When you combine nitric and hydrochloric acid, they react together, forming nitrosyl chloride and chlorine:What is the purpose of boiling solution down to a syrup? I get boiling off excess nitric but boiling down to syrup seems off to me. Just genuinely curious about that one. I don't have alot of experience but of the solutions I have done I've never heated to an actual boiling and a few hrs at near boiling has almost always worked. Of the times I have still actually had a small amount of nitric at that point urea or sulfamic acid quickly fixed it without adding very much volume at all. And I never really dilute solution much with water. I always thought more diluted ment finer gold when dropping? Again I've only done a few batches and just now passing the ounce mark on my stores gold. Almost all from ram. Fingers and bga chips off the ram and the ice chips with legs and the glass chips on the ram.
HNO3 + 3 HCl ---> NOCl + Cl2 + 2 H2O
This is why the fumes comming out of the AR dissolution have golden-yellow colour, contrary to plain nitric on silver, where mainly NO2 is produced and fumes are brown.
It take some time for this reaction to happen. Unreacted nitric acid is very hard to drive off with evaporation, but these decomposition products are gaseous/volatile, so by boiling you accomplish two things:
1. higher temperature = faster decomposition
2. higher temperature = easier driving off gases
Chlorine and NOCl are fairly water soluble, so getting rid of them is slow, and you need to physically remove water as vapor to carry them away.
As you concentrate the solution, content of chlorides and salts rises, and the boiling point also rises = harder for the nitric and its decomposition products to readsorb when once evaporated. And easier to escape from the vessel. That´s why evaporating nearly to syrup is good, but unfortunately lengthy and messy practice.
EDIT: and I forgotten one very important thing. Many metals, not only PGMs, form nitroso-complexes. These complexes hold NOx, easily said. And these could be quite stable. To decompose them, you need to heat up the solution to quite a high temperature to destabilize them and force them to liberate NO molecules, and change them with chloride ligands. This is why boil down to syrup is very helpful - if you denox this way.
You rise the temperature, so disrupting complex´s equilibrium and liberating the ligands (NO) which could then interchange for chlorides more easily.
Huge ammounts of NOCl, chlorine and HCl are evaporated, ruining your fume-hood fan extraordinarily quickly and of course, anything around the fume exhaust.
As galenrog said, sulfamic acid always works (you need to heat the solution to get nitric reacting, not only NOCl - this work at ambient temps also), and the cost is not high. Partial boildown of the solution sometimes cannot accomplish full denoxxing, even that you cannot see NOx vapors in covered beaker.