Quote Originally Posted by Rudeofus View Post
Michael, in your introductory post to this thread you wrote "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?". There is no reason I am aware of why the isoelectric point should the optimal, quite to the contrary. Looking at Ron's chart tells me that pH 8 is quite good and sure enough that's what TF-4 is set to according to their MSDS. I have no idea how they avoid the Ammonia smell at this pH, though.
Rudeofus, I'll try to clarify a few things and give some more examples from the text to show why I am confused.

Regarding the isoelectric point, again this was not about swell. The question I was asking is - is there a benefit in terms of washing the gelatin at pH values above the isoelectric point? I'm basing this on Haist pp. 634-635 (Volume 1). As the pH is increased to the isoelectric point of the gelatin, the gelatin becomes less positively charged and thus has less of an attraction for negatively charged thiosulfate and silver thiosulfate ions. At the isoelectric point, the gelatin molecules are neutral and thiosulfate is not retained. The wording is slightly vague as to whether there should be any improvement in washing as the pH is increased further and the gelatin molecules become negatively charged. But then the results of Crabtree, Eaton and Muehler are presented. Those results appear to support the notion it doesn't help to go higher than the isoelectric point unless Potassium Alum specifically is used as a hardener. "...the effect of pH of the fixing bath is of primary importance when the bath contains potassium alum." This concerns the gelatin. Regarding prints and the pH of the fixing bath: "...the rate of washing differed only during the very first minutes, after which the rate was the same regardless of the composition of the fixing bath. The washing of photographic prints involves other factors that influence the removal of thiosulfates from the photographic material." Note this concerns fresh fixing baths. things get more complicated with exhausted baths.

Under the topic of two-bath fixation there are some interesting comments. On p. 640 there is some discussion regarding the difficulty of removing thiosulfates and silver thiosulfates from the paper base of a print vs the emulsion. "Fixing baths with pH values above or below the isoelectric point of gelatin have little or no effect upon the retention of thiosulfate or silver thiosulfates by photographic prints when the retained thiosulfates are in the paper base or baryta layer." Later in that section on p. 642 - again in reference to Crabtree, Eaton, Muehler: "pH values in the range of 4.0 to 5.6 had little effect on the rate of removal of hypo and silver"

In the section on washing on p. 665 there is an interesting comment: "Any treatment that causes the emulsion layer of film or plates to swell increases the diffusion path within the emulsion and may retard slightly the washing of the emulsion free of thiosulfates or other salts. The same treatment, however, would be beneficial to the washing of paper prints. The paper emulsion layer would swell and impede slightly the washing of the emulsion layer, but most of the washing time is concerned with the elimination of the thiosulfate from the rest (baryta layer or paper base) of the paper print. A treatment, such as with an alkaline solution, that causes the paper fibers to hydrate and swell, or to rupture the paper sizing, will leave the paper fiber surfaces and channels more accessible to washing by water."

Earlier, alkaline bath treatments are discussed, but again mostly in reference to the thiosulfate and silver thiosulfate ions bound by aluminum-gelatin complexes (ie fixed in a potassium alum fixers).

It should be noted this all seems to concern sodium thiosulfate fixers, not ammonium thiosulfate rapid fixers, which could be pretty important.

Anyhow hopefully the above gives a better idea of why I find this a little unclear. Perhaps some testing of a few alkaline rapid fixers vs acidic ones would help. By testing for retained hypo, perhaps some quality data on relative washing times could be obtained.