Introduction
Aqua regia, aggregate of focused nitric and hydrochloric acids, typically one a part of the previous to 3 elements of the latter with the aid of using volume. This aggregate changed into given its name (literally, “royal water”) with the aid of using the alchemists due to its cappotential to dissolve gold. It is a pink or yellowish liquid. It is extraordinarily corrosive and might motive pores and skin burns.
Aqua regia is regularly used to dissolve gold and platinum. It and different comparable combos are utilized in analytical tactics for the answer of sure iron ores, phosphate rocks, slags, nickel-chromium alloys, antimony, selenium, and a number of the less-soluble sulfides, along with the ones of mercury, arsenic, cobalt, and lead.
History
Aqua regia first appeared in the De inventione veritatis by pseudo-Geber (after c. 1300), who produced it by adding sal ammoniac (ammonium chloride) to nitric acid.[8][d] The preparation of aqua regia by directly mixing hydrochloric acid with nitric acid only became possible after the discovery in the late sixteenth century of the process by which free hydrochloric acid can be produced.
The third of Basil Valentine’s keys (ca. 1600) shows a dragon in the foreground and a fox eating a rooster in the background. The rooster symbolizes gold (from its association with sunrise and the sun’s association with gold), and the fox represents aqua regia. The repetitive dissolving, heating, and redissolving (the rooster eating the fox eating the rooster) leads to the buildup of chlorine gas in the flask. The gold then crystallizes in the form of gold(III) chloride, whose red crystals Basil called “the rose of our masters” and “the red dragon’s blood”. The reaction was not reported again in the chemical literature until 1895.
When Germany invaded Denmark in World War II, Hungarian chemist George de Hevesy dissolved the gold Nobel Prizes of German physicists Max von Laue (1914) and James Franck (1925) in aqua regia to prevent the Nazis from confiscating them. The German government had prohibited Germans from accepting or keeping any Nobel Prize after jailed peace activist Carl von Ossietzky had received the Nobel Peace Prize in 1935. De Hevesy placed the resulting solution on a shelf in his laboratory at the Niels Bohr Institute. It was subsequently ignored by the Nazis who thought the jar—one of perhaps hundreds on the shelving—contained common chemicals. After the war, de Hevesy 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. They re-cast the medals and again presented them to Laue and Franck
Chemistry of Aqua regia
Aqua Regia is a mixture of nitric acid (HNO3) and hydrochloric acid (HCl). The two acids react together to form nitrosyl chloride (NOCl), water vapors (H2O), and chlorine gas (Cl2).
The nitrosyl chloride eventually decomposes to chlorine gas (Cl2) and nitrous oxide gas (NO). The nitrous oxide gas then gets oxidized to nitrogen dioxide gas (NO2), a reddish-brown and poisonous gas upon exposure to air.
HCl (aq) + HNO3 (aq) → NOCl (g) + 2H2O (l) + Cl2 (g)
2NOCl (g) → 2NO (g) + Cl2 (g)
2NO (g) + O2 (g) → 2NO2 (g)
Applications
Aqua regia is primarily used to produce chloroauric acid, the electrolyte in the Wohlwill process for refining the highest purity (99.999%) gold.
Aqua regia is also used in etching and in specific analytic procedures. It is also used in some laboratories to clean glassware of organic compounds and metal particles. This method is preferred among most over the more traditional chromic acid bath for cleaning NMR tubes, because no traces of paramagnetic chromium can remain to spoil spectra. While chromic acid baths are discouraged because of the high toxicity of chromium and the potential for explosions, aqua regia is itself very corrosive and has been implicated in several explosions due to mishandling.
Due to the reaction between its components resulting in its decomposition, aqua regia quickly loses its effectiveness (yet remains a strong acid), so its components are usually only mixed immediately before use.
While local regulations may vary, aqua regia may be disposed of by careful neutralization, before being poured down the sink. If there is contamination by dissolved metals, the neutralized solution should be collected for disposal.
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