Nitric acid, on its own, does not dissolve gold. Gold is classified as a noble metal, meaning it is highly resistant to chemical attack. Nitric acid is a strong oxidizing agent that readily reacts with most other metals, but gold remains largely unaffected. The inability of this acid to dissolve gold is a direct result of the metal’s unique chemical properties.
Gold’s Chemical Nobility: Why Single Acids Fail
Gold resists dissolution by single acids due to its high chemical nobility. This property is quantified by gold’s high standard reduction potential, approximately +1.50 volts for the Au3+/Au half-reaction. This highly positive value indicates that gold atoms have a very strong tendency to hold onto their electrons and resist oxidation (the loss of electrons).
To dissolve a metal, the acid must oxidize the solid metal atoms into positively charged ions. While nitric acid is a strong enough oxidizing agent to convert common metals like copper or silver into ions, it lacks the chemical force necessary to overcome gold’s electron affinity. The energy required to strip three electrons from a gold atom and form the Au3+ ion is not met by the acid alone. When gold is placed in nitric acid, the reaction quickly reaches an equilibrium that strongly favors the solid, undissolved gold metal.
This inertness means gold does not tarnish or corrode when exposed to air, water, or most common chemicals. Even concentrated nitric acid, which can dissolve almost every other metal, only converts a negligible amount of gold into its ionic form. A successful dissolution requires a chemical environment that not only oxidizes the gold but also stabilizes the resulting gold ions.
The Necessity of Aqua Regia
The solution required to dissolve gold is a unique mixture of acids known as aqua regia, which is Latin for “royal water.” This name reflects its ability to dissolve gold and other noble metals. Aqua regia is a freshly prepared combination of concentrated nitric acid and concentrated hydrochloric acid (HCl), optimally mixed in a volume ratio of one part nitric acid to three parts hydrochloric acid.
Neither acid can dissolve gold effectively alone, but their combination creates a powerful chemical environment. The mixture is extremely corrosive and volatile, immediately turning yellow or orange and producing toxic fumes. It must be handled with extreme caution and prepared immediately before use, as the mixture is unstable and loses potency over time due to decomposition.
The potent nature of aqua regia is utilized in modern applications, particularly in the refining of high-purity gold and in analytical chemistry procedures. It enables processes like the recovery of gold from electronic scrap or the purification of gold samples. Because of the concentrated nature of both acids, specialized protective equipment and a laboratory fume hood are necessary when handling the mixture.
The Two-Step Chemical Mechanism
The dissolution of gold in aqua regia relies on the acids performing two chemical functions. The mechanism begins with the oxidizing action of the nitric acid, which acts as the initial chemical aggressor. Nitric acid converts solid gold metal (Au) into gold ions (Au3+), which is the necessary first step for dissolution.
Hydrochloric acid then performs the second function: complexation. Hydrochloric acid provides a high concentration of chloride ions (Cl-) in the solution. These chloride ions immediately bond with the newly formed gold ions, creating the extremely stable complex ion known as the tetrachloroaurate anion (AuCl4-).
This complexation step drives the reaction forward, as it effectively removes the Au3+ ions from the solution as soon as they are formed. By removing the product of the first step, the chemical equilibrium of the oxidation reaction is shifted, allowing more solid gold to be oxidized by the nitric acid. The overall process is a continuous, unidirectional conversion of solid gold into the highly stable, soluble chloroaurate anion, resulting in the complete dissolution of the noble metal.