AR-insoluble black to deep purple residue from 22k gold dust, and question about assay

I purchased 26.05 g of claimed >22k gold dust from ebay. The claim initially checked out - it did not dissolve in 22k test acid.

I did several rinses with aqua regia using HNO3 in a 3:1 volumetric ratio with HCl. The first wash recovered lots of gold, but an insoluble silvery to blackish-purple residue remained. I treated it four more times; in the last two, I heated the container (a pressure tube, lightly unscrewed to allow some gas to escape) overnight in a Crock-Pot to a temperature of about 80 C with the goal of dissolving platinum, but (at least most of) the residue remained. I precipitated all the Au from the collected AR with SMB, which went nicely.

My final yield was:
20.5 g Au
5.2 g non-dissolving metals
0.35 g lost

I hope that the insoluble precipitate is some mix of osmium/iridium/rhodium/ruthenium/undissolved platinum, but there's no way to be sure. I really want to know what the composition of this precipitate is, but I have no way of knowing for sure. is there anybody who does low-cost assays of very small quantities of Pt-group metals, and what sorts of prices might I be looking at? I might send in 5 g of the gold as well just to check its purity; it stands up to 22k test acid very well but I don't know of any other way to figure out the composition myself.

Chances are it's silver chloride..

Looks like you're right. I went to this page to get an idea of how to dissolve silver chloride and do tests on it, since it's insoluble in water by itself (and even AR, apparently). It appears the trick is either ammonia or ammonium chloride.

I dissolved 4 g of NH4Cl in 10 mL H2O to encourage AgCl dissolution, then added 25 mg of my powder in a little 15 mL pressure tube sealed shut. Then I put it in a rice cooker with the cooker lid open, which heats up to a constant simmer just below boiling. After several hours at about 95 C, most but not quite all of it had dissolved. The solubility of AgCl in 4/14 = 28.6% NH4Cl by mass at that temp should be something like 7.5-8 g/cc, 4 times more than I had in the tube, so either it's just really slow to dissolve or there are other trace components. But I'm sure the bulk is indeed AgCl.

I did two tests on the solution. I put about half of it in an acidified potassium dichromate solution, which turned turbid (although not red). The other half (minus the little undissolved granules) went in a KI solution, which turned milky and slightly yellow - I suspect as it settles it will be very obvious that it's AgI that formed.

A third test involved a larger quantity (200 mg) of the powder in a crucible. I heated it with a butane torch for a while, letting it get yellow-hot and sort of crackly. Upon cooling, there were several little globules of formerly-molten material surrounded by a shiny little coating of the Ag that must have dissociated from 2AgCl --> 2Ag + Cl2. The shiny coating gave me a red color when treated with silver testing solution, confirming it is Ag.

It's a bit of a disappointment to "only" get silver rather than PGMs, but I definitely learned something today!

Silver Chloride...ugh...I hate it..

I have learned a lot about it though, through this beautiful forum.

It is soluble in:
*ammonia (quickly reacidify it, for your safety -with HCl)
*sodium thiosulfate
*HCl (Ar will as well, gsp did experiments with how much, but I dont recall seeing the exact numbers/solubility curve)
*Even strong brine will dissolve AgCl to some extent (more so if hot)
*Potassium/Sodium Cyanide
*Also strong hot sulfuric leach will remove it (unsure if it creates a double complex, or how it goes about it honestly)

..I feel like I am missing another one too...

I have used all of them except the last 2, which sulfuric will be my next acquaintance experiment with it.

Glad you got it figured out