AR question

[bswartzwelder]

I know that AR will not keep. I cannot mix up a batch today, use a little today and hope to use the rest of it in a month. My question is: What is the failure mechanism of the AR? Does the chlorine come out of solution, or does the nitric acid lose its nitrogen atoms? If the nitric loses the nitrogen, then why couldn't I mix up some AR, dissolve my gold in it, then let it sit on a shelf for a week or two to get rid of the excess nitric?

 

[Smack]

Upon mixing of concentrated hydrochloric acid and concentrated nitric acid, chemical reactions occur. These reactions result in the volatile products nitrosyl chloride and chlorine as evidenced by the fuming nature and characteristic yellow color of aqua regia. As the volatile products escape from solution, the aqua regia loses its potency.

HNO3 (aq) + 3 HCl (aq) → NOCl (g) + 2[Cl] (g) + 2 H2O (l)

Nitrosyl chloride can further decompose into nitric oxide and chlorine. This dissociation is equilibrium-limited. Therefore, in addition to nitrosyl chloride and chlorine, the fumes over aqua regia contain nitric oxide.

2 NOCl (g) → 2 NO (g) + Cl2 (g)

Because nitric oxide reacts readily with atmospheric oxygen, the gases produced also contain nitrogen dioxide, NO2.

2 NO (g) + O2 (g) → 2 NO2 (g)

pasted from wiki

 

[Gratilla]

These chemical reactions result in a decrease in oxidation potential, which is a good quantitative measure of "potency". ORP meter required.

 

[Lino1406]

Use HCl 3 HNO3 1 H2O 3 and you can keep it fresh
a long, long time
30 and more recovery procedures.

 

[Pantherlikher]

Why mix a batch of AR in the first place?...
When you dissolve something, don't you cover it with HCL and add a little nitric acid at a time and wait for the reaction to stop?...

That way you don't use too much nitric and need to bother with dealing with the "too much nitric" problem...

Just sayin

B.S.

 

[4metals]

Years ago I worked for a refinery where they made 100 gallons of aqua regia up at a time and kept it chilled with a titanium coil and a chiller. They went through the acid in about 5 or 6 working days and it never went bad.

Bad practice though, they had to use a ton of urea to kill off the excess nitric and they had to control the reaction by adding the scrap slowly to the full volume of acid. Not my favorite way of doing this.

Much better using the acids separate and being stingy with the nitric.

 

[Charles Connor]

Because nitric oxide reacts readily with atmospheric oxygen, the gases produced also contain nitrogen dioxide, NO2.

2 NO (g) + O2 (g) → 2 NO2 (g) // Just to add: This two gases are in equilibrium also in the aquos form if recipent close, so even if it is not exposed openly to the atmosfere they are all in disolution wich the concentration of oxygen from water is higher than the air... so no matter if well closed or stored...