Generally, no, especially if mixed, and ideally separate even in individual containers due to potential hazards and reactivity.
Understanding the Reactivity
When nitric acid (HNO₃) and hydrochloric acid (HCl) are mixed, typically in a 1:3 ratio (HNO₃:HCl), they form a fuming, highly corrosive mixture known as Aqua Regia. This name, Latin for "royal water," reflects its ability to dissolve noble metals like gold and platinum, which single acids cannot.
The provided reference highlights this interaction: "In summary, while you can mix nitric and hydrochloric acids in a defined ratio...". This confirms the potential for these two acids to be combined.
Storing the Mixture (Aqua Regia)
Storing the mixed solution (Aqua Regia) is highly discouraged for extended periods due to its inherent instability and hazards. The reference explicitly states: "...storing this mixture for an extended period is generally not advisable due to its reactivity and potential hazards."
Here's why storing Aqua Regia is problematic:
- Decomposition: Aqua Regia is unstable and decomposes over time, releasing corrosive and toxic gases, primarily chlorine (Cl₂) and nitrosyl chloride (NOCl). This decomposition reduces its effectiveness and builds pressure within a sealed container.
- High Reactivity: It is extremely corrosive and reactive, posing severe risks upon contact with skin, eyes, or other materials.
- Gas Evolution: The continuous production of gases can lead to pressure buildup in storage containers, potentially causing them to rupture if not properly vented.
The reference acknowledges that in some specific circumstances, temporary storage might be necessary, advising: "If you must store it, ensure it is in a safe, controlled environment and monitor it regularly." However, standard practice is to prepare Aqua Regia only just before use and dispose of any leftover material properly and promptly.
Storing Separate Containers in Proximity
Even storing separate containers of concentrated nitric acid and hydrochloric acid near each other is generally avoided in laboratory and industrial settings.
- Incompatibility: Nitric acid is a strong oxidizer, and hydrochloric acid is a strong acid. They are considered incompatible chemicals.
- Risk of Accidental Mixing: Storing them together increases the risk of accidental mixing in case of spills, container leaks, or other incidents. As established, mixing them creates the dangerous Aqua Regia.
Therefore, standard chemical storage guidelines recommend segregating these acids from each other and other incompatible substances.
General Acid Storage Guidelines
Safe storage of strong acids like nitric and hydrochloric acid requires adherence to strict safety protocols:
- Segregation: Store acids away from incompatible materials, such as bases, oxidizers, flammable substances, and reactive metals. Store different classes of acids (like oxidizing acids vs. non-oxidizing acids) separately.
- Appropriate Containers: Store acids in containers made of compatible materials (e.g., glass, specific plastics like PTFE or HDPE). Ensure containers are in good condition with tight-fitting caps.
- Secondary Containment: Use secondary containment (e.g., trays or tubs) to catch spills and prevent mixing of incompatible chemicals in case of primary container failure.
- Ventilation: Store acids in well-ventilated areas or corrosive storage cabinets to handle potential fumes.
- Labeling: Ensure all containers are clearly labeled with the chemical name, hazard warnings, and concentration.
- Cool, Dry Area: Store acids in a cool, dry area away from direct sunlight or heat sources.
Following these guidelines helps prevent dangerous reactions and ensures a safer working environment.
In conclusion, storing nitric acid with hydrochloric acid, particularly as a pre-mixed solution, is highly discouraged due to significant reactivity and safety hazards. Storing separate containers in close proximity is also ill-advised based on chemical incompatibility and the risk of forming dangerous mixtures in case of accidents.
