What is royal water?

Royal water is a mixture of concentrated hydrochloric acid and nitric acid, typically in a 3:1 ratio. It is highly corrosive and has the ability to dissolve noble metals such as gold and platinum, which are resistant to attack by either acid alone.

The reaction between hydrochloric acid and nitric acid produces nitrosyl chloride gas (NOCl), a highly reactive yellow-green gas that is responsible for the corrosive properties of royal water. It is believed that the nitrosyl chloride reacts with gold to form auric chloride, which is soluble in the aqua regia solution. Similarly, platinum in royal water forms soluble chloroplatinic acid.

The name "royal water" comes from its historical use in dissolving gold, which was considered the "royal metal." It was first described in the 13th century by the alchemist pseudo-Geber, but its origins may date back to the 8th century. Although this mixture of highly concentrated nitric and hydrochloric acids can dissolve noble metals, its corrosive, toxic nature limits its general use. Due to safety concerns, safer alternatives are often considered for processing of noble metals. Nowadays, royal water finds niche applications in specific industrial processes demanding strong metal etching or refining but must be handled with extreme caution due to its hazardous nature.