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  1. #11

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    Quote Originally Posted by Photo Engineer View Post
    Modern emulsions use up to 10% iodide, but emulsions from about 75 years ago were typically maxed out at about 3%. Fixing with Ammonium Thiosulfate is always faster regardless of emulsion type, so in a difficult situation the Ammonium salt is the way to go. Since Iodide represents only a max of about 10% of the halide present, the effect is not huge, but can be seen, particularly in exhaustion of a fix used with high iodide films.

    PE
    Thank you sir. Appreciate it.

    Bob
    "Why is there always a better way?"

  2. #12

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    Quote Originally Posted by BobNewYork View Post
    Dan: Am I wrong in thinking that today's film emulsions
    increasingly use silver iodide? If so, does that mean
    that sodium thiosulphate is a better fixer for
    modern emulsions? Bob H
    As for the amounts of silver iodide in today's emulsions over
    years ago, I think Ron has answered that question. I seem to
    recall some to-do over the Delta and T films when brought to
    market around 20 years ago. Particular note was taken of
    their iodide content. I'm quite sure there must be some
    update versions of older emulsions which do not have
    significantly increased iodide content. Certainly not
    the increase seen in the T grain films.

    Sodium has no affinity for silver so takes no part in the
    removing of silver. Ammonia in solution does have some
    affinity. I've read of it being used alone as a fixer in the
    case of chlorided only silver emulsions. The chloride of
    silver is the most soluble of the silver halides.

    At the other extreme is silver iodide. It and silver sulfide
    are two most insoluble salts of silver. The ammonium ion
    has nearly no effect upon the iodide while the thiosulphate
    ion with it's greater affinity for silver will, in some surplus,
    clear the silver from an emulsion.

    In all cases, chloride, bromide, or iodide, the ammonium
    ion acts as a fixer; the more so with the chloride and least
    so with the iodide. Temporary though is it's attachment to
    silver because the thiosulfate ion, with it's much greater
    affinity for silver, 'steals' the silver from the ammonium
    ion. Both the ammonium and thiosulphate ions are
    active in attaching silver where rapid fixers are
    used. So Ron, on a technicality, is correct.
    Rapid fixers are faster in all cases. Dan

  3. #13
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    Well, to get a bit more specific and technical, Sodium takes no part in the fixing reaction other than being a balancing positively charged ion in the fixer. Ammonium ion takes part in the fixing reaction by being a silver complexing agent itself. So, both Ammonium and Thiosulfate ions take part in the fixing action and their action is superadditive. In addition, the Ammonium ion is a positively charged ion in the fixer, and it is about 2/3 the size of a Sodium ion and much much smaller than Thiosulfate.

    Therefore, wherever Ammonium ion replaces Sodium, the complex of Silver + Thiosulfate is smaller by a rather huge amount, and therefore diffuses out of the coating more rapidly.

    You therefore have several things on your side in Ammonium Thiosulfate fixes. The superadditivity, size and diffusivity effects all add up to increase fixing rate.

    Halide ions of any sort slow fix rate down, and therefore buildup of halide with use slows down fixation by helping to exhaust the fix. The fix is exhausted two ways, by buildup of halide ions and by use of Thiosulfate ions. But, in Ammonium Thiosulfate fixes, since both Ammonium ion and Thiosulfate ion are complexing agents, the fixer has an effective concentration of useful complexing agent of about 2x that of a comparable Sodium Thiosulfate fix. It can be even larger, depending on pH and the nature of the 5 or so complexes that can form between the various reactants.

    By analogy, larger ions slow down fixation and therefore you must NEVER mix a fix with Potassium Thiosulfate or Calcium Thiosulfate or Magnesium Thiosulfate, as these large positive ions will effectively poison a fix.

    There is fixing in a larger expanded version.

    PE

  4. #14

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    WOW! I had to ask, didn't I

    Appreciate the insights guys. So it Ammonium all the way for me.

    BTW PE - someone on another thread who shall remain nameless, is accusing you of suggesting that C-41 fixer, (buffered Ammonium Thio as far as I know) is not suitable for B&W work. Why would that be?

    Bob H
    "Why is there always a better way?"

  5. #15
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    Bob;

    The C-41 fixer is usable for color and B&W. Many B&W fixers are not suitable for color. It is a question of pH.

    Now, to go further, the C-41 RA fixer may not be suitable for some B&W films because of the chemical composition of the fixer and the hardness (or lack thereoof) of certain films.

    So, it may depend on fixer and film.

    PE

  6. #16
    RPC
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    So, for B&W, should regular C-41 fixer be used at normal strength for both film and paper?

    RPC

  7. #17
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    Well, what is the "normal" concentration of C-41 fixer for paper. There are no suggestions by Kodak. And, due to the different level of silver in B&W and color, what is the normal concentration for B&W film?

    You can use it, but you are on your own and you must use your own testing to determine that.

    PE

  8. #18
    RPC
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    By normal, I meant working solution strength for C-41. For B&W, would you suggest starting testing at that strength for film and paper and just adjust times for 2x clearing? I was thinking that for paper it might be diluted some, perhaps half working solution strength.

    RPC

  9. #19
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    IDK your paper, film or workflow. The only way to be sure is by testing with the retained hypo and retained silver tests. You would have to do that. Otherwise there are no suggestions anywhere for the correct values of color for use with B&W.

    PE

  10. #20

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    OK - starting to breathe a little easier!!

    I've done the tests with Tri-X, TMX and TMY, (the older version) and they were fine. Also done then with Polymax, Polymax-Art, Polycontrast, MGIV and MGWT - fine again. I use the C41 fixer replenisher diluted 1+4 for both film and paper - though I'm sure a higher dilution would be fine for paper.

    Thanks PE - feeling better!!

    Bob H
    "Why is there always a better way?"

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