Nitric acid (\(\text{HNO}_3\)) is a highly corrosive mineral acid and a powerful oxidizing agent used in various industrial and laboratory processes. Its chemical properties make storage a high-risk activity requiring strict safety protocols. Improperly stored nitric acid can initiate violent reactions, leading to fire, explosion, or the release of highly toxic gases. Adhering to storage guidelines is essential to mitigate the danger posed by this reactive compound.
Understanding Nitric Acid’s Reactive Nature
Nitric acid presents a dual chemical hazard, functioning simultaneously as a strong acid and a potent oxidizer. As a strong acid, it readily donates protons, making it intensely corrosive to human tissues and many metals. This corrosive action causes severe chemical burns upon contact with the skin or eyes.
Its strong oxidizing capability is equally hazardous, meaning it accepts electrons from other substances, supporting and accelerating combustion. Nitric acid reacts violently with reducing agents, organic materials, and combustible substances, potentially resulting in spontaneous ignition or explosion. Concentrated nitric acid can also decompose when exposed to light or heat, releasing toxic, reddish-brown nitrogen dioxide (\(\text{NO}_2\)) fumes that pose an inhalation risk.
Container and Material Specifications
Storage vessels must be constructed from chemically compatible materials to prevent degradation and failure. Approved materials include chemically inert borosilicate glass and certain plastics like high-density polyethylene (HDPE) or fluorinated polymers such as PTFE (Teflon). HDPE is generally suitable for concentrations up to 70%; 304-grade stainless steel containers are also used, relying on the metal’s passive oxide layer for corrosion resistance.
Containers must feature tight-fitting closures to prevent the escape of corrosive fumes, but they should be designed to manage pressure buildup. Since nitric acid can slowly decompose or off-gas, unrelieved pressure may cause the container to rupture, particularly plastic vessels. Each container must display a durable, GHS-compliant label that clearly identifies the chemical, hazard warnings, and safety precautions.
Strict Segregation Guidelines
Nitric acid must be physically separated from incompatible chemical classes to eliminate the risk of violent reactions. It should never be stored near flammable liquids, such as organic solvents, or combustible materials, including paper and wood, because its oxidizing nature can ignite these substances. Even small leaks can initiate a dangerous reaction if the acid contacts an incompatible material.
Segregation must also occur within the category of corrosives; for instance, nitric acid must be separated from bases like sodium hydroxide and organic acids like acetic acid. Storing nitric acid with other oxidizing acids, such as perchloric acid, is permissible, but they must be kept in separate secondary containment trays to prevent mixing. Physical separation is often achieved using dedicated, non-metallic or epoxy-coated corrosive storage cabinets.
Storage Environment and Physical Controls
The storage environment requires specific physical controls to ensure long-term stability and safety. Storage areas must be cool and protected from direct sunlight or heat sources, as elevated temperatures accelerate the acid’s decomposition and the release of toxic nitrogen dioxide gas. Maintaining the temperature below \(23^\circ\text{C}\) (\(73.4^\circ\text{F}\)) significantly slows this degradation process.
The storage location must be well-ventilated, ideally with a mechanical negative pressure system, to prevent the accumulation of corrosive vapors. Corrosive storage cabinets should be connected to exhaust ventilation to draw fumes away from the workspace. A noncombustible secondary containment system, such as spill trays or bunding, must be implemented beneath all containers. This containment should be sized to hold 110% of the largest single container’s volume or 10% of the total volume stored, whichever is greater, to capture a complete spill and prevent its spread.
Immediate Response and Disposal Procedures
Contingency planning involves having appropriate materials and trained personnel ready to respond immediately to spills. Small spills (typically less than one liter) can be neutralized using agents like sodium carbonate or calcium hydroxide, followed by absorption with an inert material such as vermiculite or sand. Avoid using combustible absorbent materials like sawdust, which can react violently with the strong oxidizer.
All personnel involved in spill cleanup must wear appropriate personal protective equipment (PPE), including chemical-resistant gloves, splash goggles, and an apron. For larger spills, or those beyond the capacity of on-site personnel, the area must be immediately evacuated, and a specialized hazardous materials (HazMat) team must be called. Waste nitric acid, including neutralized cleanup materials, must be collected in properly labeled, compatible containers. This waste must never be mixed with organic waste streams, as this combination is highly reactive and can lead to an explosion; disposal must be handled by licensed hazardous waste disposal contractors in accordance with federal and state regulations.