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Aqua Regia



Aqua regia (/ˈreɪɡiə, ˈriːdʒiə/; from Latin, literally "regal water" or "royal water") is a mixture of nitric acid and hydrochloric acid, optimally in a molar ratio of 1:3.[b] Aqua regia is a fuming liquid. Freshly prepared aqua regia is colorless, but it turns yellow, orange or red within seconds from the formation of nitrosyl chloride and nitrogen dioxide. It was named by alchemists because it can dissolve the noble metals gold and platinum, though not all metals.




aqua regia



as evidenced by the fuming nature and characteristic yellow color of aqua regia. As the volatile products escape from solution, aqua regia loses its potency. Nitrosyl chloride (NOCl) can further decompose into nitric oxide (NO) and elemental chlorine (Cl2):


This dissociation is equilibrium-limited. Therefore, in addition to nitrosyl chloride and chlorine, the fumes over aqua regia also contain nitric oxide (NO). Because nitric oxide readily reacts with atmospheric oxygen, the gases produced also contain nitrogen dioxide, NO2 (red fume):


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.[1] 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.[2]


Because its components react quickly, 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.[3][4]


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.[5] The equation for the reduction of oxidized gold (Au3+) by sulfur dioxide (SO2) is the following:


Experimental evidence reveals that the reaction of platinum with aqua regia is considerably more complex. The initial reactions produce a mixture of chloroplatinous acid (H2[PtCl4]) and nitrosoplatinic chloride ([NO]2[PtCl4]). 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:


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.


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:[6]


Yep! He dissolved them in aqua regia! The Nazi soldiers completely ignored the beakers of yellow liquid, and after World War II was over, de Hevesy found the solutions unscathed. He reprecipitated the gold and returned it to the Swedish Academy in Stockholm to be re-cast into new Nobel medals and returned to their rightful owners. So not only does aqua regia dissolve noble metals, it also dissolves Nobel medals!


THE DANGERS OF AQUA REGIA: When adding aqua regia to ore, there can be use a red gas given off; THIS RED GAS CAN KILL Sometimes the ore will bubble over, so watch it carefully. Use the aqua regia in an open area and it to the ore very slowly.


Aqua regia cannot be boiled or simmered in the house or where it is not well ventilated. This can cause serious illness or KILL YOU! Use a vent hood with an exhaust fan or simmer in the open.


Does your research require the use of Aqua regia or Piranha solutions? These are two types of acidic washes used for very specific purposes and have very important workplace health and safety considerations.


Aqua regia and Piranha solutions should never be stored in closed containers, as gas evolution can lead to pressure build-up and an explosion. For more information on how to handle these solutions safely in your laboratory, review the new standard operating procedure (SOP) templates now available for Aqua Regia and Acid Piranha solution on the Chemical SOPs page.


When Germany invaded Denmark on April 9th, 1940, Niels Bohr knew he had to hide the medals entrusted to him, or else the prizes would be seized. After discussing the option of burying them, Bohr and Hungarian chemist, George de Hevesy, decided instead to dissolve the gold medals in nitro-hydrochloric acid, also known as aqua regia. Both awards liquefied into an orange colored substance and were placed inside a flask stored on a laboratory shelf in the Niels Bohr Institute. De Hevesy fled Copenhagen for Sweden in 1943, leaving the flask behind.


After the war, De Hevesy returned to find the solution of aqua regia undisturbed. He then precipitated the gold out of the acid. He sent the gold to the Nobel Society. The Nobel Society was able to recast the prize medals. These recast medals were presented to Franck and von Laue in 1952.


Computer circuit board scrap was first treated with one part concentrated nitric acid and two parts water at 70 degrees C for 1 h. This step dissolved the base metals, thereby liberating the chips from the boards. After solid-liquid separation, the chips, intermixed with some metallic flakes and tin oxide precipitate, were mechanically crushed to liberate the base and precious metals contained within the protective plastic or ceramic chip cases. The base metals in this crushed product were dissolved by leaching again with the same type of nitric acid-water solution. The remaining solid constituents, crushed chips and resin, plus solid particles of gold, were leached with aqua regia at various times and temperatures. Gold was precipitated from the leachate with ferrous sulphate.


The primary use of aqua regia is the production of chloroauric acid, the electrolyte in the Wohlwill process for refining gold. Although gold is typically an inert metal, it will dissolve in aqua regia due to the unique action of nitric and hydrochloric acid. Nitric acid is a powerful oxidising agent, capable of converting small amounts of gold to its ionic form, Au3+. Once this ionic form is present in the solution, the hydrochloric acid provides a source of chlorine anions which react with the gold cations to form tetrachloroaurate(III) anions. As the reaction with hydrochloric acid is an equilibrium reaction favouring the formation of chloroaurate anions (AuCl4-), the gold ions are removed from solution making room for more oxidation to occur. And as the solution is so acidic, the chloroaurate anions are swiftly protonated to form chloroauric acid. Using this method, it is possible to produce gold with a jaw-dropping purity of 99.999%. Due to its reactivity and strength, aqua regia it can also dissolve platinum in a similar way.


Another common use for aqua regia is the deep cleaning the tubes used in nuclear magnetic resonance, or NMR, spectroscopy. It is very efficient at this as it is able to remove all traces of the paramagnetic element chromium, which can contaminate spectra and ruin research.


In the acid method of refining gold, the acids and the fumes that are generated by the acids are exceptionally corrosive (less than 1 second of exposure is all that is needed to cause a reaction that will rust even the highest grades of stainless steel). Despite this (and because of this), it is necessary to use vacuum and transfer pumps to move and help filter this acid. Most transfer pumps cannot work without priming (a dangerous and inadvisable procedure when working with acid). Vacuum pumps, because they are made from metal, corrode within days or weeks. The Shor ARVT Pump is different. It&#;19.959;s made for daily use in both transferring and filtering acid in large volume. This is the heart of our acid refining systems (our smallest aqua regia [acid] system will handle 500 ounces of gold a day).


When refining using the aqua regia method, it is necessary to add urea before the gold is precipitated. Typically, especially with nitric-acid-based aqua regia, there are free nitrogen ions in solution. This can cause some gold to redissolve after it has precipitated. Adding urea neutralizes free nitrogen ions and prevents gold from redissolving.


Nitric acid is highly corrosive and highly toxic nitric oxide fumes when used in the aqua regia process of refining precious metal. Additionally, it is expensive and difficult to obtain. By using Subzero instead of nitric acid, nitric oxide fumes are completely eliminated. Subzero can be comfortably stored for extended periods of time without deterioration or hazmat concerns. Subzero is a safe and economical replacement for nitric acid when making aqua regia. 041b061a72


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