The only substance that can effectively and practically dissolve gold is aqua regia.
Aqua regia, Latin for "royal water," is a highly corrosive mixture of acids renowned for its ability to dissolve noble metals like gold and platinum, which are otherwise resistant to dissolution by single acids.
Composition of Aqua Regia
Aqua regia is a powerful oxidizing agent and complexing agent, making it uniquely capable of dissolving gold. It is composed of two specific concentrated acids mixed in a precise ratio.
- Hydrochloric Acid (HCl): Also known as muriatic acid.
- Nitric Acid (HNO₃): Often referred to as aquafortis.
The standard and most effective composition of aqua regia is a blend of concentrated hydrochloric acid and concentrated nitric acid, typically in a 2:1 ratio by volume, respectively. This unique combination allows for the dissolution of gold through a synergistic effect where neither acid can dissolve gold on its own. The nitric acid oxidizes the gold to gold ions, and the hydrochloric acid then reacts with these ions to form tetrachloroaurate(III) anions, preventing the gold ions from being reduced back to solid gold.
Here's a breakdown of its components:
| Component | Common Name | Ratio (by volume) |
|---|---|---|
| Hydrochloric Acid (HCl) | Muriatic Acid | 2 parts |
| Nitric Acid (HNO₃) | Aquafortis | 1 part |
This powerful mixture is the most useful and important vehicle for dissolving gold, as detailed in historical scientific accounts such as those found in Scientific American.
Applications and Importance
Aqua regia is crucial in various fields, including:
- Gold Refining: Essential for purifying gold, separating it from other metals.
- Assaying: Used to analyze the purity of gold and other precious metals.
- Electroplating: Preparing gold solutions for electroplating processes.
- Chemistry Research: Employed in laboratories for specific chemical reactions and material analysis.
Due to its extreme corrosiveness, aqua regia must be handled with great care and proper safety precautions.
