Nitric acid -PRODUCTION

Industrial nitric acid production uses the Ostwald process. The combined Ostwald and Haber processes are extremely efficient, requiring only air and natural gas feedstocks.

The Ostwald process’ technical innovation creates the proper conditions under which anhydrous ammonia burns to nitric oxide (NO) instead of dinitrogen (N2). The nitric oxide is then oxidized, often with atmospheric oxygen, to nitrogen dioxide (NO2):

2 NO + O2 → 2 NO2

The dioxide then disproportionates in water to nitric acid and the nitric oxide feedstock:

3 NO2 + H2O → 2 HNO3 + NO

The net reaction is maximal oxidation of ammonia:

NH3 + 2 O2 → HNO3 + H2O

Dissolved nitrogen oxides are either stripped (in the case of white fuming nitric acid) or remain in solution to form red fuming nitric acid.

Commercial grade nitric acid solutions are usually between 52% and 68% nitric acid by mass, the maximum distillable concentration. Further dehydration to 98% can be achieved with concentrated H2SO4.Historically, higher acid concentrations were also produced by dissolving additional nitrogen dioxide in the acid, but the last plant in the United States ceased using that process in 2012.

More recently, electrochemical means have been developed to produce anhydrous acid from concentrated nitric acid feedstock.

Laboratory synthesis

Laboratory-scale nitric acid syntheses abound. Most take inspiration from the industrial techniques.

A wide variety of nitrate salts metathesize with sulfuric acid (H2SO4) – for example, sodium nitrate:

NaNO3 + H2SO4 → HNO3 + NaHSO4

Distillation at nitric acid’s 83 °C boiling point then separates the solid metal-salt residue. The resulting acid solution is the 68.5% azeotrope, and can be further concentrated (as in industry) with either sulfuric acid or magnesium nitrate.

Alternatively, thermal decomposition of copper(II) nitrate gives nitrogen dioxide and oxygen gases; these are then passed through water or hydrogen peroxide as in the Ostwald process:

2 Cu(NO3)2 → 2 CuO + 4 NO2 + O2

2 NO2 + H2O → HNO2 + HNO3 or 2 NO2 + H2O2 → 2 HNO3