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  1. #1
    Rafal Lukawiecki's Avatar
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    Buffering Neutral Fixer

    I'd like to mix a neutral rapid fixer, relatively low-odour, that can keep its pH about-neutral when used after a citric acid stop bath. I've researched APUG, and I can see many references to a buffered fixer by Ryuji, however, all of those referenced pages seem to have been deleted, and are not to be found elsewhere. I suppose there were reasons why that recipe was removed from APUG and from the public domain, which is why I am not posting those thread references, and I am not asking for it. The only Ryuji recipe for a neutral fixer is http://www.apug.org/forums/forum226/...pid-fixer.html, which is similar to the below one, MF-2, except it uses 5g sodium metabisulfite instead of boric acid, but it seems to me that it may not have much of a buffer. Other fixer discussions do not mention buffering much.

    However, I found the following, somewhat alkaline, recipe MF-2 on Flickr:

    Ammonium Thiosulfate - 120g (200mL of 60% solution)
    Sodium Sulfite - 15g
    Borax - 5g
    Boric Acid - 5g
    Water to make 1L (pH 8.0)

    I remember reading somewhere that borax, triethanolamine, or disodium phosphate would make good buffers for a fixer, and I am aware of Ryuji's use of salicylic acid combined with triethanolamine as a buffer (and a chelating agent I think) for his developers.

    I could go ahead and adjust the proportions of borax, metaborate, and either boric acid or sodium metabisulfite until I get pH of about 7, but I am concerned that in doing so I would not be making a good buffer, but just using up the buffering ability of such a mix to arrive at a desired pH, which sounds counterproductive to me. Do I need a quick lesson in pH 7 buffers suitable for fixers?

    Could the collective chemical mind of APUG make a suggestion as to a sensible neutral buffer to use for an ammonium thiosulfate/sodium sulfite rapid fixer?

    As an adjunct to this, I am curious about a buffer for a citric acid stop bath. I guess I do need a lesson in buffers.
    Last edited by Rafal Lukawiecki; 03-10-2013 at 12:11 PM. Click to view previous post history. Reason: Clarified grammar
    Rafal Lukawiecki
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  2. #2

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    What is your reason for needing this? By the way you might want to try TF-5 which is very close to neutral. PE was involved in its design.

  3. #3
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    Before you load your fixer with all kinds of extra compounds, go read what "ionic strength" means and what that means to fixers. Long story short you can really slow down fixer action if you add too much extra stuff. That's why most fixers don't include much buffering and why the weak buffering that is there comes from compounds which are needed anyway: Sulfite/Bisulfite.

    If you are concerned about pH stability, think about a wash step before fixing. Citric Acid stop bathes are usually pretty dilute so they shouldn't mess up too much. I'd be more concerned about Ammonia smell from a pH 7 rapid fixer ...
    Trying to be the best of whatever I am, even if what I am is no good.

  4. #4

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    The pH would have to be considerably higher than 7 before any ammonia gas is released.

    The pH of an ammonium thiosulfate solution 60% is already between 7.5 aqnd 9 add to this the addition of sodium sulfite and you already have a buffered fixer without the need for any additional buffering. Fixer will wash out faster if the fixerr is slightly basic.

    Acidic fixers are really only needed when they must contain alum as a hardening agent.
    Last edited by Gerald C Koch; 03-10-2013 at 07:09 PM. Click to view previous post history.
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  5. #5
    Rafal Lukawiecki's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    What is your reason for needing this? By the way you might want to try TF-5 which is very close to neutral.
    Michael, I should have explained my reasons, but I didn't want to bore anyone with the details of my current process, which I am aiming to optimise—tinkering to save time, I suppose, and to cut down on acetic acid fumes, though I don't want to replace them with plenty of ammonia, either, hence the desire to stay neutral, rather than alkaline. As Gerald points out, I believe I have no need for an acidic fixer, as I have no need for hardeners. Secondly, I tend to tone, in KRST, at the end of my printing session, sometimes twice in a day, and I would like to make that step even more immediate at times. Sine I tested, I know my current fixing is good, in terms of residual silver halides, so I should be able to dispense with, or shorten, the rinse prior to KRST, which is currently necessary due to the acid in the fixer, which Se does not like. However, I want to stay with a rapid fixer, hence ammonium thio in a neutral setting appeals to me. Finally, I am comfortable with a citric acid stop, and I want to continue using it, without pushing the fixer into acidic territory, hence my request for a suggestion of a buffer. For what it is worth, I fix twice, 45 s each plus 15 s drain time, and I like using the first fixer till it has about 3 g Ag/l, hence the safety net of the 2nd fixer bath to make the argentothiosulfates easier to wash out. I also use a sodium sulfite washing aid after KRST. In the long run, I am keen to experiment with removing the second fix bath, since I think the thiosulfate in KRST, and then a fresh washing aid, perhaps slightly prolonged from my current 3 min to 5 or so, may do a good enough job of getting rid of the fixing byproducts. But first, I just want to try with a neutral one.

    Unfortunately, shipping TF-5 to Ireland is prohibitively expensive, due to weight, and I cannot find it sold here...

    Quote Originally Posted by Gerald C Koch View Post
    The pH of an ammonium thiosulfate solution 60% is already between 7.5 aqnd 9 add to this the addition of sodium sulfite and you already have a buffered fixer without the need for any additional buffering.
    Gerald, would you be able to estimate how many ml of citric acid stop would it take to bring 1l of a mix of (ammonium thiosulfate 120g + sodium sulfite 15g + sodium metabisulfite 5g) below pH 7, say to 6.5?

    Quote Originally Posted by Rudeofus View Post
    Before you load your fixer with all kinds of extra compounds, go read what "ionic strength" means and what that means to fixers. Long story short you can really slow down fixer action if you add too much extra stuff.
    Rudeofus, thank you for your kind suggestion to read up on ionic strength. I have spend an hour or so picking up on the subject on Wikipedia, and I have browsed a Kodak patent that talk about this metric in the context of fixers. I will be reviewing Mees later today, to find out more, as I am just reading that amazing book. All I can understand, so far, is that increase in ionic strength will affect photographic material gelatin swell which may also reduce fixing rate. What I am unable to determine, however, is how significant would that effect be, especially with a buffer designed for the purpose.

    Since you know about ionic strength issues in fixing, may I ask you, please, to estimate what would be the impact, in terms of how much I would have to extend fixing time when using a buffer such as 5g Borax + 5g Boric Acid in 1l of the fixer, such as the one mentioned above? Do you have any other suggestions as to where I should read more about ionic strength and fixers?
    Rafal Lukawiecki
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  6. #6

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    Gerald, would you be able to estimate how many ml of citric acid stop would it take to bring 1l of a mix of (ammonium thiosulfate 120g + sodium sulfite 15g + sodium metabisulfite 5g) below pH 7, say to 6.5?
    Such a calculation would be very hard to do as it involves 5 variables which includes the pH of the ammonium thiosulfate 60% solution which is variable from batch to batch. But the amount of citric acid carryover should be small and of little consequence. Just let the prints drain well before putting them in the fixer.

    If you are concerned about the pH of the fixer it is easy to check it periodically with some pH paper of the appropriate range. If it becomes too acidic then just add a small amout of base.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

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  7. #7
    Rafal Lukawiecki's Avatar
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    Thank you, Gerald. I will keep notes on the evolving pH of the fixer as I work with it, what an obviously simple suggestions, many thanks, indeed. I had no idea that ammonium thiosulfate 60% solution comes at different pH, batch-to-batch. I have assumed it to be more-or-less constant. I suppose this means even Ryuji's recipe needs to be adjusted to get it into pH 7 upon making, I suppose by varying the amount of sodium metabisulfite.

    I also want to thank Rudi for his PM suggestions re ionic strength.

    Still, why do I have a feeling I am rediscovering something that must have been done by quite a few others?
    Rafal Lukawiecki
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  8. #8

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    Hi Rafal, I don't know how much processing you do, but it might be worth considering the option of replenishing your fixer.

    This is the standard method used in high-volume processing. Whatever is wrong with the tank solution is fixed with the right amount of the right replenisher. (Replenisher volume is normally based on area of the material being processed.)

    Since you indicated that you could mix to a certain pH aim, and you know how to limit your first fixer to a certain silver-content, I'm thinking that you could probably customize your own fixer replenisher. (I wouldn't normally suggest this idea, but it sounds like you're having fun formulating.) Essentially you would make a fix replenisher that counteracts the pH-lowering effect of the stop bath.

    If you're interested, and want some suggestions on how, I'll be glad to kibitz.

    ps. of course, Gerald's suggestion is much simpler and gets right to the point.

  9. #9
    Rafal Lukawiecki's Avatar
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    Thank you, Bill, for making the suggestion about replenishment. I have researched it in the past, and I understand that fixer replenishment only works in conjunction with a silver recovery option, as no amount of replenisher will remove the silver that is already in the fixer, unless you remove the solution itself—if this is incorrect, please let me know, as I am eager to know.

    Having said that, I do use a silver recovery device (Silver Magnet from JNanian) to remove a good bit of the silver from the spent fixer. However, what is left gets taken away, as it is has been contaminated with stop bath carryover. I'd be happy to research replenishment too, but for now I am more focused on making a switch from acidic to a neutral one. I'll report my findings as to the pH stability when I have some data, for now I am using up the remainder of my acid fixer. Once that is under control, I would love to recycle/replenish my fixer, as anything I can reasonably do to minimise waste without creating something worse, is high on my agenda.
    Last edited by Rafal Lukawiecki; 03-11-2013 at 02:55 PM. Click to view previous post history. Reason: Grammar, clarified.
    Rafal Lukawiecki
    See rafal.net | Read rafal.net/articles

  10. #10

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    Quote Originally Posted by Rafal Lukawiecki View Post
    as no amount of replenisher will remove the silver that is already in the fixer, unless you remove the solution itself—now, if this is incorrect, please let me know, as I am eager to know.
    Well, although this is correct, it's not the whole story. When you replenish, you don't REMOVE silver, but the fresh replenisher being added will dilute byproducts; thus you control silver concentration by changing the replenishment rate (solution being removed changes volume, but not concentration).

    I don't know that there would be any advantage to replenishment other than the tanks staying in a steady-state condition. You're already doing good by using a 2-bath fixer and John's Silver Magnet. (Anybody doing any volume without something like the Magnet, or better, is missing out in my opinion.)

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