The aqua regia ratio is 3:1, specifically referring to three parts of hydrochloric acid to one part of nitric acid by volume. This potent mixture is renowned for its ability to dissolve noble metals like gold and platinum, which single acids cannot.
Understanding Aqua Regia
Aqua regia, Latin for "royal water," is a highly corrosive mixture of concentrated acids. Its unique dissolving capabilities stem from the synergistic action of its two components:
- Hydrochloric Acid (HCl): Provides chloride ions, which complex with metal ions, particularly gold, to form stable chloroaurate anions. This removes gold ions from the solution, allowing more gold to dissolve.
- Nitric Acid (HNO₃): Acts as a powerful oxidizing agent, dissolving the noble metal and forming metal ions.
The precise ratio is critical for this synergistic effect, enabling aqua regia to achieve what neither acid can accomplish alone.
The Exact Ratio Explained
As per the definition, aqua regia is composed of hydrochloric acid and nitric acid in a specific volumetric proportion.
- Ratio: 3 parts Hydrochloric Acid : 1 part Nitric Acid
This means that for every 3 units of volume of hydrochloric acid used, 1 unit of volume of nitric acid is added to form the mixture.
Here's a breakdown of the components and their respective parts:
| Component | Ratio Part (by volume) | Description |
|---|---|---|
| Hydrochloric Acid | 3 | Provides chloride ions to complex with metal ions. |
| Nitric Acid | 1 | Acts as an oxidizing agent, dissolving the metal. |
Why This Specific Ratio?
The 3:1 ratio is crucial because it balances the oxidative power of nitric acid with the complexing ability of hydrochloric acid. Nitric acid oxidizes gold to gold ions (Au³⁺), while hydrochloric acid reacts with these gold ions to form tetrachloroaurate(III) anions ([AuCl₄]⁻). This complexation effectively removes gold ions from the solution, shifting the equilibrium and allowing more gold to be oxidized and dissolved. Without the correct ratio, either the oxidation would be insufficient, or the complexation would not be optimized, limiting the mixture's effectiveness.
