Nitric acid

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Nitric Acid
Chemical formula HNO3
OTP appearance clear, oily liquid 
Molar Mass(g/mol) 63.01 
Melting Point(°C) -42 
Boiling Point(°C) 83 
Density(g/cc) 1.51 
Solubility in water(g/L) misc 
Water azeotrope(%w/w) 68 
Water azeotrope bp(°C) 120.5
NFPA 704

Nitric acid is the central exchange for nitrate ions. Nitrates can in general be converted to nitric acid via sulfuric acid, and most compounds can be nitrated by exposure to nitric acid. This makes nitric acid an essential source of oxidizers.



  • Nitration
    Nitric acid is used for nitrating other compounds, such as converting cellulose to nitrocellulose:
    3 HNO3 + C6H10O5 C6H7O5(NO2)3 + 3 H2O
  • Action on metals
    Alone, nitric acid attacks most metals, including tin, copper, silver, magnesium, manganese and zinc. In concert with hydrochloric acid it will dissolve noble metals such as gold and platinum. These properties make nitric acid a useful metal etchant.



From sulfuric acid and nitrates

Nitric acid is easily produced by combining equimolar amounts of sulfuric acid with nitrate feedstocks such as potassium nitrate, which results in an equilibrium between the -bisulfate and the -nitrate. The reaction is driven forward by distilling off the nitric acid.


The above process produces the -bisulfate and not the sulfate. Adding additional -nitrate could potentially produce additional nitric acid for the same amount of sulfuric acid:


But this requires greater purity reactants, more concentrated sulfuric acid, and results in a less-soluble (cleanable) -sulfate.

dry materials

This method requires 98% sulfuric acid, and relatively "dry" nitrates, but produces 96%+ nitric acid.

  1. Gather 85 ubm sodium nitrate or 101ubm potassium nitrate
  2. Gather 98 ubm of sulfuric acid (or molar equivalent)
  3. Combine the -nitrate and acid in an alembic
  4. Heat alembic to 85°C
  5. Collect distillate. It is pure nitric acid
  6. Neutralize the remaining liquid in the alembic and discard it. It is primarily the bisulfate salt of the nitrate compound.

This produces pure (96%) nitric acid.

hydrous materials

This method can use dilute sulfuric acid and/or hydrated nitrates, and produces 68% (azeotropic) nitric acid.

  1. Gather 85ubm sodium nitrate or 101ubm potassium nitrate
  2. Gather equimolar amount of sulfuric acid
  3. Combine the -nitrate, along with a minimum of water, in the alembic
  4. Slowly add the acid to the nitrate solution
  5. Heat alembic to 105°C
  6. Wait until no more condensate forms
  7. Discard distillate. It is primarily water.
  8. Heat alembic to 125°C
  9. Collect distillate. It is pure nitric acid
  10. Neutralize the remaining liquid in the alembic and discard it. It is primarily the bisulfate salt of the nitrate compound.

From phosphoric acid and nitrates

Heat a mixture of sodium nitrate and phosphoric acid yielding nitric acid and sodium dihydrogen phosphate

H3PO4 + 3 NaNO3 Na3PO4 + 3 HNO3 // Comes out as either azeotropic nitric acid or mixed oxides and water in the boiling flask

From nitrogen dioxide

cf. Ostwald process
and air

Nitric acid is produced from bubbling nitrogen dioxide and air through water in a cycle. At any given point in the cycle, there is some amount of nitric acid and some amount of nitrogen oxides.

NET: 4 NO2 + O2 + 2 H2O 4 HNO3
Step Reaction Consumed Produced Total Nitric Acid
Absorption of nitrogen dioxide 6 NO2 + 3 H2O 3 HNO3 + 3 HNO2 6 NO2 + 3 H2O 2 HNO3 + 3 HNO2 2
Recombination 3 HNO2(aq)
warm heat}
HNO3 + 2 NO(g) + H2O
3 HNO2 HNO3 + 2 NO2 + H2O 3
Oxidation 2 NO(g) + O2(g) 2 NO2 2 NO2 + O2 2 NO2 4 (via ⅓ previous reactions)
and hydrogen peroxide

The reaction is much faster and more direct if hydrogen peroxide is used instead of water:

2 NO2 + H2O2 2 HNO3



Nitric acid forms an azeotrope with water between 67 and 68 percent nitric acid. Up to 67% or so, distillation works fine: the very dilute acid is boiled, providing a closer-to-the-azeotrope condensate. This can be iterated, or performed under vacuum, but it will only get the solution up to 67% or so nitric acid.


There are methods to go beyond the azeotropic limit.

sulfuric acid

Sulfuric acid can be used to "hold" the water back from boiling. A mixture of near-azeotropic nitric acid, combined with 50% by mass 98% sulfuric acid will distill off pure nitric acid and leave dilute sulfuric acid behind. Since sulfuric acid is (somewhat) easier to concentrate, this represents a net gain.


A lossy but lower-energy solution might involve a common-ion approach. Anydrous magnesium nitrate (Mg(NO3)2) added to azeotropic nitric acid will affect the solution in exactly the same way as sulfuric acid (50% by mass breaks the azeotrope) but is very difficult to obtain. Magnesium nitrate most commonly occurs as the hexahydrate, and decomposes before it dehydrates. Thus, the nitrate needs to be made in situ: Add magnesium carbonate to the azeotropic nitric acid, producing magnesium nitrate and sub-azeotropic nitric acid. But this time the entirety of the acid may be separated by distillation.

HNO3 + H2O + MgCO3 Mg(NO3)2 + HNO3 + H2O + CO2


When produced from mixed oxides of nitrogen, nitric acid will often contain impurities of nitrous acid HNO2.

nitrous acid removal

via urea

If the nitric acid has already nitrated something (more ammonia, for instance) and has thus become nitrates, adding urea decomposes nitrous acid to water and inert gases while leaving nitrates unreacted.[1] However, any nitric acid still present will nitrate the urea forming an explosive precipitate, so be careful.

CO(NH2)2 + HNO2 2 N2 + 3 H2O + CO2
CO(NH2)2 + HNO3 CO(NH2)2•HNO3

via potassium dichromate

Potassium dichromate can oxidize nitrous acid[2]

3 HNO2 + 2 K2Cr2O7 3 HNO3 + 2 K2CrO4 + Cr2O3




See Also


  1. US patent 2139142A "Purification of materials containing nitrite", 1934
    Link courtesy Google patents
  2. Millon, M. E. (1844) "Purification of Nitric Acid"
    American Journal of Pharmacy IX Merrihew & Thompson, Printers
    link courtesy Google Books.