That's an interesting observation Loveall. I have always found that no matter how often I filter the acid liquid going from a-b (also including manske steps) there always seems to be more dark stuff in the filter no matter how many times I filter. I don't filter the base liquids unless collecting precipitate, usually just a decant/siphon.

The pH is obviously varying a lot when I'm doing these but I have tended to go quite a lot lower (<4) and often even lower. I was not paying attention to it at all when acidifying really - most of the time I was watching pH in detail was during the basing for separations and pH reversals, never the acidifying.

This is probably the reason the acid phase cleaning filters always seem to leave yet more stuff no matter how many iterations! I suspect you are right that controlling it and keeping it in a tighter range is a good idea.

**OT
As an aside, I bioassayed 300mg of my 'full spectrum' and it kicked my butt! Bit strong for me but quite stomach trouble-free apart from a sudden urge-to-purge around t+2hr but the purge was done and dusted v quickly with no nausea build-up or die-down like my full rue 'tea' experiences.