"Royal water" in chemistry refers to aqua regia, a highly corrosive and fuming yellow or reddish liquid renowned for its extraordinary ability to dissolve noble metals, such as gold and platinum, which are typically resistant to dissolution by individual acids.
Composition of Aqua Regia
Aqua regia is a specialized mixture, traditionally prepared by combining concentrated hydrochloric acid (HCl) and nitric acid (HNO₃). The specific ratio of these components is crucial for its effectiveness and unique properties.
| Component | Chemical Formula | Ratio (by volume) |
|---|---|---|
| Hydrochloric Acid | HCl | 3 parts |
| Nitric Acid | HNO₃ | 1 part (approximately) |
This precise 3:1 mixture of hydrochloric acid to nitric acid creates a powerful synergistic effect. The combined action makes aqua regia far more reactive than either acid alone, enabling it to attack and dissolve metals that are otherwise inert.
Why is it Called "Royal Water"?
The name aqua regia is Latin for "royal water." This fitting title was given due to its exceptional capability to dissolve "royal" or noble metals like gold (often considered the "king of metals") and platinum. Prior to its discovery, no single acid was known to effectively dissolve gold.
How Does Aqua Regia Work?
The potent action of aqua regia stems from a complex interplay between its constituent acids:
- Nitric Acid (HNO₃) acts as a powerful oxidizing agent. It oxidizes the noble metal (e.g., gold) to its respective ions (e.g., Au³⁺).
- Hydrochloric Acid (HCl) then plays a crucial role by reacting with these metal ions to form stable anionic chloro-complexes (e.g., tetrachloroaurate(III) ions, [AuCl₄]⁻).
This formation of stable complexes effectively removes the metal ions from the solution, preventing them from being reduced back to their elemental form. This continuous removal drives the equilibrium, allowing further oxidation and dissolution of the noble metal to occur, thus making it soluble.
Key Applications of Aqua Regia
Given its unique dissolving power, aqua regia finds several critical applications, primarily in industries and laboratories dealing with noble metals:
- Gold Refining and Recovery: It is widely used to purify gold, especially in the recovery of gold from electronic waste, jewelry, or dental work.
- Etching and Cleaning: In laboratories, it can be used for etching in microfabrication processes and for effectively cleaning glassware contaminated with organic compounds and metal particles.
- Platinum Group Metals (PGM) Processing: Similar to gold, it is an essential reagent in the dissolution and processing of platinum and palladium.
Safety Considerations
Aqua regia is an extremely corrosive and dangerous substance. Handling it requires strict adherence to safety protocols. It should always be prepared in a fume hood and used with appropriate personal protective equipment (PPE), including chemical-resistant gloves, eye protection, and a lab coat. Due to its unstable nature and the gases it produces, it should never be stored for extended periods. Users are typically advised to prepare it fresh before each use. For more detailed safety information regarding this powerful solution, refer to chemical safety guidelines for aqua regia.
