Aqua Regia: The Only Acid That Dissolves Gold

Aqua regia (Latin: "royal water" or "king's water") is a fuming, corrosive mixture of hydrochloric acid (HCl) and nitric acid (HNO₃), typically in a 3:1 ratio by volume. It is one of the few chemical mixtures capable of dissolving gold and platinum — metals that resist attack by any individual acid. ## How It Works Neither acid alone can dissolve gold. The mechanism requires both working in concert: 1. Nitric acid (the oxidizer) attacks the gold surface, removing electrons and creating gold ions (Au³⁺) 2. Hydrochloric acid provides chloride ions (Cl⁻) that immediately bind to the gold ions, forming chloroauric acid (HAuCl₄) — a stable, soluble complex 3. By continuously removing gold ions from the surface (stabilizing them as chloroaurate), the reaction is driven forward Without chloride ions to stabilize the dissolved gold, the reaction would quickly reach equilibrium and stop. The two acids create a synergistic system that neither can achieve alone. ## Historical Significance The mixture was known to medieval alchemists, who named it for its ability to dissolve the "royal" (noble) metals that resisted all other chemical attacks. Its discovery was an important step in understanding that gold, while extremely resistant to corrosion, was not truly indestructible. During World War II, Hungarian chemist George de Hevesy dissolved the gold Nobel Prize medals of Max von Laue and James Franck in aqua regia to prevent their confiscation by Nazi forces occupying Copenhagen. After the war, the gold was precipitated from solution and recast into new medals by the Nobel Foundation. ## Practical Use Aqua regia is used in gold refining, precious metal recovery from electronics, and analytical chemistry. In gold recovery from electronic waste, it dissolves gold after base metals have been removed with nitric acid alone. The dissolved gold is then precipitated back to metallic form using sodium metabisulfite (SMB) or other reducing agents. **Safety:** Aqua regia produces toxic fumes including nitrogen dioxide (NO₂) and chlorine gas (Cl₂). All work must be performed outdoors or under a fume hood with proper PPE. Gold Recovery from Laptop Hard Drives via Aqua Regia: Process and Economics