What Is the Connection between Nitric Acid and Sulfuric Acid?

Nitric acid and sulfuric acid are two of the most important industrial chemicals and are produced in huge amounts. They are connected in a number of ways, in terms of their manufacture and their uses. 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 contributors to acid rain.

An early method of making sulfuric acid was by 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 sulfate: H₂SO4 + NaNO₃ → NaHSO₄ + HNO₃. The mixture is heated and the nitric acid, which boils at 181.4°F (83°C), is produced as a vapor that is condensed in a receiver.

These methods of production have been superseded in the industrial context, but the distillation of a metal nitrate with sulfuric acid may still be employed to produce small amounts of nitric acid in the laboratory. The hot nitric acid vapor tends to partially decompose into nitrogen dioxide, water and oxygen, resulting in an impure acid. To obtain the pure acid, distillation is performed 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 manufactured commercially by the catalytic oxidation of ammonia, forming nitrogen dioxide, which is dissolved in water to produce the acid. The raw 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 very highly concentrated nitric acid — often called “fuming nitric acid” — is required it can be produced by distillation under reduced pressure with concentrated sulfuric acid, which removes the water.

The biggest 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 widely used fertilizers. Sulfuric acid is reacted with phosphate rock or bone meal to produce “superphosphate,” which provides phosphorus, another essential element for plants. This acid is also used in the production of ammonium sulfate, a common fertilizer.

Another major industrial application for nitric acid and sulfuric acid is in the production of explosives. The majority of explosives in military and commercial use are synthesized by nitrating organic compounds; in this process, nitronium (NO₂⁺) ions are used to replace hydroxyl (OH) groups with nitro (NO₂) groups. The NO₂⁺ ions are provided by combining nitric acid and sulfuric acid, which react as follows: HNO₃ + 2H₂SO₄ → H₃O⁺ + NO₂⁺ + 2HSO₄^-.

For some explosives, for example trinitrotoluene (TNT), the mixture must be water-free. 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 by combination with SO₃ to produce more sulfuric acid. The latter method is generally preferred, as oleum is easily manufactured by the same process that produces most sulfuric acid. Fuming nitric acid is relatively costly to produce, dangerous to transport and prone to decomposition unless kept refrigerated.

Aside from their industrial applications, nitric acid and sulfuric acid are among the most commonly used laboratory reagents. Both are useful in procedures requiring strong acids. Sulfuric acid is a powerful dehydrating agent that can be used for drying some freshly prepared gases and in reactions involving the removal of water from compounds. It can also be used to release other acids from their salts. Nitric acid is useful as an oxidizing agent, as a nitrating agent and is widely used in chemical analysis, especially in identifying metals present in soil and mineral samples.

Sulfuric and nitric acids are also major pollutants and contributors to acid rain. Nitric acid is formed when the gas nitrogen dioxide — found in vehicle exhaust and produced naturally by the action of lightning — dissolves in water. Sulfuric acid is formed when sulfur dioxide — produced by the burning of fossil fuels and naturally by volcanoes — reacts with oxygen to form sulfur trioxide, which dissolves in water to produce sulfuric acid. Despite the natural production of these compounds, man-made sources account for most acid rain. Sulfuric acid is the larger contributor, as sulfur dioxide is a more common pollutant than nitrogen dioxide.