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aqua regia(äk`wə rē`jēə) [Lat.,=royal water], corrosive, fuming yellow liquid prepared by mixing one volume of concentrated nitric acid with three to four volumes of concentrated hydrochloric acid. It was so named by the alchemists because it dissolves gold and platinum, the "royal" metals, which do not dissolve in nitric or hydrochloric acid alone. Its fumes and yellow color are caused by reaction of nitric acid, HNO3, with hydrogen chloride, HCl, to form nitrosyl chloride, NOCl, chlorine, Cl2, and water; both chlorine and nitrosyl chloride are yellow-colored and volatile. The nitrosyl chloride further decomposes to nitric oxide, NO, and chlorine. Nitric acid is a powerful oxidizing agent (see oxidation and reductionoxidation and reduction,
complementary chemical reactions characterized by the loss or gain, respectively, of one or more electrons by an atom or molecule. Originally the term oxidation
..... Click the link for more information. ), but the chemical equilibriumchemical equilibrium,
state of balance in which two opposing reversible chemical reactions proceed at constant equal rates with no net change in the system. For example, when hydrogen gas, H2, and iodine gas, I2
..... Click the link for more information. for its reaction with gold, Au, only permits formation of a tiny amount of Au+3 ion, so the amount of gold dissolved in pure nitric acid is undetectable. The presence of chloride ion, Cl−, allows formation of the stable chloraurate complex ioncomplex ion,
charged molecular aggregate (see ion), consisting of a metallic atom or ion to which is attached one or more electron-donating molecules. In some complex ions, such as sulfate, SO4−2
..... Click the link for more information. , AuCl4−. Because of the high concentration of chloride ion in aqua regia, the Au+3 is reacted almost as soon as it is formed, keeping its concentration low; this allows oxidation of more Au to Au+3, and the gold is dissolved. The gold may also react directly with the free chlorine in aqua regia, since chlorine is a powerful oxidizing agent.
a mixture of concentrated hydrochloric and nitric acids, usually one part nitric acid and three parts hydrochloric acid. Aqua regia is a yellow liquid with the odor of nitrogen oxides and chlorine. It has strong oxidizing properties owing to the liberation of chlorine in the reactions
3HCl + HNO3 = Cl2 + NOCl + 2H2O
2NOCl = 2NO + Cl2
Aqua regia dissolves all metals with the exception of silver, rhodium, and iridium. It even dissolves gold—hence the name “aqua regia,” given by alchemists who considered gold the king of metals. Aqua regia is used as a laboratory reagent in such processes as the refining of gold and platinum and the production of metal chlorides.