aqua regia

[skwiff]

if i was to form a solution of aqua regia using diluted acids (5 mol nitric and hydrochloric instead of 15 and 12 moles respcetifily) would it still be able to dissolve gold? the reason being is that if the acid is diluted should be some what safer than the 'pure' stuff.

 

 

[elementcollector1]

I believe it should, but it would dissolve the gold much slower.

[skwiff]

I believe it should, but it would dissolve the gold much slower.

that is to be understood ^^

[Dekan]

if i was to form a solution of aqua regia using diluted acids (5 mol nitric and hydrochloric instead of 15 and 12 moles respcetifily) would it still be able to dissolve gold? the reason being is that if the acid is diluted should be some what safer than the 'pure' stuff.

 

 

 

I wonder, why is gold dissolved by Aqua Regia. This is just a mix of nitric acid and hydrochloric acid. And each of these acids, by itself, won't dissolve gold. Yet together, somehow, they can. This seems strange. Is the gold affected by the chlorine, in the hydrochloric acid? If so, shouldn't the hydrochloric acid work by itself - why is the nitric acid needed - it's got no chlorine in it.

[skwiff]

I wonder, why is gold dissolved by Aqua Regia. This is just a mix of nitric acid and hydrochloric acid. And each of these acids, by itself, won't dissolve gold. Yet together, somehow, they can. This seems strange. Is the gold affected by the chlorine, in the hydrochloric acid? If so, shouldn't the hydrochloric acid work by itself - why is the nitric acid needed - it's got no chlorine in it.

 

in simple terms. the HNO2 forms ions of gold but in an equalibrilum so it does not dissolve, the HCl then bonds to the gold ions to form AuCl3 and this can not reattach to the gold, then the HNO2 ionises more gold and the process continues untill the gold is dissolved

Edited October 11, 2011 by skwiff

[Anders Hoveland]

The mix of concentrated nitric and hydrochloric acids in aqua regia exists in an equilibrium. The equilibrium only begins moving to the right when the acids are very concentrated. Even heating 30% conc HCl with NaNO3 will give off some brown fumes.

 

HNO3 + (3)HCl <==> (2)H2O + NOCl + Cl2

 

(2)NOCl + H2O --> NO + NO2 + (2)HCl[aq]

 

 

Aqueous solution of chlorine can attack gold.

 

(2)Au + (3)Cl2 + (2)Cl[-] --> (2)AuCl4[-]

 

 

An interesting way to reduce gold(III) chloride back to elemental gold is to use an alkaline solution of hydrogen peroxide. Although usually oxidizer, in some reactions H2O2 can act as a reducing agent.

 

(2)AuCl4[-] + (3)H2O2 + (6)OH[-] --> (2)Au + (8)Cl[-] + (6)H2O + (3)O2

 

The gold usually separates out in a finely divided state, and appears brown by reflected light and greenish blue by transmitted light. If very dilte solutions are used, the gold sometimes separates out forming a yellowish film on the sides of the test tube.