Understanding Aqua Regia and its Composition

Aqua regia is a highly corrosive, fuming liquid mixture that is famously known for its ability to dissolve noble metals like gold and platinum, which are otherwise resistant to dissolution by individual acids.

The name "aqua regia" is Latin for "royal water" because of its ability to dissolve gold, the "royal" metal.

Components of Aqua Regia

Aqua regia is freshly prepared by mixing concentrated hydrochloric acid ($\text{HCl}$) and concentrated nitric acid ($\text{HNO}_3$). The specific ratio of these two acids is crucial for its unique properties.

The Specific Ratio of Acids in Aqua Regia

The correct ratio for preparing aqua regia is three parts of concentrated hydrochloric acid to one part of concentrated nitric acid.

Mathematically, this ratio is expressed as:

Ratio of Concentrated $\text{HCl}$ : Concentrated $\text{HNO}_3$ = $3 : 1$

This specific mixture creates a synergistic effect. Nitric acid acts as an oxidizing agent, and hydrochloric acid provides chloride ions. The combination leads to the formation of highly reactive species, such as nitrosyl chloride ($\text{NOCl}$) and nascent chlorine ($\text{Cl}$), which are responsible for dissolving noble metals.

For example, the dissolution of gold ($\text{Au}$) in aqua regia can be represented by the following overall equation:

$\text{Au(s)} + 3\text{HNO}_3\text{(aq)} + 4\text{HCl(aq)} \rightarrow \text{HAuCl}_4\text{(aq)} + 3\text{NO}_2\text{(g)} + 3\text{H}_2\text{O(l)}$

Or, more commonly presented focusing on the reactive species formed:

$\text{HNO}_3 + 3\text{HCl} \rightarrow \text{NOCl} + \text{Cl}_2 + 2\text{H}_2\text{O}$

Then, the gold reacts with the chloride ions ($\text{Cl}^-$) and chlorine ($\text{Cl}_2$) formed:

$\text{Au} + \text{HNO}_3 + 4\text{HCl} \rightarrow \text{H}[\text{AuCl}_4] + \text{NO} + 2\text{H}_2\text{O}$ (This represents complex reactions leading to the formation of tetrachloroaurate(III) anion, $\text{HAuCl}_4$ is chloroauric acid)

The question asks for the ratio of concentrated hydrochloric acid to concentrated nitric acid. Based on chemical principles and standard preparation methods, this ratio is 3:1.

Conclusion

The ratio of concentrated hydrochloric acid and concentrated nitric acid in freshly prepared aqua regia is 3:1. This unique ratio is essential for its powerful solvent properties towards noble metals.

Additional Information on Aqua Regia

Freshly prepared aqua regia is essential because its active components, like nitrosyl chloride, decompose over time. Aqua regia is extremely corrosive and should be handled with extreme caution in a well-ventilated area due to the release of toxic fumes.

While it dissolves gold and platinum, it cannot dissolve all metals. For example, it does not dissolve metals like rhodium, iridium, osmium, palladium, and ruthenium easily, although some might be attacked slowly.