Non-Chemical Evaporation Question

[Noxx]

I have a quick question for you guys.

Hoke, and some other sources, recommend to add more HCl after each evaporation step (down to a syrup).

I used to add water instead of HCl and achieved good results. Is there a difference between both ?

I suppose that when you always add water, HCl and HNO3 are driven off during evaporation and your HAuCl4 is converted to AuCl3...

Thanks

 

[Lou]

To answer your question, the purpose of adding the HCl to the remaining nitric solution is to form nitroysl chloride, NOCl, which is the 'active' ingredient in aqua regia and what actually dissolves the gold.

This NOCl is quite unstable to heat, and decomposes to Cl2 and NO2 gas. It is the NO2 that you wish to drive off because it removes the oxidizing properties of the mixture and prevents interference between reducing agents. There is a difference to adding water and adding HCl. Adding HCl drives formation of NOCl which is far easier to remove; adding water just helps to distill off the nitric acid, which is easier to distill off that water.

You'll never ever have pure AuCl3 in aqueous solution--it will always be HAuCl4 or watered complex to some extent so long as there is water, because AuCl3 is a Lewis acid (weak in comparison to AlCl3 though) but water is sufficiently acidic to protonate it.

 

[butcher]

just by adding Hcl to hot syrup, you sometimes see a gas cloud of NOx form,also the concentrated nitric left in solution may form azeotrope with water,the HCl may help to change this also I dont Know, Hoke suggests adding some sulfuric at last evaporation also, not just to sulfate any lead but also to help eliminate nitric.
also boiling I supposed we were also boiling out some of the Hcl acid from solution and we needed acidic solution for precipitation.
I have no chemical Knowledge to back my thinking, just observations.
Thanks Lou for the insight on what chemically is going on in the solution, that made alot of sense.