| Aqua Regia |
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HOW IT WORKS Aqua regia works to dissolve gold, even though neither constituent acid will do so alone, because, in combination, each acid performs a different task. Nitric acid is a powerful oxidizer, which will actually dissolve a ''tiny'' (virtually undetectable) amount of gold, forming gold ions (Au3+). The hydrochloric acid provides a ready supply of chloride ions (Cl-), which react with the gold to produce chloraurate anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction which favors formation of chloraurate anions (AuCl4-). This results in a removal of gold ions from solution and allows further oxidation of gold to take place, and so, the gold is dissolved. In addition, gold may be oxidized by the free chlorine present in aqua regia. Appropriate Equations are: :Au ''(s)'' + 3 NO3- ''(aq)'' + 6 H+ ''(aq)'' → Au3+ ''(aq)'' + 3 NO2 ''(g)'' + 3 H2O ''(l)'' :Au3+ ''(aq)'' + 4 Cl- ''(aq)'' → AuCl4- ''(aq)'' The oxidation reaction can also be written with Nitric Oxide as the product rather than Nitrogen Dioxide . :Au ''(s)'' + NO3- ''(aq)'' + 4 H+ ''(aq)'' → Au3+ ''(aq)'' + NO ''(g)'' + 2 H2O ''(l)'' 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. :Pt ''(s)'' + 4 NO 3- ''(aq)'' + 8 H+ ''(aq)'' → Pt4+ ''(aq)'' + 4 NO2 ''(g)'' + 4 H2O ''(l)'' :3Pt ''(s)'' + 4 NO 3- ''(aq)'' + 16 H+ ''(aq)'' → 3Pt4+ ''(aq)'' + 4 NO ''(g)'' + 8 H2O ''(l)'' The oxidized platinum ion then reacts with chloride ions resulting in the chloroplatinate ion. :Pt4+ ''(aq)'' + 6 Cl- ''(aq)'' → PtCl62- ''(aq)'' Experimental evidence reveals that the reaction of platinum with aqua regia is considerably more complex. The initial reactions produce a mixture of chloroplatinous acid (H2PtCl4) and nitrosoplatinic chloride ((NO)2PtCl4). 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. :Pt ''(s)'' + 2 HNO3 ''(aq)'' + 4 HCl ''(aq)'' → (NO)2PtCl4 ''(s)'' + 3 H2O ''(l)'' + 1/2 O2 ''(g)'' :(NO)2PtCl4 ''(s)'' + 2 HCl ''(aq)'' → H2PtCl4 ''(aq)'' + NOCl ''(g)'' The chloroplatinous acid can be oxidized to Chloroplatinic Acid by saturating the solution with chlorine while heating. :H2PtCl4 ''(aq)'' + Cl2 ''(g)'' → H2PtCl6 ''(aq)'' DECOMPOSITION OF AQUA REGIA Upon mixing of concentrated hydochloric acid and concentrated nitric acid, chemical reactions will begin to occur. These reactions result in volatile products (nitrosyl chloride and Chlorine ) that result in the fuming nature of aqua regia. The yellow color of the products also results in the characteristic color of aqua regia. As the volatile products escape from solution, the aqua regia will lose its potency. :HNO3 ''(aq)'' + 3 HCl ''(aq)'' → NOCl ''(g)'' + Cl2 ''(g)'' + 2 H2O ''(l)'' Nitrosyl chloride can further decompose into Nitric Oxide and chlorine. This dissociation is equilibrium-limited. Therefore, the fumes over aqua regia contain nitrosyl chloride, nitric oxide, and chlorine. :2 NOCl ''(g)'' → 2 NO ''(g)'' + Cl2 ''(g)'' HISTORY Hydrochloric acid was first discovered around the year 800 by the Islamic Alchemist Jabir Ibn Hayyan (Gaber) , by mixing Common Salt with Vitriol ( Sulfuric Acid ). Jabir's invention of gold-dissolving Aqua Regia , consisting of hydrochloric acid and Nitric Acid , contributed to the effort of alchemists to find the Philosopher's Stone . When Germany invaded Denmark in World War II , the Hungarian chemist George De Hevesy dissolved the gold Nobel Prize s of Max Von Laue and James Franck into aqua regia to prevent the Nazis from stealing them. He placed the resulting solution on a shelf in his laboratory at the Niels Bohr Institute . After the war, he returned to find the solution undisturbed and precipitated the gold out of the acid. The gold was returned to the Royal Swedish Academy of Sciences and the Nobel Foundation presented new medals to Laue and Franck. {Link without Title} REFERENCES EXTERNAL LINKS |