Aqua regia (8007-56-5) Physical and Chemical Properties
Aqua regia
A concentrated nitric/hydrochloric acid mixture used in specialized metal dissolution, etching and analytical workflows; highly corrosive and strongly oxidizing, requiring controlled handling and corrosion‑resistant equipment.
| CAS Number | 8007-56-5 |
| Family | Hydrochloric–nitric acid mixture (inorganic acid mixture) |
| Typical Form | Yellow fuming liquid |
| Common Grades | EP |
Aqua regia is an inorganic acid mixture composed principally of concentrated hydrochloric acid and concentrated nitric acid. It is classed functionally as a strong acid and a powerful oxidizing mixture rather than a discrete molecular substance; the mixture contains multiple reactive molecular and ionic species (including species reported as nitrosyl chloride and free chlorine) that together produce oxidative chlorination chemistry capable of dissolving noble metals. Structurally this material is non-stoichiometric and best described as an acid–oxidant system formed by mixing \(\mathrm{HCl}\) and \(\mathrm{HNO_3}\); simple molecular representations (for example, a single molecular formula) reflect an aggregate composition rather than a single covalent structure.
Electronically, aqua regia combines the proton acidity of \(\mathrm{HCl}\) and \(\mathrm{HNO_3}\) with oxidizing equivalents that enable halogenation and electron abstraction reactions. The resulting chemistry is highly oxidative and strongly acidic: it chlorinates and oxidizes metals and organic substrates, often generating gaseous products (chlorine, nitrosyl chloride, nitrogen oxides) and heat. The mixture is polar and strongly hydrophilic; it dissolves in water with evolution of heat and emits corrosive, irritating fumes. Because its active oxidizing species are produced in situ, properties such as oxidizing strength and fuming behavior vary with formulation ratio and temperature.
Practically, aqua regia is prepared immediately prior to use by mixing concentrated \(\mathrm{HNO_3}\) with concentrated \(\mathrm{HCl}\) in typical volumetric ratios of about 1:3 or 1:4 (\(\mathrm{HNO_3}:\mathrm{HCl}\)). It is widely used where aggressive oxidative chloride chemistry is required—most notably to dissolve gold and platinum group metals, in metal recovery and refining, and in specific laboratory etching or cleaning operations—because the combination of chlorine-based chlorinating agents and nitric-oxidation enables reactions not accessible to either acid alone. Common commercial grades reported for this substance include: EP.
Overview and Composition
Qualitative Composition
Aqua regia is a mixture of concentrated nitric acid and concentrated hydrochloric acid, typically prepared by combining one part \(\mathrm{HNO_3}\) with three or four parts \(\mathrm{HCl}\). The composition is non-stoichiometric: reaction between the two acids produces a dynamic equilibrium of species, including chlorinating oxidants such as nitrosyl chloride and molecular chlorine in addition to solvated protons and chloride ions. The input aggregate composition is represented by the computed molecular formula Cl3H4NO3 and aggregate descriptors derived from the mixture; these reflect the combined atom counts of the component acids rather than a discrete molecule.
Reported identifiers and computed descriptors associated with the aggregate composition include: - Molecular formula: Cl3H4NO3 - Molecular weight: 172.39 - Computed IUPAC-like string: nitric acid;trihydrochloride - SMILES: N+(O)[O-].Cl.Cl.Cl - InChI: InChI=1S/3ClH.HNO3/c;;;2-1(3)4/h3*1H;(H,2,3,4) - InChIKey: NICDRCVJGXLKSF-UHFFFAOYSA-N
(These descriptors are aggregate/computational representations for the mixed system and should be interpreted as mixture-level identifiers.)
Appearance and Typical Form
Aqua regia typically appears as a fuming liquid with colors reported in the range yellow, red, or gold, depending on composition and concentration. It emits a strong, pungent odor; vapors and fumes are irritating to eyes and mucous membranes. The liquid is highly corrosive to metals and organic tissue. It is miscible with water, and dissolution in water is exothermic; contact with moist air produces fumes.
Chemical Properties
Reactivity and Corrosive Behavior
Aqua regia is both a strong acid mixture and a strong oxidizing agent. Its reactivity characteristics include: - Dissolution of elemental metals, including gold and platinum, via oxidative chlorination pathways; dissolution proceeds through formation of soluble chloro-complexes of noble metals. - Oxidizing behavior that can initiate or catalyze decomposition and polymerization of susceptible organic compounds. - Generation of corrosive and toxic gases during reactions or decomposition (notably nitrosyl chloride, chlorine, and nitrogen oxides). - Exothermic reactions on dilution or on reaction with bases; it is soluble in water with release of heat and will fume on exposure to air.
Selected computed/aggregate properties (as reported for the mixture representation): - Molecular weight: 172.39 - Exact mass: 170.925676 - Monoisotopic mass: 170.925676 - Topological polar surface area (TPSA): 66.1 - Hydrogen bond donor count: 4 - Hydrogen bond acceptor count: 3 - Rotatable bond count: 0 - Heavy atom count: 7 - Formal charge: 0 - Complexity: 24.8 - Covalently-bonded unit count: 4
(Note: these values are computational descriptors for the aggregate representation and serve to describe polarity and hydrogen-bonding capacity of the constituent acid species.)
Compatibility and Incompatibilities
Aqua regia reacts vigorously or dangerously with a wide range of substances. Key incompatibilities and reaction partners include: - Strong bases and inorganic hydroxides — exothermic neutralization producing heat and salts. - Cyanide salts — reaction can release hydrogen cyanide gas. - Sulfites, nitrites, thiosulfates, dithionites — reactions can generate toxic gases such as sulfur oxides and hydrogen sulfide and other gaseous products. - Carbonates and bicarbonates — rapid CO2 evolution and foaming. - Organic materials (including reducing organics, mercaptans, dithiocarbamates, isocyanates, nitriles and many others) — may undergo violent oxidation, polymerization or produce toxic/flammable gases. - Metals other than noble metals — corrosion and, when in contact with reactive metals, evolution of flammable hydrogen gas; containers/substrates must be selected for high corrosion resistance. - Some firefighting foams and materials may react with the material to release corrosive or toxic gases.
Aqua regia is chemically unstable over time; reactive halogen species evolve and the oxidizing potency changes after standing. Containers should be chemically resistant and dry; contamination with water in storage containers or with incompatible materials can produce vigorous or hazardous reactions.
Usage and Safety
Industrial and Commercial Use Contexts
Aqua regia is used wherever aggressive oxidizing-chlorinating chemistry is required. Typical applications include: - Dissolution and processing of noble metals (gold, platinum group metals) for analytical sample preparation, metal refining and recovery. - Laboratory etching, cleaning, and digestion of refractory metal samples prior to analysis. - Specialty cleaning operations where strong oxidative removal of metal films or organics is necessary.
In practice the substance is selected for tasks that exploit its combined acidity and oxidative chlorination capacity; handling and process design must account for fuming, corrosivity and gas evolution.
Hazards and Handling Considerations
Hazards - Corrosive: causes severe skin burns and eye damage; corrosive to metals and many materials. - Toxic vapors: inhalation of fumes (chlorine, nitrosyl chloride, nitrogen oxides) can cause severe respiratory irritation, pulmonary injury or toxic pneumonitis. - Reactive: generates heat on dilution and can produce toxic/flammable gases on reaction with incompatible substances. - Non-combustible but may decompose when heated to produce corrosive and toxic fumes; contact of corrosive acids with metals can produce flammable hydrogen.
Safe handling and storage (high-level guidance) - Use in a chemical fume hood or equivalent local exhaust ventilation to control fumes; avoid confined-area accumulation of vapors. - Employ appropriate personal protective equipment: chemical-resistant gloves, full-face splash goggles or face shield, acid-resistant clothing and apron; for emergency or spill response, positive-pressure self-contained breathing apparatus (SCBA) and chemical protective suits may be required. - Store in compatible, corrosion-resistant containers in a cool, well-ventilated area; avoid contamination with water, bases, organics, or incompatible solids. Do not store in containers that may react (e.g., reactive metals). - Prepare fresh solutions as required; do not store large volumes for extended periods due to decomposition and gas evolution.
Emergency response and first aid (concise) - In case of skin or eye contact: immediately flush with copious amounts of water for at least 20–30 minutes and seek emergency medical attention; remove contaminated clothing and shoes while flushing. - If inhaled: move the exposed person to fresh air; administer oxygen if breathing is difficult; seek immediate medical attention. - For spills: evacuate and isolate area; keep upwind and prevent runoff to waterways and sewers; use dry, inert absorbents (dry earth or sand) to contain and collect small spills and place into suitable containers for disposal; avoid water application that can spread or react with spilled material. - Firefighting: material is non-combustible but can produce toxic fumes when heated. For small fires use CO2, dry chemical or dry sand; for larger fires use water spray or fog and cooling of containers from maximum safe distance. Do not allow water runoff to enter drains; dike for later disposal.
Transport and regulatory identifiers (selected) - CAS: 8007-56-5 - European Community (EC) number: 690-364-2 - UN number (transport): UN1798 - UNII: X3TT5X989E
For detailed, product-specific hazard, transport and regulatory information consult the product Safety Data Sheet (SDS) and applicable local regulations prior to procurement, use, storage or transport.
