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Nitric and sulfuric acid are important industrial chemicals used in fertilizer and explosives production, as well as laboratory procedures. Nitric acid is produced commercially by catalytic oxidation of ammonia, while sulfuric acid is produced by burning fossil fuels. Both acids are major pollutants and contribute to acid rain.
Nitric acid and sulfuric acid are two of the most important industrial chemicals and are produced in huge quantities. They are connected in various ways, in terms of manufacture and use. Both acids are vital to the fertilizer and explosives industries and are among the most widely used laboratory reagents. They are also serious pollutants and contribute to acid rain.
An early method of producing sulfuric acid was heating saltpeter (NaNO3) and sulfur with steam. Nitric acid (HNO3) was first produced by distilling saltpeter with concentrated sulfuric acid (H2SO4). The two chemicals react to produce nitric acid and sodium hydrogen sulfide: H2SO4 + NaNO3 → NaHSO4 + HNO3. The mixture is heated and nitric acid, boiling at 181.4°C (83°F), is produced as a vapor which is condensed in a receiver.
These production methods have been outdated in the industrial context, but the distillation of a metal nitrate with sulfuric acid can still be used to produce small quantities of nitric acid in the laboratory. Hot nitric acid vapor tends to partially decompose into nitrogen dioxide, water and oxygen, resulting in an impure acid. To obtain the pure acid, the distillation is done under reduced pressure, so that less heat is required. The apparatus used must be all glass, as nitric acid rapidly destroys rubber, cork and most plastics.
Today, nitric acid is produced commercially by the catalytic oxidation of ammonia, forming nitrogen dioxide, which is dissolved in water to produce the acid. The crude acid can be distilled to produce concentrations up to 68.5%, at which point it forms an azeotrope with water. This is the commercial nitric acid used for most applications and as a laboratory reagent. Higher concentrations cannot be obtained by simple distillation. When anhydrous or highly concentrated nitric acid is required – often called “fuming nitric acid” – it can be made by distillation under reduced pressure with concentrated sulfuric acid, which removes the water.
The largest use for both nitric acid and sulfuric acid is in the fertilizer industry. Nitric acid is used in the production of nitrate fertilizers, which release essential nitrogen in a form that can be easily absorbed by plants. Ammonium nitrate is one of the most used fertilizers. Sulfuric acid is reacted with rock phosphate or bone meal to produce “superphosphate,” which supplies phosphorus, another essential element for plants. This acid is also used in the production of ammonium sulphate, a common fertilizer.
Another major industrial application for nitric acid and sulfuric acid is in the manufacture of explosives. Most explosives in military and commercial use are synthesized by nitration of organic compounds; in this process, nitronium ions (NO2+) are used to replace the hydroxyl groups (OH) with nitro groups (NO2). NO2+ ions are provided by the combination of nitric acid and sulfuric acid, which react as follows: HNO3 + 2H2SO4 → H3O+ + NO2+ + 2HSO4-.
For some explosives, such as trinitrotoluene (TNT), the mixture must be free of water. This can be ensured by using anhydrous, or “fuming,” nitric acid. Alternatively, commercial nitric acid can be used with oleum, made by dissolving sulfur trioxide (SO3) in concentrated sulfuric acid, so that the water is removed in combination with SO3 to produce more sulfuric acid. The latter method is generally preferred, as oleum is easily made by the same process that makes most sulfuric acid. Fuming nitric acid is relatively expensive to produce, dangerous to transport, and prone to decomposition if not refrigerated.
Aside from their industrial applications, nitric acid and sulfuric acid are among the most commonly used laboratory reagents. Both are useful in procedures that require strong acids. Sulfuric acid is a powerful dehydrating agent that can be used to dry some freshly prepared gases and in reactions involving the removal of water from compounds. It can also be used to free other acids from their salts. Nitric acid is useful as an oxidizing agent, as a nitrating agent, and is used extensively in chemical analysis, especially in the identification of metals present in soil and mineral samples.
Sulfuric and nitric acids are also major pollutants and contribute to acid rain. Nitric acid is formed when the gaseous nitrogen dioxide, found in vehicle exhaust and naturally produced by the action of lightning, dissolves in water. Sulfuric acid is formed when sulfur dioxide – produced by burning fossil fuels and naturally from volcanoes – reacts with oxygen to form sulfur dioxide, which dissolves in water to produce sulfuric acid. Despite the natural production of these compounds, man-made sources account for the majority of acid rain. Sulfuric acid is the largest contributor, as sulfur dioxide is a more common pollutant than nitrogen dioxide.
