AQUA REGIA, a mixture of one volume of concentrated nitric acid and three volumes of concentrated hydrochloric acid. It was originally given the name which it still retains, by the alchemists because of its power to dissolve gold—the king of metals.
When aqua regia dissolves gold, the metal is converted into auric chloride, which combines with more hydrogen chlo ride to form aurichloric acid (chloroauric acid), which crystallizes from solution as HAuC14, Aqua regia also dissolves platinum, and on concentrating the solution with excess of hydrochloric acid, deliquescent crystals of platinichloric or chloroplatinic acid, are obtained. Evaporation of the aqua regia solution leads to the isolation of an intermediate nitrosoplatinic chloride, or Palladium is also readily dissolved by aqua regia, but the other four metals of the platinum group (iridium, osmium, rhodium and ruthenium) are less readily attacked by this solvent. (See PLATINUM METALS.) In qualitative analysis aqua regia is also a useful solvent for certain of the less soluble sulphides : it attacks arsenious and mercuric sulphides with the production of the chlorides of arsenic, and mercury, It also dissolves the sulphides of cobalt and nickel, forming the corresponding chlorides, and The solvent action is due to an interaction between nitric and hydrochloric acids whereby chlorine is produced : 3HC1= -1- NOCH- On warming aqua regia the two gases (chlorine and nitrosyl chloride) are evolved, and if passed through concentrated sulphuric acid, chlorine escapes and nitrosyl chloride is absorbed as nitrosyl sulphate : = S02(OH) •O.NO.
When the solution of nitrosyl sulphate is dropped on to sodium chloride, pure nitrosyl chloride is generated as an orange-yellow gas with pungent suffocating odour, easily condensed in a freezing mixture to a red liquid, boiling at 5.6°C and solidifying at —60°C to a lemon-yellow solid. This gas has no action on gold although it attacks mercury. (G. T. M.)