Alkaline fixer/wash times for FB paper
I did a quick search and didn't really find anything so hopefully this thread isn't totally repetitive. An APUG member asked me a question about fixers recently and it reminded me of a subject I've wanted to discuss.
Topic note: This concerns non-hardening, Ammonium Thiosulfate rapid fixers.
We're told all things being equal, an alkaline fixer will wash out of paper significantly faster than an acidic one and that a hypo clearing agent isn't needed. Does anyone have hard data on this? I'm asking because based on what I think I understand from Haist I'm not convinced. However the chemistry is quite complex so I could easily be wrong.
When a Potassium Alum hardener is included in a necessarily acidic fixer, it is true that a lower pH fixer is harder to wash out. I wonder if the notion alkaline fixers wash faster is an oversimplified extension of this (ie wash rates keep increasing as pH is elevated).
1. The emulsion
Given a non hardening Ammonium Thiosulfate fixer, my understanding is the most efficient washing will occur if the fixer pH is increased to the isoelectric point of the gelatin (~4.9), with no real benefits above that point. Am I wrong?
2. The paper base
My understanding is the rate of washing the thiosulfate and silver thiosulfate complexes out of the paper fibers, baryta etc. is essentially unrelated to the pH of the fixer. Am I wrong? This would also appear to contradict the hypo clearing non-requirement with alkaline fixers.
If I'm reasonably correct about (1) and (2) above, faster paper washing would not be an advantage when using an alkaline fixer. The remaining benefits are no image bleaching with overfixing and possibly higher (?) capacity.