Aqua regia is a highly corrosive mixture of concentrated acids, historically named “royal water” because of its unique ability to dissolve noble metals like gold and platinum. This fuming, yellowish-orange liquid is typically prepared by combining one part nitric acid with three parts hydrochloric acid by volume. Its application spans centuries, from early alchemical experiments to modern industrial processes and specialized scientific work.
How Aqua Regia Dissolves Noble Metals
The dissolving power of aqua regia comes from the synergistic action of its two components, as neither nitric acid nor hydrochloric acid can dissolve gold alone. Nitric acid is a powerful oxidizing agent, which initially converts a minuscule amount of the metal, such as solid gold, into metal ions.
The hydrochloric acid then supplies a large concentration of chloride ions. These chloride ions immediately bond with the newly formed metal ions, creating highly stable, soluble complex ions, such as the tetrachloroaurate anion (AuCl4-). By removing the gold ions as a stable complex, the hydrochloric acid shifts the chemical equilibrium. This prevents the metal ions from reducing back onto the metal surface, allowing the nitric acid to continuously oxidize the remaining solid metal.
The initial mixing of the two acids also generates highly reactive compounds, including nitrosyl chloride (NOCl) and elemental chlorine gas (Cl2). These compounds contribute significantly to the mixture’s aggressive nature and oxidizing potential. The resulting solution, often chloroauric acid (HAuCl4) when gold is dissolved, keeps the noble metal in a stable liquid solution.
Industrial Use in Precious Metal Recovery
Aqua regia’s ability to dissolve gold and platinum makes it a central tool in the precious metal refining industry. This method is effective for recovering gold from scrap materials, including old jewelry, dental alloys, and electronic waste (e-waste). The process separates gold from less valuable base metals and other impurities present in the source material.
The refining process begins by dissolving the metal source in the acid mixture, converting the gold into a soluble form. Once dissolved, the liquid is separated from any undissolved solid residues, such as insoluble silver chloride. A chemical reducing agent, such as sodium metabisulfite, is then added to the solution.
This reducing agent selectively reverses the chemical reaction for the gold ions, causing pure gold powder to precipitate out of the liquid. The resulting fine gold powder is filtered, washed to remove residual chemicals, and finally melted into high-purity gold bars. Because the process releases toxic gases like nitrogen dioxide and chlorine, industrial use requires strict safety measures, including specialized fume hoods and personal protective equipment.
Niche Applications in Science and Research
Beyond large-scale refining, aqua regia finds specialized uses in scientific laboratories where complete material breakdown is necessary. One significant application is in analytical chemistry, particularly for preparing samples for trace element analysis. Complex materials like geological samples, soil, or difficult-to-dissolve alloys must be broken down so that all trace elements are released into a liquid solution.
Aqua regia is frequently used for this “digestion” process before the sample is analyzed using sophisticated instruments like Inductively Coupled Plasma Mass Spectrometry (ICP-MS). This ensures the instrument measures the total concentration of elements, not just the easily soluble portions. The mixture is also a powerful cleaning agent for specialized scientific glassware that must be free of contaminants.
It is employed to remove stubborn organic residues or trace metal films from laboratory equipment, such as nuclear magnetic resonance (NMR) tubes. The cleaning application is restricted to glass and must use freshly prepared solution, as aqua regia loses potency over time. Storing it in a sealed container can build up dangerous pressure.