Which metal is used to displace gold from sodium dicyanoaurate?

Answer 1

Long version of answer: From what I know, dicyanoaurate(I) is a very stable complex because of its 2 cyanides. Cyanides are amongst the strongest pi-acids I know, and they tend to stick and not let go of any metal encountered. So, another metal wouldn't be the solution here; another metal wouldn't break the very strong bonding between the gold(I) and the cyanides. The cyanides themselves are very stable, so we cannot break them. We are now left with the gold ion itself. Gold ions are not so stable as gold tends thermodynamically to become Au^(0), (see for yourself in the Electrochemical Series). So the trick here is to provide the gold ions enough electron (and chances to meet them), so they could become zero-charged, instead of 1+. This is achieved by introducing a reducing gas over the solid dicyano complex. The gas that is traditionally used is hydrogen. It turns into protons by leaving its electrons at the gold ion, turning the latter into neutral gold atom. The cyanides are attracted to the protons to produce hydrogen cyanide, that leaves the system in gaseous state, and the complex is broken. Then, neutral gold atoms find one another and precipitate at the bottom of the oven (this process is performed by heating the dicyano sample) as solid, very pure metallic gold.

Short version of answer: Another metal is not used here. Hydrogen gas turns the dicyano complex into metallic solid (which precipitates as solid particles) and hydrogen cyanide, that leaves the system as a gas.