How royal water dissolves gold

Aqua regia

Gold is a precious metal whose appreciation and reputation for being crisis-proof are no coincidence. On the one hand, gold is a relatively rare precious metal. As a rule, only a few grams of gold are contained per ton of ore, one speaks here of ppm (parts per million) and speaks of the one to low two-digit range (e.g. Olympic Dam / Australia only contains approx. 0.5 grams of gold per Ton).

HNO3 + 3 HCl ---> NOCl + 2 Clnasc. + 2 H2O

On the other hand, gold is considered to be inert and insensitive to acids. The precious metal gold is not considered indestructible for nothing. With one exception. Aqua regia simply makes gold disappear in the truest sense of the word.

Aqua regis - the royal water

However, aqua regia is not magic or hocus-pocus, but simple chemistry. The special feature: aqua regia consists of one part each of concentrated nitric acid and three parts of concentrated hydrochloric acid. Only this mixture and its special properties make it possible to dissolve gold. Chlorine compounds are formed from the oxidizing nitric acid and the non-oxidative hydrochloric acid.

NOCl (nitrosyl chloride) and the nascent chlorine are responsible for the extraordinary behavior of aqua regia towards precious metals. In addition to gold, the aqua regis also reacts with palladium or platinum. The exception is silver, the oxidation layer of which protects the metal from the effects of aqua regia.

Aqua regia - use then and now

Today the composition of metals can be checked in different ways. This was not the case in the past. For a long time, aqua regia remained one of the few ways to check the fineness. An application that still satisfies the mixture of nitric and hydrochloric acids. At the same time, it still appears in chemistry laboratories to break down samples.

In the past, aqua regia was used in a diluted form in medicine. Incidentally, the gold dissolved in aqua regia is not lost. It can be regained even after years - as demonstrated by the Nobel Prize medals of physicists Max von Laue and James Franck, which outlasted time in Königswasser during the occupation of Denmark.