Dissolving gold produced by the aqua regia chemical refining process Aqua regia dissolves
gold, although neither constituent acid will do so alone. Nitric acid is a powerful oxidizer, which will dissolve a very small quantity of gold, forming gold(III)
ions (). The hydrochloric acid provides a ready supply of chloride ions (), which react with the gold ions to produce tetrachloroaurate(III)
anions (), also in solution. The reaction with hydrochloric acid is an equilibrium reaction that favors formation of tetrachloroaurate(III) anions. This results in a removal of gold ions from solution and allows further oxidation of gold to take place. The gold dissolves to become
chloroauric acid. In addition, gold may be dissolved by the chlorine present in aqua regia. Appropriate
equations are: : Au + 3 + 4 HCl + 3 + + 2 or : Au + + 4 HCl + NO + + . Solid
tetrachloroauric acid may be isolated by evaporating the excess aqua regia, and decomposing the residual nitric acid by repeatedly heating the solution with additional hydrochloric acid. That step
reduces nitric acid (see
decomposition of aqua regia). If elemental gold is desired, it may be selectively
reduced with reducing agents such as
sulfur dioxide,
hydrazine,
oxalic acid, etc. The
equation for the
reduction of
oxidized gold () by sulfur dioxide () is the following: :
Dissolving platinum Similar equations can be written for
platinum. As with gold, the oxidation reaction can be written with either nitric oxide or nitrogen dioxide as the nitrogen oxide product: : : The oxidized platinum ion then reacts with chloride ions resulting in the chloroplatinate ion: : Experimental evidence reveals that the reaction of platinum with aqua regia is considerably more complex. The initial reactions produce a mixture of
chloroplatinous acid () and nitrosoplatinic chloride (). The nitrosoplatinic chloride is a solid product. If full dissolution of the platinum is desired, repeated extractions of the residual solids with concentrated hydrochloric acid must be performed: : and : The chloroplatinous acid can be oxidized to
chloroplatinic acid by saturating the solution with molecular chlorine () while heating: : Dissolving platinum solids in aqua regia was the mode of discovery for the densest metals,
iridium and
osmium, both of which are found in platinum ores and are not dissolved by aqua regia, instead collecting as insoluble metallic powder (elemental Ir, Os) on the base of the vessel.
Precipitating dissolved platinum As a practical matter, when platinum group metals are purified through dissolution in aqua regia, gold (commonly associated with PGMs) is
precipitated by treatment with
iron(II) chloride. Platinum in the filtrate, as hexachloroplatinate(IV), is converted to
ammonium hexachloroplatinate by the addition of
ammonium chloride. This ammonium salt is extremely insoluble, and it can be filtered off. Ignition (strong heating) converts it to platinum metal: : Unprecipitated hexachloroplatinate(IV) is reduced with elemental
zinc, and a similar method is suitable for small scale recovery of platinum from laboratory residues.
Reaction with tin Aqua regia reacts with
tin to form
tin(IV) chloride, containing tin in its highest oxidation state: :
Reaction with other substances It can react with
iron pyrite to form
Iron(III) chloride: : ==History==