I considered sand, Kirk. Look again. Sand falls to the bottom and will show even in distilled water. Besides, I have seen tap water with sand in it.
I have seen no sign of iron in pHPlus. I'm fortunate that my well is not full of sulfur and iron. All my neighbors have it. Or maybe the black stuff that comes up is something else. Some say it's worth money.
I won't say anyone should not use USP carbonate. By all means, someone doing commercial work will use the most reliable source and the customer will pay for it. I do say that it may not be the wisest thing to believe that what you get at P. F. is uniformly any better than pHPlus. When you look at labels and see that one is for swimming pools and the other for darkrooms, is that a guarantee that the two are not the same? The only ones I have heard from here who have run tests have found that it is not any such guarantee. If you really are that afraid, you must test each new batch and take nobody's word for it.
What I used to get as A&H Washing Soda years ago was the stuff we used also for flushing auto radiators and such tasks. I can't get the current stuff here in rural West Virginia.