Gold is a metal prized for its brilliant luster and resistance to tarnish, a stability that chemists classify as a “noble metal.” This inherent chemical resistance makes gold impervious to most corrosive substances, including strong single acids like nitric or hydrochloric acid. While the common belief that gold does not dissolve in acid is true, a specific, carefully formulated chemical mixture overcomes this natural inertia. This unique solution is the only common way to successfully dissolve gold, transforming the solid metal into a liquid compound.
Gold’s Chemical Inertia
Gold’s durability stems from its electronic structure and its low position on the chemical reactivity series. This means it has a weak tendency to lose electrons and undergo oxidation. Its stable atomic structure features a filled d-orbital and tightly bound valence electrons.
This configuration makes it difficult for a single acid to react with the gold atoms. A typical acid reaction requires the metal to be oxidized into a positive ion, but the energy required to strip electrons from gold is too high for most common acids to provide. Acids like sulfuric or hydrochloric acid alone fail to dissolve the metal because they lack the necessary oxidizing power to initiate the reaction.
The Unique Power of Aqua Regia
The mixture capable of dissolving gold is a highly corrosive substance called aqua regia, which translates from Latin as “royal water.” Alchemists named it this because it was potent enough to dissolve gold, the “king of metals.” Aqua regia is formed by combining concentrated nitric acid and concentrated hydrochloric acid.
The optimal ratio is typically one part nitric acid to three parts hydrochloric acid by volume. This specific combination is the key to its power, as neither acid can dissolve gold on its own. Upon mixing, the two acids react to produce highly volatile and reactive components, including nitrosyl chloride and free chlorine gas. This reaction gives freshly prepared aqua regia its characteristic yellow-orange color and fuming property.
The Chemistry of Dissolution
The ability of aqua regia to dissolve gold relies on a sophisticated two-step chemical mechanism, where the acids perform synergistic roles.
The first step involves concentrated nitric acid, which acts as a strong oxidizing agent. This acid initially attacks the gold surface, oxidizing solid gold metal into gold ions (\(\text{Au}^{3+}\)). If nitric acid were used alone, this reaction would quickly stop because the \(\text{Au}^{3+}\) ions would immediately be reduced back into solid gold metal, making the reaction reversible.
The second element is the role of the hydrochloric acid, which supplies a high concentration of chloride ions (\(\text{Cl}^{-}\)) to the solution. These chloride ions immediately react with the newly formed gold ions. They bond with the gold ions to form a very stable, soluble complex ion called the tetrachloroaurate ion (\(\text{[AuCl}_4]^-\)).
By forming this stable complex, the chloride ions effectively remove the \(\text{Au}^{3+}\) ions from the solution, preventing their reduction back to solid gold. This constant removal of the gold ions drives the initial oxidation reaction forward. This allows the nitric acid to continuously attack and dissolve the gold until the entire sample is consumed, leaving behind a liquid solution called chloroauric acid.
Practical Uses of Dissolved Gold
The ability to dissolve gold is a necessary process in several industrial and scientific applications.
- Refining: Dissolving raw gold alloy in aqua regia allows for the separation of gold from less noble metals like silver, which precipitates out as an insoluble chloride salt. This process, such as the Wohlwill process, produces gold purity exceeding 99.999%.
- Assaying: Dissolved gold is used in the analysis of gold purity in geological samples or scrap metal. By dissolving a sample, chemists can accurately determine its exact gold content for valuation in the precious metals market.
- Urban Mining: The process is utilized to recover gold from electronic waste, such as circuit boards and computer components.
- Manufacturing: The dissolved gold compounds are used in the production of gold plating solutions and specialized electronic components.
The resulting liquid solution of gold salts is then processed to recover the pure metal through a controlled chemical reduction. This recovery is a more environmentally sound way to reclaim the metal compared to traditional mining.