A slightly acidic saturated solution of Ammonium Chloride, since that is what you are going to need to precipitate the PGMs.
Why go through all the extra work to dry it?
It's also available on Ebay at a very reasonable price. It's hardly worth making it, it's so cheap.
I produce it as a byproduct. After you finish digesting your PMs, you have to boil off your excess HCl. I boil it off into Ammonium Hydroxide, thus making the Ammonium Chloride that I need.
Everything but the squeal.