Aqua regia

Aqua regia is a freshly mixed, highly corrosive solution of concentrated hydrochloric and nitric acids. It dissolves gold and platinum by combining oxidation with chloride complex formation.

Nitrosyl chloride: Nitrosyl chloride is a reactive gas with formula NOCl, formed in aqua regia from nitric oxide and chloride-containing species. Its formation is part of the mixture’s changing chemistry and contributes to its reactivity.
Hydrochloric acid: Hydrochloric acid is an aqueous solution of hydrogen chloride, a strong acid that supplies hydrogen and chloride ions. Its chloride ions stabilize dissolved gold and platinum as soluble complexes.
Gold refining: Gold refining separates gold from other materials and impurities to produce metal of higher purity. Aqua regia can dissolve gold for subsequent separation and recovery.
Hydrofluoric acid: Hydrofluoric acid is an aqueous solution of hydrogen fluoride that attacks glass and many silicate materials. Unlike aqua regia’s attack on noble metals, hydrofluoric acid is notably effective against silica.
Chemical burn: A chemical burn is tissue damage caused by contact with a corrosive or otherwise harmful chemical. Aqua regia can cause severe burns on contact with skin or eyes.
Chlorine: Chlorine is a reactive halogen element that exists as a diatomic gas under ordinary conditions. Aqua regia can generate chlorine, an oxidant that helps attack noble metals.
Nitric acid: Nitric acid is a strong acid and oxidizing agent with formula HNO₃. It provides much of the oxidizing power that attacks otherwise resistant metals.
Fire assay: Fire assay is a high-temperature analytical method for measuring precious metals in ores and materials. Aqua regia digestion can follow assay procedures to prepare precious-metal samples for analysis.
Piranha solution: Piranha solution is a strongly oxidizing mixture of sulfuric acid and hydrogen peroxide used to clean organic residues. It is another hazardous cleaning mixture, but targets organic contamination rather than noble metals.
Nitrogen dioxide: Nitrogen dioxide is a toxic, reddish-brown gas with formula NO₂, produced in several oxidation reactions. Aqua regia can release toxic nitrogen oxides, including nitrogen dioxide.

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SuperacidCompared with: Its metal-dissolving power comes from reactive chemistry, not classification as a superacid.