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.
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.