Gold is famous for its chemical stability, maintaining its brilliant metallic state even when exposed to most single, concentrated acids. Dissolving gold is not a simple task and requires highly specialized, aggressive, and hazardous chemical environments to break down the metal’s structure. This process is typically undertaken in a controlled setting for applications like refining, assaying, or recovering the metal from electronic waste. Because these chemical reactions involve significant risks, expert knowledge and stringent safety protocols are necessary.
The Primary Method: Aqua Regia
The most recognized and effective chemical method for dissolving gold is aqua regia, which translates from Latin to “royal water.” This solution is a freshly prepared, highly corrosive mixture of concentrated nitric acid and concentrated hydrochloric acid. The acids are typically combined in a volumetric ratio of one part nitric acid to three parts hydrochloric acid for optimal performance. The mixture must be created immediately before use because it rapidly degrades, losing potency as its volatile components escape.
The power of aqua regia lies in the synergy of the two acids, each playing a distinct chemical role. Nitric acid acts as a powerful oxidizing agent, converting solid gold into gold ions. Simultaneously, the hydrochloric acid provides an abundant supply of chloride ions. These chloride ions immediately react with the newly formed gold ions to create the extremely stable tetrachloroaurate(III) anion.
The formation of this stable complex ion effectively removes the gold ions from the solution, driving the oxidation reaction forward. This mechanism allows the nitric acid to continue oxidizing the gold metal, leading to the complete dissolution of the gold into a liquid form known as chloroauric acid. The mixing of the acids also produces highly reactive gases, including nitrosyl chloride and chlorine gas, which further aid the dissolution process.
Essential Safety and Handling Procedures
Working with concentrated acids and the resulting aqua regia solution necessitates extreme caution due to the severe health and physical hazards involved. The process releases highly toxic gases, including chlorine, nitrosyl chloride, and nitrogen dioxide. Inhalation of these fumes can cause chemical burns to the respiratory tract, leading to symptoms like coughing, shortness of breath, and potentially fatal pulmonary edema.
All operations must be conducted exclusively within a certified, high-efficiency chemical fume hood to capture and safely vent these toxic fumes. The operator must wear comprehensive Personal Protective Equipment (PPE) to prevent direct contact with the corrosive liquid. This PPE includes safety goggles and a face shield, a lab coat, and specialized chemical-resistant gloves, such as heavy-duty neoprene or butyl rubber.
Aqua regia should never be stored; it must be prepared immediately before use and only in the smallest practical quantity required for the task. Storing the solution, especially in a sealed container, is hazardous because the continuous evolution of gases can cause a dangerous pressure buildup and possible explosion. Spent solutions containing dissolved metals must never be poured down the drain, but instead must be neutralized and managed as regulated hazardous waste according to strict guidelines.
Alternative Chemical and Electrical Techniques
While aqua regia is the standard for small- to medium-scale refining, other chemical and electrical methods are employed in specialized or industrial settings.
Chlorination
One group of alternatives involves chlorination, which uses chlorine gas or combinations of hydrochloric acid and a strong oxidant like sodium hypochlorite (bleach). These chloride-based systems dissolve gold by forming soluble gold chloride complexes. They are highly corrosive and must be managed carefully.
Cyanide Leaching
Another industrial method is cyanide leaching, which accounts for the vast majority of global gold extraction from low-grade ores. This process uses an alkaline solution of sodium cyanide to form a soluble gold-cyanide complex. Despite its high efficiency, the extreme toxicity of cyanide makes this method highly regulated and entirely unsuitable for non-industrial or general public use.
Electrolysis
Electrolysis offers a different approach, dissolving gold using an electric current in an electrolyte solution, such as in the Wohlwill process for high-purity refining. This electrochemical technique is used to selectively dissolve gold and platinum group metals from an impure anode into a solution. This process is highly controlled and yields gold with exceptional purity, often exceeding 99.99%.
Recovering Gold from Solution
Once gold has been dissolved into a liquid solution, it exists as a salt, such as chloroauric acid, and must be chemically reduced to recover the solid metal. This second step, known as precipitation, involves adding a reducing agent that converts the dissolved gold ions back into solid elemental gold. The most common and selective chemical used for this purpose is Sodium Metabisulfite (SMB).
When SMB is dissolved in water, it forms the active reducing species that reacts with the chloroauric acid. The gold is converted into a fine, dark-brown powder, which is the visual cue for successful precipitation. Another effective reducing agent is ferrous sulfate, also known as copperas, which is highly selective for gold over other metals that might be in the solution.
After the gold has completely settled, the resulting powder, often called gold mud or gold sponge, is separated from the waste liquid through filtration. The recovered gold powder must then be thoroughly washed with clean water to remove any residual acid and chemical salts. The final step involves drying the powder and melting it at a high temperature, typically over 1064°C, to coalesce the fine particles into a solid, shiny gold bead or nugget.