Aqua regia is primarily used to dissolve noble metals, specifically gold and platinum, which are insoluble in most acids. It finds common application in laboratory settings for this purpose.
Key Uses of Aqua Regia
Here's a breakdown of aqua regia's main applications:
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Dissolving Noble Metals: Aqua regia's most well-known use is dissolving gold (Au) and platinum (Pt). Neither of these metals dissolves in nitric acid or hydrochloric acid alone, but they do dissolve in a mixture of the two. The nitric acid acts as an oxidizer and is used to form gold ions ($Au^{3+}$). The hydrochloric acid is then used to react with the gold ions to form tetrachloroaurate(III) anions ($[AuCl_4]^−$), also in solution. The reaction with hydrochloric acid is an equilibrium reaction that favors the formation of tetrachloroaurate(III) anions. This reaction results in the removal of gold ions from solution and allows the further oxidation of gold to take place. The appropriate reaction equation is:
$Au + 3 HNO_3 + 4 HCl \rightleftharpoons [AuCl_4]^- + 3NO_2 + H_3O^+ + 2 H_2O$
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Metal Recovery: Aqua regia can be used to recover gold from scrap materials or electronic waste, making it valuable in recycling processes.
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Cleaning Glassware: In microfabrication and microelectronics labs, aqua regia is sometimes used to clean glassware by removing trace amounts of organic compounds. However, this is a dangerous application and should be done with extreme caution and proper personal protective equipment.
Why Aqua Regia Works
The unique dissolving power of aqua regia comes from the synergistic effect of nitric and hydrochloric acids:
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Nitric Acid ($HNO_3$): Acts as a powerful oxidizer. It helps to form metal ions from the solid metal.
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Hydrochloric Acid ($HCl$): Forms complexes with the metal ions, preventing them from plating out of solution. This drives the dissolution reaction forward. For example, with gold, hydrochloric acid forms tetrachloroaurate(III) anions ($[AuCl_4]^−$), which are soluble.
Important Safety Considerations
Working with aqua regia requires extreme caution due to its highly corrosive and reactive nature:
- Always wear appropriate personal protective equipment (PPE): This includes safety goggles, gloves (acid-resistant), and a lab coat.
- Work in a well-ventilated area: Aqua regia releases toxic fumes, including nitrogen dioxide and chlorine.
- Never store aqua regia in a closed container: The pressure buildup from released gases can cause explosions.
- Handle with care and follow proper disposal procedures: Aqua regia should be neutralized and disposed of according to local regulations.
