Nitric Acid (HNO₃): The Oxidizing Acid Behind Fertilizers, Explosives, and Gold Refining

Nitric acid (HNO₃) is a colorless to pale-yellow fuming liquid and one of the most important industrial acids. It is both a strong acid (fully ionized in water) and a powerful oxidizing agent, capable of dissolving most metals and oxidizing many non-metals including carbon, sulfur, and phosphorus. Concentrated nitric acid yellows over time due to decomposition into nitrogen dioxide (NO₂). ## The Ostwald Process Over 99% of commercial nitric acid is produced via the Ostwald process, developed by Wilhelm Ostwald in 1902: 1. Ammonia (NH₃) is catalytically oxidized over a platinum-rhodium gauze at ~900°C: 4NH₃ + 5O₂ → 4NO + 6H₂O 2. Nitric oxide cools and oxidizes further: 2NO + O₂ → 2NO₂ 3. NO₂ is absorbed in water: 3NO₂ + H₂O → 2HNO₃ + NO (the NO is recycled) The resulting acid is typically 50–68% concentration; further distillation can produce fuming (98%) nitric acid. ## Uses Over 75% of global production feeds into ammonium nitrate fertilizer, making nitric acid a cornerstone of modern agriculture. The remainder serves explosives manufacturing (TNT, RDX, nitroglycerin all require nitration), dyes, polyurethane precursors, and metal etching. In gold recovery and e-waste processing, dilute nitric acid selectively dissolves base metals (copper, iron, nickel) while leaving gold intact — the preparatory step before Aqua Regia: The Only Acid That Dissolves Gold dissolves the gold itself. ## Aqua Regia Connection Aqua Regia: The Only Acid That Dissolves Gold is a 1:3 mixture of nitric acid and hydrochloric acid. Neither acid alone can dissolve gold, but combined, the nitric acid oxidizes gold atoms while the hydrochloric acid provides chloride ions that form soluble chloroauric acid (HAuCl₄). This synergy is why aqua regia has been central to gold refining for over 700 years. ## Safety Nitric acid causes severe chemical burns on contact and reacts violently with organic materials. When concentrated acid contacts most metals, it releases toxic nitrogen dioxide fumes (orange-brown gas). Red fuming nitric acid (RFNA), used in rocketry, is among the most aggressive oxidizers in practical use. All handling requires chemical-resistant gloves, face shield, and fume ventilation.