[piesforyou]

Hi. I analyse gold from geological samples by dissolving gold using aqua regia and (eventually!) running through an ICP-MS.

After dissolving the rock material (and gold contained in the rock) in aqua regia, the gold is present as AuCl₄^- (so wikipedia tells me). However, I then run the solution through an ion-exchange resin, and afterwards the elutant is dried down to be re-dissolved in HCl. Aqua regia is not used at this stage. So my question is, after drying the solution down, how is the HCl able to dissolve the gold? And is it possible to do anything to the residue (heat it for too long, for example) which would change the gold to a state where the HCl can't dissolve it?

BTW, the technique is detailed in
Pitcairn, I. K., P. E. Warwick, et al. (2006). "Method for ultra-low-level analysis of gold in rocks." Analytical chemistry 78(4): 1290-5.

[Fleaker]

Gold is most commonly in the +3 state. AuCl₃ is the crust you have left over after elution--it is a Lewis acid which readily accepts that HCl molecule to make the stable chloroauric acid you're familiar with in your assays. The only way to make AuCl₃ or HAuCl₄ insoluble in HCl is to overheat it--the compound then decomposes to gold, chlorine, and mixed gold-chlorine vapours (i.e. Au₂Cl₆, a dimer, in the presence of excess chlorine).