Why does Aqua Reiga dissolve Gold but neither of its compoents does?

[RyanJ]

Why does Aqua Reiga dissolve Gold but neither of its compoents does?

 

There must be a reason - as far as I knw neither Hydrochloric acid nor Nitric Acid dissolve gold and if they do then they don't to it at any impressive rate. So does anyone had a reason why Aqua Reiga does?

 

Cheers,

 

Ryan Jones

[jdurg]

Gold requires a strong oxidizing agent in order for it to dissolve. HCl alone is simply not a very good oxidizing agent. As a result, the gold is not able to be oxidized into the +3 oxidation state. In HNO3, the nitrate ion is able to work in conjunction with the hydrogen ions to oxidize gold to the +3 state, however this does not happen to any appreciable amount. The equillibrium of the reaction where gold becomes a +3 ion lies far to the left. So if you are somehow able to remove the Au+3 ion from solution, then the reaction will move over to the right thanks to LeChateillers(sp?) principle.

 

One way of removing the Au+3 ion is to complex it. If you can lock up the Au+3 into a complex, it will remove the ion from solution and allow the equillibrium to move over. One way of doing this is add chloride ions or cyanide ions to solution. If you add cyanide ions to an acidic solution, however, you get cyanide gas which is bad. Instead, if you add a chloride salt or hydrochloric acid, you get plenty of Cl- in solution. Now, when the little bits of Au+3 form, they can complex with the Cl- ions to form the VERY stable and very soluble AuCl4(-) complex. This allows the equillibrium to move over to the right and allows the gold to dissolve.

 

So HCl alone and HNO3 alone really don't dissolve gold becuase the equillibrium is shifted towards the side of solid gold metal. If you mix them together, you allow a stable complex of gold salts to form which allows the equillibrium to shift to the right hence dissolving the gold.

[RyanJ]

Ha, I think I get it. So the qualities of each acid are useful in one way but along prove to be useless in this case but when mixed they compament each other (Forming a stable ion) allowing the gold to be dissolved

 

Are there any other acid combinations like this? I've never heared of any others but I'd be interested to know if there are any.

 

Cheers and thanks!

 

Ryan Jones

[akcapr]

H2O2 and H2SO4--> H2SO5 + H20. H2SO5 is more corrosive than just H2SO4 due to oxidizing capability. I guess that is kindof an example of what you were asking.

[Alchemist]

You can also get red fuming nitric acid or fuming sulfuric acid they are more corrosive then nitric acid and sulfuric acid!

[jdurg]

Still, fuming nitric acids won't dissolve gold, and neither will Caro's acid. The reason for this is simply because the Au ions don't have anything to stabilize them. As a result, they won't go into solution. In the presence of the CN- or Cl- ions, however, VERY stable complex ions are formed which allows the gold to dissolve. Gold will dissolve in a solution of a cyanide salt because the Au(CN)2[-2] complex ion is VERY stable. Therefore, the equillibrium of Au => Au+1 will shift to the right as the Au+ ion is removed from solution. The CN- ion is able to allow atmospheric oxygen to oxidize gold which kind of shows that gold is actually pretty reactive. (Which one would think it would be as it has a lone 6s1 electron).

 

So gold will dissolve pretty readily as long as a complex ion can be formed. Now you might ask why gold won't dissolve in a chloride only solution then. This is because atmospheric oxygen is only able to oxidize gold to the Au+1 oxidation state, and the stable complex formed with a chloride ion will only occur with gold in the Au+3 oxidation state.

[RyanJ]

So gold will dissolve pretty readily as long as a complex ion can be formed. Now you might ask why gold won't dissolve in a chloride only solution then. This is because atmospheric oxygen is only able to oxidize gold to the Au+1 oxidation state' date=' and the stable complex formed with a chloride ion will only occur with gold in the Au+3 oxidation state.[/quote']

 

I see, so it does actually dissolve in nitric but then it forms an unstable ion and comes back out of solution? Makes perfect sence if thats what you are saying

 

Cheers,

 

Ryan Jones

[jdurg]

Yeah, but only a handful of atoms worth. The equillibrium is FAAAAAAAAAAAAAR to the left in that reaction. It's just that when a complexing agent gets involved, then once those gold atoms go into solution they stay there and more will move over to take its place. The net result being the dissolution of gold.

[RyanJ]

Yeah, but only a handful of atoms worth. The equillibrium is FAAAAAAAAAAAAAR to the left in that reaction. It's just that when a complexing agent gets involved, then once those gold atoms go into solution they stay there and more will move over to take its place. The net result being the dissolution of gold.

 

Ah, I see thanks jdurg!

 

Cheers,

 

Ryan Jones

[YT2095]

Are there any other acid combinations like this? I've never heared of any others but I'd be interested to know if there are any.

 

Cheers and thanks!

 

Ryan Jones

 

I seem to remember hearing that Nitrosyl Chloride will also dissolve Gold.

[RyanJ]

I seem to remember hearing that Nitrosyl Chloride will also dissolve Gold.

 

It will? I also know what Cyanide will dissolve gold because it too also forms a complex that is stable

 

Cheers,

 

Ryan Jones

[jdurg]

Cyanide's do dissolve gold because they form a stable Au(CN)2(-) complex. The presence of the Cyanide ion allows atmospheric oxygen to oxidize gold to the Au+1 state.

 

Nitrosyl Chloride (NOCl) will dissolve gold because in solution the NOCl will break down into nitrogen oxides and chloride ions. It's basically a compacted aqua regia.

[RyanJ]

Cyanide's do dissolve gold because they form a stable Au(CN)2(-) complex. The presence of the Cyanide ion allows atmospheric oxygen to oxidize gold to the Au+1 state.

 

Nitrosyl Chloride (NOCl) will dissolve gold because in solution the NOCl will break down into nitrogen oxides and chloride ions. It's basically a compacted aqua regia.

 

Ok, thanks all and Mr Chemistry Expert

 

Cheers,

 

Ryan Jones