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What Is the Ostwald Process?

By Phil Riddel
Updated May 17, 2024
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The Ostwald process is a method used for the industrial production of nitric acid, patented by the German/Latvian chemist Willhelm Ostwald in 1902 and first implemented in 1908. In this process, nitric acid is synthesized by the oxidation of ammonia. Prior to the introduction of the Ostwald process, all nitric acid was produced by distilling saltpeter — sodium nitrate (NaNO3) or potassium nitrate (KNO3) — with concentrated sulfuric acid. The Ostwald process now accounts for all industrial production of nitric acid, a chemical crucial to the fertilizer and explosives industries.

The first synthesis of nitric acid — by heating a mixture of saltpeter, copper sulfate and alum — is generally attributed to the Arabic alchemist Jabir ibn Hayyan Geber sometime in the 8th century, but there is some uncertainty about this. In the mid 17th century, the German chemist Johann Rudolf Glauber produced the acid by distilling saltpeter with sulfuric acid. Nitric acid was of interest mainly because of its ability to dissolve most metals until the discovery, in 1847, of nitroglycerin. Soon after this point, with the opening up of a new range of explosives made by the nitration of organic compounds, nitric acid — and its precursor, saltpeter — were much in demand. Until the early 20th century, all nitric acid production was from saltpeter.

In 1901, Willhelm Ostwald, a German chemist born in Latvia, developed a method of synthesizing nitric acid from the oxidation of ammonia by catalysis. The process takes place in three steps. Firstly, a mixture of one part ammonia (NH3) gas and 10 parts air is fed into the catalytic chamber where, at a temperature of 1292 to 1472 °F (700 to 800 °C) and using a platinum catalyst, the ammonia combines with oxygen (O2) to produce nitric oxide (NO): 4NH3 + 5O2 → 4NO + 6H2O. Secondly, in the oxidation chamber, at a temperature of 122 °F (50 °C), nitric oxide is combined with oxygen to produce nitrogen dioxide: 2NO + O2 → 2NO2. Finally, in the absorption chamber, the nitrogen dioxide is dissolved in water, giving nitric acid (HNO3) and nitric oxide, which can then be recycled: 3NO2 + H2O → 2HNO3 + NO.

The Ostwald process produces nitric acid as an aqueous solution of about 60% concentration. By distillation, the concentration is increased to 68.5%, giving the reagent-grade nitric acid that is used for most purposes. This acid is an azeotrope of nitric acid and water, meaning that the two compounds boil at the same temperature — 251.6 °F (122 °C), and so it cannot be further concentrated by simple distillation. If higher concentrations are required, they can be obtained by distillation with concentrated sulfuric acid — which absorbs the water — or directly by the combination of nitrogen dioxide, water and oxygen at high pressure.

This chemical process would reduce the reliance on diminishing reserves of saltpeter, but required a source of ammonia, which at that time was not readily available in large amounts. The ammonia problem was solved by the development of the Haber process, in which this compound was synthesized using atmospheric nitrogen and hydrogen from natural gas. The Ostwald process quickly took over as the main means of nitric acid production.

These two industrial processes between them enabled the cheap production of nitric acid in huge amounts. This in turn led to increased agricultural productivity, as nitrate fertilizers could be produced cheaply in bulk quantities. It also, however, prolonged World War I, as Germany — cut off from most supplies of saltpeter during the war — was able to continue producing explosives in large quantities.

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