How does aqua regia help separate platinum and palladium from other platinum-group metals?

In the classical procedure for refining platinum-group metals, aqua regia serves as a selective solvent to initiate the separation process.1

When a mineral concentrate containing these metals is leached with aqua regia, the mixture effectively dissolves both platinum and palladium, while the other platinum-group metals—specifically rhodium, iridium, ruthenium, and osmium—remain as solids in the leach residue.1

Once this initial separation is achieved, the dissolved platinum and palladium can be isolated from the solution through sequential chemical precipitation:
* Platinum: It is precipitated from the solution by adding ammonium chloride, filtered, and then processed into a pure metallic form.1
* Palladium: After the platinum is removed, the palladium remains in the solution and is subsequently precipitated by the addition of ammonia.1

This method leverages the differing chemical solubilities of these elements to isolate the platinum and palladium from the remaining, more resistant metals, which require further specialized treatments such as fusion with molten salts to be solubilized.1