Oh, I just found an error in my post above. That is 10^-6 moles of Iridium / mole of silver. Aaaak. Sorry about that error. Thanks for the catch.
I should also have added to that post, now that I see it again, that the Iridium must be made up in a special solution to prevent decomposition, and that addition must be at an optimum time during the run of silver. This time is often determined by trial and error and then running comparative reciprocity tests.