Aqua regia, Latin for “royal water,” is a highly corrosive and powerful chemical mixture. This fuming liquid earned its regal name from alchemists due to its unique ability to dissolve noble metals, such as gold and platinum, which are otherwise resistant to dissolution by individual acids. This property has made aqua regia a valuable tool in various scientific and industrial applications.
Its Unique Composition
Aqua regia is a precise combination of two strong acids: concentrated nitric acid (HNO₃) and concentrated hydrochloric acid (HCl). For optimal effectiveness, these acids are typically mixed in a 1:3 molar ratio of nitric acid to hydrochloric acid. This ratio ensures efficient synergistic chemical reactions.
When freshly prepared, aqua regia is often colorless but quickly changes to a yellow, orange, or reddish hue. This color change is due to the formation of volatile products like nitrosyl chloride (NOCl) and chlorine gas (Cl₂), which also contribute to its fuming nature. Due to these gases and ongoing chemical reactions, aqua regia is usually prepared immediately before use, as its potency diminishes over time.
How It Works
Aqua regia’s dissolving power, particularly for noble metals like gold, stems from the interplay between its two acidic components. Neither nitric acid nor hydrochloric acid can dissolve gold independently. Nitric acid acts as an oxidizing agent, converting a small quantity of the noble metal into its ionic form, such as gold(III) ions (Au³⁺). This initial oxidation step is crucial for dissolution.
Hydrochloric acid then plays a vital role by providing chloride ions (Cl⁻). These chloride ions react with the newly formed metal ions, creating stable complex anions, specifically tetrachloroaurate(III) anions (AuCl₄⁻) in the case of gold. The formation of these stable complexes effectively removes the metal ions from the solution, preventing them from reverting to their solid metallic state. This continuous removal allows nitric acid to oxidize more of the noble metal, leading to its complete dissolution.
Key Applications
Aqua regia’s ability to dissolve noble metals makes it valuable across various industries and scientific disciplines. A primary application is in the refining and purification of gold and platinum. This process, often part of the Wohlwill process, yields high-purity gold. The mixture produces chloroauric acid, a key electrolyte in refining operations.
Aqua regia also finds use in etching processes, such as in microfabrication and the creation of printed circuit boards (PCBs). It selectively dissolves metal traces, facilitating intricate circuitry patterns. Laboratories also use aqua regia for cleaning glassware, removing stubborn organic compounds and trace metal contaminants. It is also employed in analytical chemistry for determining metals by extracting trace elements from diverse samples.
Handling and Safety
Handling aqua regia demands strict safety protocols due to its highly corrosive and reactive nature. It poses severe hazards, including corrosive effects on skin, eyes, and the respiratory system, capable of causing tissue destruction. The mixture also releases toxic fumes, such as nitrogen oxides, chlorine gas, and nitrosyl chloride, which are poisonous if inhaled.
Due to the rapid production of gases, aqua regia should never be stored in a sealed container, as pressure buildup can lead to an explosion. All work must be conducted within a chemical fume hood to ensure proper ventilation and contain hazardous fumes. Personal protective equipment, including a lab coat, safety goggles, and appropriate chemical-resistant gloves, is essential. Avoid mixing aqua regia with organic materials, as this can cause a violent reaction or explosion. Therefore, aqua regia should only be prepared in small, fresh batches just before use and and handled by trained professionals.