How to Make Nitric Acid from Ammonia: A Step-by-Step Overview

Turning ammonia into nitric acid sounds complicated, but the basic idea is pretty straightforward. You start with ammonia gas, which is exposed to oxygen in the presence of a special catalyst, usually platinum, at high temperatures. This makes a gas called nitrogen monoxide. That gas then reacts with more oxygen to form nitrogen dioxide. Finally, when nitrogen dioxide is bubbled through water, it produces nitric acid.

You can think of it like a chain of changes: ammonia → nitrogen monoxide → nitrogen dioxide → nitric acid. This process is mostly used in factories because it requires precise control over heat, pressure, and the catalyst. At home or in small labs, it’s not safe to try because the gases are toxic and the reactions are highly exothermic. Factories use this to make fertilizers and other chemicals you probably use every day, like ammonium nitrate, showing how common nitric acid really is.

Making nitric acid (HNO₃) from ammonia (NH₃) is industrialized through the Ostwald process—a three-stage catalytic sequence that relies on controlled oxidation and acid-base reactions, with each step engineered to maximize efficiency and safety, and rooted in the chemical properties of NH₃ (a reducing agent) and HNO₃ (a strong oxidizing acid). This process is the global standard for HNO₃ production, as it leverages NH₃’s availability (synthesized via the Haber-Bosch process from atmospheric nitrogen and hydrogen) and avoids the inefficiencies of older methods.

The first stage is the catalytic oxidation of NH₃ to nitric oxide (NO)—the most critical step, as it converts NH₃ (a nitrogen-containing base) into a reactive nitrogen oxide intermediate. A mixture of NH₃ and preheated air (with a precise NH₃ concentration of ~10%, to prevent explosions) is passed over a heated catalyst—typically a fine gauze of platinum-rhodium alloy (90% Pt, 10% Rh)—at temperatures of 800–900°C. Platinum’s surface provides active sites where NH₃ molecules adsorb, weakening their N-H bonds, while rhodium enhances the catalyst’s stability and selectivity for NO over unwanted byproducts (e.g., nitrogen gas, N₂). The exothermic reaction (4NH₃ + 5O₂ → 4NO + 6H₂O) generates enough heat to maintain the catalyst temperature without external input, a key engineering efficiency. Without this catalyst, the reaction would be too slow to be industrial, as breaking NH₃’s strong covalent bonds requires excessive energy.

In the second stage, NO is oxidized to nitrogen dioxide (NO₂)—a spontaneous reaction that occurs at lower temperatures (200–300°C) in stainless steel absorption towers, with no catalyst needed. Here, NO reacts with excess oxygen from air: 2NO + O₂ → 2NO₂. This step is designed to proceed rapidly, as NO is unstable and prone to decomposition; the tower’s design ensures maximum contact between gas and oxygen to minimize NO loss.

The third stage converts NO₂ to HNO₃ via reaction with water. NO₂ is introduced into a series of packed absorption towers, where it reacts with a controlled flow of water (and recycled NO) to form HNO₃: 3NO₂ + H₂O → 2HNO₃ + NO. The NO produced is recycled back to the second stage to be reoxidized to NO₂, creating a closed loop that boosts overall yield to ~95%. The resulting HNO₃ is a 68–70% aqueous solution (the azeotropic point, where further concentration via distillation requires a dehydrating agent like sulfuric acid).

A common misunderstanding is assuming NH₃ can be directly converted to HNO₃ in a single step; the process requires intermediate nitrogen oxides because direct oxidation of NH₃ to HNO₃ would release excessive heat and produce toxic byproducts. Another misconception is neglecting the catalyst’s role—cheaper metals (e.g., copper) cannot replace platinum-rhodium, as they degrade in the high-temperature, acidic environment or produce N₂ instead of NO, reducing yield.

For chemical engineers, this process is vital to supplying HNO₃ for fertilizer production (via ammonium nitrate), electronics manufacturing (metal etching), and pharmaceuticals. Its reliance on NH₃ links it to the Haber-Bosch process, creating a supply chain that supports global agriculture and industry. Understanding the Ostwald process also illustrates how catalysis and reaction engineering enable the conversion of a simple base (NH₃) into a high-value acid (HNO₃), balancing efficiency, safety, and sustainability—critical for meeting global chemical demand.

When considering the industrial production of nitric acid from ammonia, the process relies on a sequence of controlled chemical reactions known as the Ostwald process. The first step involves oxidizing ammonia gas using a platinum-rhodium catalyst at high temperatures, typically around 800–900°C. This reaction converts ammonia (NH₃) into nitric oxide (NO), a key intermediate. The efficiency of this step is crucial because it determines how much of the ammonia is successfully transformed without producing unwanted byproducts.

Once nitric oxide is formed, it reacts rapidly with oxygen in the air to produce nitrogen dioxide (NO₂). This stage is exothermic and requires precise handling to avoid energy losses and ensure safety. The nitrogen dioxide gas is then absorbed in water, producing nitric acid (HNO₃) with a concentration that can be further refined for industrial applications. This absorption step highlights the solubility characteristics of nitrogen dioxide in water and explains why large-scale plants often use packed towers or similar setups to maximize contact between the gas and liquid phases.

In practical terms, this process is the backbone of fertilizer production, supplying ammonium nitrate and other nitrogen-based compounds essential for agriculture. The same chemistry also supports the manufacture of explosives, dyes, and various industrial chemicals. Understanding each step—from catalytic oxidation to gas absorption—illustrates not only the chemical principles but also the operational considerations such as catalyst longevity, temperature control, and gas flow management. This sequence shows why nitric acid production is both a highly technical and commercially vital chemical process.

Converting ammonia (NH₃) into nitric acid (HNO₃) is a cornerstone of industrial chemistry, primarily achieved through the Ostwald process, which elegantly combines catalytic oxidation, gas-phase reactions, and aqueous absorption. The process begins with ammonia being oxidized over a platinum-rhodium catalyst at 800–900°C, where it reacts with oxygen to form nitrogen monoxide (NO) and water (4NH₃ + 5O₂ → 4NO + 6H₂O). This step exploits the catalyst’s ability to lower the activation energy, enabling efficient conversion under high temperatures that favor the exothermic reaction. The nitrogen monoxide produced is a colorless gas but rapidly reacts with additional oxygen at lower temperatures (200–400°C) to form nitrogen dioxide (NO₂), a reddish-brown gas with strong oxidizing properties (2NO + O₂ → 2NO₂).

The nitrogen dioxide is then absorbed into water under controlled pressure, initiating a series of reactions that yield nitric acid and regenerate nitrogen monoxide (3NO₂ + H₂O → 2HNO₃ + NO). This cyclic regeneration of NO is critical for minimizing waste and improving process efficiency, reflecting principles of sustainable chemistry. The reaction’s thermodynamics favor nitric acid formation, as the absorption of NO₂ in water is exothermic, releasing heat that must be managed to prevent runaway reactions.

Industrially, this method underpins the production of fertilizers like ammonium nitrate, which supports global agriculture by providing essential nitrogen to crops. It also enables the synthesis of explosives, plastics, and dyes, demonstrating nitric acid’s versatility in chemical manufacturing. However, the process demands robust engineering to handle corrosive gases and high temperatures, requiring specialized materials for reactors and piping. From a broader perspective, the Ostwald process exemplifies how fundamental chemical principles—catalysis, oxidation, and equilibrium—can be harnessed to address societal needs, from food production to material science, while highlighting the ongoing challenge of balancing industrial efficiency with environmental safety.