Organic Aqua Regia

While noble metals such as platinum and palladium are becoming increasingly important, the world has limited supplies of these metals so that it is vitally important that industry can recycle these metals efficiently.

Many of the transition metals have negative standard reduction potentials, indicating that these metals will dissolve in dilute acid, eg, clean chromium will dissolve in dilute hydrochloric acid

2 x [Cr(s) ----> Cr³⁺ + 3e] E^o = 0.74V
3 x [2e + 2H⁺ ----> H₂(g)] E^o = 0.00
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2Cr(s) + 6H⁺ -----> 2Cr³⁺ + 3H₂(g) E^o = 0.74V

Some transition metals have positive reduction potentials, so they are poorer reducing agents than hydrogen, and are difficult to dissolve in acid. These metals are referred to as the noble metals and include silver, gold and platinum as well as ruthenium, rhodium, palladium, osmium, and iridium. Dissolving the noble metals requires the use of an oxidizing agent and sometimes a complexing agent. The most common reagent used to dissolve noble metals is aqua regia.

Aqua regia is a highly corrosive, fuming yellow or red solution formed when 1 part of concentrated nitric acid is added to 3 parts of concentrated hydrochloric acid. The nitric acid part is a powerful oxidizer, it oxidizers the noble metal atoms to cations. The hydrochloric acid provides a supply of chloride anions which react with the noble metal cations. For example, gold can be dissolved in aqua regia:

• gold reacts with nitric acid to form gold (III) ions:

Au(s) + 3NO₃^-(aq) + 6H⁺(aq) -----> Au³⁺(aq) + 3NO₂(g) + 3H₂O(l)

• gold (III) ions react with chloride ions to form chloroaurate anions:

Au³⁺(aq) + 4Cl^-(aq) -----> AuCl₄^-(aq)

However, aqua regia will dissolve all the metals together which introduces impurities into the recycling process. Georgia Institute of Technology scientists have developed a new organic solvent process that may solve this problem since the concentration of each component of the solvent can be adjusted to preferentially dissolve gold or palladium, but will not dissolve platinum. This solvent has been dubbed organic aqua regia.

Reference
Wei Lin, Rong-Wei Zhang, Seung-Soon Jang, Ching-Ping Wong, Jung-Il Hong. 'Organic Aqua Regia'-Powerful Liquids for Dissolving Noble Metals. Angewandte Chemie, 2010; 122 (43): 8101 DOI: 10.1002/ange.201001244

Further Reading
http://www.ausetute.com.au/acidbase.html
http://www.ausetute.com.au/trmetals.html
http://www.ausetute.com.au/ligands.html
http://www.ausetute.com.au/redox.html
http://www.ausetute.com.au/calcelemf.html

Study Questions

1. What is meant by the term transition metal?
2. What are the typical properties of a transition metal?
3. Refer to the equation above for the reaction between chromium and hydrochloric acid. Is this an example of a redox reaction? Explain your answer.
4. Refer to the equation above for the reaction between gold and nitric acid. Has the gold been oxidised or reduced? Explain your answer.
5. Refer to the equation above for the reaction between gold and nitric acid. Has nitrogen been oxidised or reduced? Explain your answer.
6. Could the reaction between gold and nitric acid be described as a redox reaction? Explain your answer.
7. Refer to the equation for the reaction between gold (III) ions and chloride ions. Is this a redox reaction? Explain your answer.
8. Write suitable equations for dissolving the following metals in aqua regia:

• iron
• tin
• copper
  
• silver
• platinum
• palladium