Why does a chemical compound precipitate metals differently than metal

[BernieM]

In a solution of aqua regia (or other acids), if there is a precious metal dissolved, along with other metal ( such as other precious metals or base metals), and you add a powdered metal, you precipitate all the metals lower than it on the reactivity list.

If however, you use a chemical compound (such as sodium metabisulfate to precipitate gold or ammonium chloride for platinum), it predominantly only precipitates one particular metal.

Why is the compound selective?

 

[Yanick]

Your first example is redox chemistry and your precipitating M⁰ which was reduced by whatever reducing agent you added to your solution, the electrons don't care where they go as long as they are lower in energy and so on. Whereas in the second case you are doing a so-called Metathesis/Exchange Reaction and you are precipitating M^a_|b|A^b_|a|.

 

[BernieM]

I see my error now, thinking that it was all valence electron exchanges. Thanks.