Nitric acid (\(\text{HNO}_3\)) is a strong mineral acid used in industries for manufacturing fertilizers, explosives, metal cleaning, and etching. As a common laboratory and industrial chemical, questions frequently arise regarding its inherent dangers, particularly whether it poses a flammability risk. Understanding the true chemical properties of this substance is essential for ensuring safety in any environment where it is used.
Flammability Status and Chemical Definition
Nitric acid is classified as a non-combustible and non-flammable substance under typical conditions. The liquid form, often commercially available between 68% and 70% concentration, does not readily ignite or sustain a fire and possesses no measurable flash point. This absence of flammability distinguishes it from true fuels, which are defined by their ability to vaporize and combust. This characteristic is represented on the National Fire Protection Association (NFPA) 704 hazard diamond, where the red section for flammability is scored at zero.
The strong, colorless to yellow liquid is a monoprotic acid, meaning it releases a single proton when dissolved in water, making it highly corrosive. Even fuming nitric acid, a more concentrated form above 86%, remains non-flammable. However, dismissing the fire risk based solely on this non-combustible nature is an error, as its primary danger lies in its intense chemical reactivity.
The Hazard of Extreme Oxidizing Properties
The true fire hazard associated with nitric acid stems from its classification as a powerful oxidizer. An oxidizer readily supplies oxygen to a chemical reaction, drastically lowering the ignition temperature of surrounding materials and accelerating their combustion. Nitric acid does not burn itself, but it can make other substances burn much hotter and faster than they normally would. This property is why the NFPA 704 diamond often carries a special hazard designation of “OX” in the white section.
Contact between concentrated nitric acid and incompatible materials can lead to violent, spontaneous combustion or explosive reactions. Materials such as wood, paper, solvents, clothing, turpentine, and charcoal are highly susceptible to this reaction. Mixing the acid with powdered metals, alcohols, or bases can generate intense heat and release flammable gases, such as hydrogen, creating a genuine fire or explosion risk. The violent nature of these reactions can lead to deflagration, where a rapid burn creates a supersonic pressure wave.
Safe Handling and Storage Protocols
The oxidizing hazard of nitric acid mandates strict safety protocols for both handling and storage. Personnel must wear appropriate personal protective equipment (PPE), including chemical-resistant gloves, a face shield over safety goggles, and an acid-resistant apron or lab coat. Due to the acid’s tendency to emit toxic nitrogen oxide fumes, particularly when heated or concentrated, all work must be conducted within a chemical fume hood or an area with robust local exhaust ventilation.
Storage requires segregation, which is the most important measure to prevent accidental reactions. Nitric acid must be stored entirely separate from incompatible materials, including organic compounds, flammable liquids, reducing agents, and strong bases. The containers themselves must be constructed from compatible materials, such as glass or specific plastics like high-density polyethylene (HDPE) or polytetrafluoroethylene (PTFE), as the acid will corrode and potentially breach metal containers. Storage areas should be cool, dry, and well-ventilated, positioned away from direct sunlight or external heat sources to prevent decomposition and pressure buildup.