Aqua regia

Water regia used to clean testets.

The aqua regia (from the Latin aqua regia, 'aqua regia') is a highly corrosive and fuming solution, yellow in color, formed by the mixture of concentrated nitric acid and concentrated hydrochloric acid, in a ratio of one to three parts by volume.

It is one of the few mixtures capable of dissolving gold, platinum and other metals. It was called that way because it can dissolve those called royal, royal, or noble metals. It is used in some analytical procedures. Aqua regia is not very stable, so it must be prepared just before use.

Coordination and displacement of balance

Although aqua regia dissolves these metals, none of its constituent acids can do so on its own. Nitric acid is a powerful oxidant that can dissolve a miniscule (virtually undetectable) amount of gold, forming gold ions. Hydrochloric acid, for its part, provides chloride ions, which coordinates the gold ions, pulling the gold out of solution. This allows the gold to continue to oxidize, so the gold ends up dissolving.

Aqua regia is a powerful solvent due to the combined effect of H⁺, NO₃^- ions, and Cl ^- in solution (oxidation is favored by the complexing capacity of the Cl^- ion). All three ions react with gold atoms, for example, to form water, nitrogen dioxide, and the stable tetrachloroaurate(III) complex ion (AuCl₄^-), which remains in solution.

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History

Alchemist Jabir Ibn Hayyan.

Hydrochloric acid was discovered around the year 800 by the Persian alchemist Jabir ibn Hayyan (Geber), by mixing common salt with vitriol (sulfuric acid). Jabir's invention, which managed to dissolve gold in aqua regia, contributed to the efforts of the first alchemists in their search for the philosopher's stone.

The German alchemist Andreas Libau (1549-1616), known by the Latinized name of Libavius, published the book Alchemy in 1597, in which, apart from describing medieval achievements in alchemy, he described for the first time the process of making aqua regia.