[gainer]

We all know from watching the various CSI shows on the boob tube that identification of microscopic traces of anything is now a piece of cake.

A saturated solution of MC is clear as distilled water. I have no idea how much chloride-bromide it takes to fail the ammoniacal silver nitrate test. I am quite sure that a higher grade would be required for analytical work, but I'm also quite sure from my own long experience that for use in film and paper developers, MC is fine.
Now PE has thrown a new monkey wrench into the works. What if there is potassium in the MC and I want to use it to make fixer? First, I looked all through my copy of The Theory of the Photographic Process and could find no admonitions or even mention of the effects of potassium in fixer. Maybe there is a later edition than the third, but I think they would have known about it by then. But suppose we must avoid potassium in the fixer at all cost. How would it be removed from borax, especially if it is in the form of potassium tetraborate? Also, how much washing between developer and fixer would be required to prevent carryover from a potassium-containing developer, of which there are many? Perhaps PE was referring to color developer.

I must repeat my request that you guys also test what you buy to make sure you are getting what you think you got. I have used MC long enough to know it does what I expect of borax, but have you used it long enough to know that it does not do what you expect of it, or tested yours against MC to see if you could use it?