Oxidation is a chemical process where an atom, molecule, or ion loses electrons, increasing its oxidation state. Gold, prized for its enduring luster, is classified as a noble metal, meaning it resists oxidation under normal environmental conditions. While the immediate answer to whether gold can be oxidized is generally no, pure gold can be forced to surrender its electrons under very specific and powerful laboratory or industrial circumstances.
The Chemistry of Gold’s Resistance
Gold’s exceptional stability stems from its unique atomic structure and placement on the periodic table. As a noble metal, gold possesses an electron configuration that makes its outer electrons tightly bound to the nucleus. This tight binding is enhanced by relativistic effects, which pull the electrons closer to the nucleus, making it extremely difficult for a gold atom to lose an electron and form a positive ion.
The resistance to oxidation is quantified by gold’s standard reduction potential, which measures the tendency of a chemical species to gain electrons. For the reaction forming the gold ion (\(\text{Au}^{3+}\)), the standard reduction potential is highly positive, approximately \(+1.498\) volts. This high positive value indicates that gold strongly prefers to remain in its metallic, non-oxidized form. Consequently, common environmental oxidizers like oxygen, water, and most acids cannot supply enough power to overcome gold’s inherent stability.
Distinguishing Tarnish from Surface Contamination
The discoloration often observed on gold jewelry is a common source of confusion. Pure gold (24-karat) does not tarnish because it does not react with oxygen or sulfur compounds in the air. However, jewelry gold, such as 10-karat or 14-karat, is an alloy mixed with other metals like copper, silver, or zinc to increase hardness and reduce cost.
The dark film that appears on these pieces is the oxidation or sulfidation of the base metals within the alloy. For instance, silver reacts with hydrogen sulfide in the air to form black silver sulfide, and copper can oxidize to form various discolored compounds. Since the gold content in lower karat items is less than 75 percent, the more reactive metals tarnish, resulting in a dull appearance often mistaken for oxidized gold. This discoloration is a reaction of the alloy, not the pure gold component.
Surface contamination is another factor that mimics tarnishing. Residue from cosmetics, lotions, perfumes, or dirt can build up on the metal, trapping moisture and accelerating the tarnishing of the alloyed metals. A simple cleaning process usually removes these contaminants and restores the item’s original luster, confirming the gold itself was not chemically altered.
Extreme Conditions for Gold Reactivity
To overcome gold’s powerful chemical resistance, highly aggressive conditions and specialized chemical agents are required. The most famous example is aqua regia, a potent mixture of concentrated nitric acid and hydrochloric acid, typically combined in a 1:3 ratio. Neither acid alone can dissolve gold, but together they work synergistically to force the metal to oxidize.
The nitric acid acts as a strong oxidizing agent, forcing gold atoms to form \(\text{Au}^{3+}\) ions. The hydrochloric acid then provides chloride ions (\(\text{Cl}^-\)) that immediately complex with the gold ions, creating the stable, soluble tetrachloroaurate anion (\(\text{AuCl}_4^-\)). This continuous removal and stabilization of the gold ions drives the oxidation reaction forward until the gold is completely dissolved.
Gold can also be oxidized using electrochemical methods. This involves applying a high electrical potential to a gold electrode submerged in an electrolyte solution. When a sufficiently high voltage (typically above \(+1.8\) volts) is applied, the gold surface is forced to react with water or hydroxide ions. This process forms a thin, unstable layer of gold oxide or hydroxide, demonstrating that while gold is immune to everyday oxidation, it is not chemically indestructible under engineered conditions.