Nitric acid (HNO₃) is a powerful chemical compound that appears as a colorless liquid in its pure form. However, samples may develop a yellow or reddish-brown tint over time due to decomposition into nitrogen oxides. It is known for its strong reactivity and is an important chemical used in various industrial and laboratory settings.
Understanding Nitric Acid’s Nature
Nitric acid is a highly corrosive mineral acid, characterized by a sharp, pungent odor. Commercially, it is often available as a 68% solution in water, known as “concentrated nitric acid.” It is a strong acid, fully dissociating into hydronium and nitrate ions in aqueous solutions. The density of nitric acid varies with concentration, with anhydrous nitric acid having a density of approximately 1.512 g/cm³.
Beyond its acidic nature, nitric acid functions as a powerful oxidizing agent. Its oxidizing capability stems from its ability to readily accept electrons from other substances. When it reacts, nitric acid is reduced to products like nitrogen monoxide (NO) or nitrogen dioxide (NO₂). This oxidizing property allows it to react with many non-metallic compounds, sometimes with explosive force.
Nitric acid exhibits significant reactivity with most metals, excluding noble metals like gold and platinum, and certain alloys. These reactions can produce heat and gases, including flammable hydrogen. It also reacts vigorously with organic materials, a process that can be explosive due to the formation of less stable byproducts. This reactivity contributes to its corrosive effects on living tissue, as it can carry out acid hydrolysis with proteins and fats.
Key Uses Across Industries
Nitric acid is a widely used chemical with diverse applications across many industries. Its primary industrial use is in fertilizer production, accounting for a significant portion of global consumption. When neutralized with ammonia, it forms ammonium nitrate, a common fertilizer component. Other fertilizers, such as calcium nitrate, also rely on nitric acid.
The compound plays a substantial role in the explosives industry. It is used to create various explosives, including nitroglycerin and trinitrotoluene (TNT), through a process called nitration. This involves introducing a nitro group to organic molecules, leading to shock- and thermally-sensitive compounds. Fuming nitric acid, with concentrations around 98% HNO₃, is often preferred for these applications due to its predictable behavior.
Beyond fertilizers and explosives, nitric acid is used in:
- Nylon production.
- Metal etching and as a cleaning agent.
- Laboratories as a reagent for analytical purposes, a catalyst, and an oxidizer.
- Rocket propellants as an oxidizer.
- Purification of precious metals like gold, silver, and platinum.
Safety Considerations
Nitric acid poses significant safety hazards due to its corrosive and oxidizing properties. Direct contact with skin, eyes, or mucous membranes can result in severe chemical burns. Concentrated nitric acid may stain human skin yellow upon contact, a reaction with keratin. Inhalation of its vapors or mists can cause irritation to the nose, throat, and lungs, potentially leading to coughing, shortness of breath, and fluid accumulation in the lungs (pulmonary edema).
Proper handling is essential to mitigate these risks. All operations involving nitric acid should be conducted in a certified chemical fume hood to prevent vapor inhalation. Personal protective equipment (PPE) is necessary, including chemical-resistant gloves, safety goggles, and a lab coat or acid-resistant apron. For concentrated solutions, specific glove materials like neoprene may offer better protection than standard nitrile gloves.
Storing nitric acid requires careful attention to prevent hazardous reactions. It should be kept in well-ventilated areas, away from direct sunlight, and separated from incompatible substances like organic materials, reducing agents, and metals. Containers should be tightly closed and made of compatible materials like high-density polyethylene (HDPE), fluorinated plastics, or glass, avoiding metal containers. In case of skin contact, flush the affected area with large quantities of water for at least 15 minutes and seek immediate medical attention. For eye exposure, continuously flush with water while holding eyelids open is crucial until medical personnel arrive.