia on aluminium oxide at 250-300°C or with methane on aluminium-palladium catalyst at 350-500°C.
  • 100% nitric acid is made by rectification of triple mixtures. The mixture of water acid with dehydrating agent (sulfuric acid or magnesium nitrate Mg(NO3)2) is distilled to give vapours of 100% nitric acid that are condensed. Then the dehydrating agent is regenerated and recycled.
  • Another method to manufacture concentrated nitric acid is based on interaction of liquid N2O4 with pure oxygen and water under the pressure 5MPa. This process results in 97-98% nitric acid containing up to 30% of nitrogen oxides, which can be removed by following rectification.

    2N2O4(liquid) + 2H2O + O2 → 4HNO3 + 78.8KJ

  • The most simple and commonly used laboratory procedure for making of 100% nitric acid is the reaction on any anhydrous nitrate with concentrated sulfuric acid. Rectification of this mixture results in 100% acid:

    2NaNO3 + H2SO4 → 2HNO3 + Na2SO4

Uses.

  • as a raw material in simple and complex fertilizers making. Sodium nitrate NaNO3, potassium nitrate KNO3 and ammonium nitrate NH4NO3 are produced from nitric acid.
  • as an oxidizing agent in binary rocket fuels (e.g. in the mixture with 1,1-dimethylhydrazine);
  • as a nitrating agent in explosives making: trinitroglycerol or dynamite, trinitrotoluene or TNT, hexogene and others;
  • as a reagent sulfiric acid production by nitrose method;
  • to manufacture nitrocellulose;
  • as a reagent in production of nitro- substituted organic compouds. These derivatives are widely used as precursors of dyes, pharmaceuticals and other fine organic synthesis products.
  • for metal and semiconductor materials etching;

Manufacturer(s) JSC Azot
JSC Azot Novomoskovsk
Chemical structure of nitric acid

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