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  1. #21
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    Most products today are made with bone (ossein) gelatin from cows. The isoelectric point is about 4.5 and this is the point of minimum swell. So, the gelatin is "tight" in acid fixers and diffusion in and out of chemistry and salts is restricted. At an alkaline pH, gelatin is "looser" due to higher swell and thus chemistry diffuses inward and outward more easily. Fixing is faster.

    However, development can continue in an alkaline fix unless you either use a stop bath or a running water rinse.

    Ammonia odor can be a problem for some users of TF-4 and thus it was reformulated to remove the solids in the bottle and to prevent (or at least reduce) the ammonia odor. Thus we have TF_%>

    PE

  2. #22
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    Quote Originally Posted by Michael R 1974 View Post
    I didn't say the isoelectric point/pH had anything to do with gelatin swell. The discussion in Haist regarding the isoelectric point has to do with the affinity for thiosulfate and silver thiosulfate complexes.

    Swell is discussed from a diffusion perspective.
    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.
    Quote Originally Posted by Michael R 1974 View Post
    But I can't agree everything they say about developers and fixers (developers in particular) is necessarily sound.
    It is more sound and better tested than most of the data or info I gather from forum or news group threads by a wide margin. Fact is that film development and fixation is a huge topic that occupied dozens of the brightest minds in the chemistry world, and few of their findings were ever made public. Many pieces of information published in the FDC were gathered informally and could not be publicly attributed to their sources, it is well known (ask Ron) how furiously Kodak defended and still defends their trade secrets from becoming public.

    Look at Mark Overton's heroic and ongoing effort in replicating Xtol to see how much work you can sink into one simple developer formula, and look at the sheer number of formulas in the FDC alone. It is quite plausible that Bill&Steve became so immersed in and overwhelmed by their effort that they drew some fast conclusions or let their opinion creep into their pile of accumulated facts. I still challenge everyone condemning the FDC and DCB to show me something better which is at least remotely up to date.
    Trying to be the best of whatever I am, even if what I am is no good.

  3. #23

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    It's surprising how little information there is about washing papers. The reason may be the complexity of the issue. The paper, the fixer composition, the fixing time, the temperature, the washing apparatus, the pH, and a host of other things are all involved. Mees and James has a short section which is reasonably informative. Important factors they mention include: 1. Fix no longer than 10 minutes; 2. Keep the pH above the isoelectric point to retard absorption by the gelatine; 3. Use a high turbulence, minimum water washer that maintains fresh water across the print; 4. Ammonium thiosulfate fixers wash out faster than sodium thiosulfate; 5. Hardeners retard washing; 6. Salt treatments, like HCA, can reduce washing times; 7. Paper is a lot harder to wash than film.

    Ref: C. E. K. Mees and T. H. James, "Modern Photographic Process" Macmillan, 1966. pp 403-405.

  4. #24

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    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.

  5. #25
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    Mason gives a very good description of washing, including the differential equation that shows how it takes place. Haist shows the effects of pH on washing and I have shown the effects of pH on swell both here and in my book. Swell is maximized at pH 9 and minimized at pH 4.5. Washing and fixing are best at pH 9 and worst at pH 4.5. Kodak sold fixers that worked at both pH values. At these different swell values, the fixers must be totally different in order to contain hardeners.

    Michael has it backwards wrt pH vs swell and fix and wash rate.

    Anchell and Troop is much more thoroughly researched compared to Anchell. Bill Troop actually talked to Grant Haist to confirm many of the statements in A&T. The soap analogy by Anchell is reasonable but not entirely accurate.

    PE

  6. #26

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    Quote Originally Posted by Photo Engineer View Post
    Haist shows the effects of pH on washing
    Where? Are you referring to the alkaline after-bath treatments (Kodalk, Sodium bicarbonate etc)?

    If we can point to something specific I'll be much more comfortable with the argument for alkaline fixers and shorter washing times.

  7. #27

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    One problem with simple formulas seems to be that the effectiveness of washing FB papers (not so much film) seems to decrease markedly after an initial period where most of the hypo is removed. The initial period follows a simple exponential rule very well, but then the washing slows because of the way thiosulfate is absorbed into the paper fibers. I would expect RC papers to behave more like film. The curves given in the above reference show that thiosulfate is essentially never fully removed from FB papers, although acceptable levels are achieved in an hour.

  8. #28
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    Michael, that should read "Haist describes the effect of pH on washing". His publications were heavily redacted by EK. This was a case of my knowing what he wrote and what he published. Sorry, but you still have things backwards.

    As for FB papers, the paper fibers have an affinity for hypo and other salts, and therefore washing is less efficient. The exposed Baryta also has some effect. There are articles out there on this.

    The point is, that an alkaline pH speeds up fixing and washing and this is regardless of where it is applied after the stop or rinse step.

    Kodak has sold an alkaline hardening fixer BTW, which was described elsewhere on APUG. the washing information in Mason has been reported at least 2x by myself here. We keep going over and over this same thing. It is really being worried to exhaustion IMHO.

    PE

  9. #29

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    I haven't said anything about fix rate, just wash rate. I don't know what I have backwards regarding pH and swell. If you're referring to the longer diffusion path slightly slowing down the washing of the emulsion layer as swell increases, I'm just pointing out it does say that in the text. The only definitive reference to an alkaline pH increasing wash efficiency is in the section on alkaline baths which I cited above. I guess what you're saying is the effect of an alkaline bath can essentially be combined with the fixing process in an alkaline fixer. Fair enough I guess.

    I'm sorry if this is repetitive but when I read the other threads, and various other articles and books I did not find anything conclusive enough. I don't think it is a waste of time to challenge things like this. Photographic processing is a field full of myths, traditions often lacking any supporting evidence, and plain bad information. The fact companies like Kodak are not involved in research anymore has not helped. For a long time now it has been up to individuals, some skilled, some not, to come up with new commercial products and formulas. That's nice, but how are we to know if these things actually work as claimed?

  10. #30
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    Quote Originally Posted by Michael R 1974 View Post
    \ 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. .
    This is where you are wrong. You equate this to swell, wash time and charge. Charge does go down as pH goes up, but swell goes down as does diffusion.

    Photography is full of chemistry, and the best I can say is that there is a lot of bad or approximate information out there.

    PE

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