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  1. #1
    Rafal Lukawiecki's Avatar
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    How Does Sodium Sulfite Wash-Aid Increase Fixer Capacity?

    Following a discussion started on another thread, I would like to get a better understanding as to why the use of a potent, unoxidised sodium sulfite wash aid, that follows fixing, seems to be considered by Ilford, Digitial Truth, Ryuji Suzuki, and a few other authors as an agent that increases the effective capacity of a fixer when used for archival/optimum permanence of fibre-based prints.

    I am not a chemist. However, with my appreciation of the basics of photochemistry, I would like to ask those who know more if it is feasible that sodium sulfite "completes" an action that would otherwise have to be performed by a 2nd fixer bath (in 2-stage fixing set-up), or that can be performed by a single bath, as long as that one is fairly new, ie. containing no more than 0.5g silver per litre working solution (ca 10 8x10 sheets). Hypothesised mechanisms of action of sodium sulfite include:
    • desorbing complex, soluble and nonsoluble argentothiosulfates which form on the paper emulsion when the fixer is no longer entirely fresh,
    • desorbing nonsoluble monoargentomonothiosulfates which form as soon as fixer has reacted with silver halides,
    • coverting nonsoluble argentothiosulfates into soluble ones.

    For example, in Basic Photographic Materials and Processes by Stroebel, Compton, Current, and Zakia, 2nd ed, page 228 I read "The hypo-clearing agents consist of salts that act on the relatively small amount of thiosulfate or complexes remaining after a short washing, replacing them with a radical that is more easily removed by further washing. A 2% sodium sulfite solution has been found to be one of the best for this purpose." What radicals do they mean?

    I realise that there may be contrasting opinions on this matter, and one was mentioned by others, notably Doremus Scudder, on that thread, for which I am very grateful, and I humbly hope to read more of. I do not aim to agree or disagree with anyone, and I am not looking for advice on what my process should or shouldn't be—I am purely interested in the reasons for, and against, this action of sodium sulfite. To make my question more realistic, I am assuming the use of a rapid-style ammonium thiosulfate fixer, preferably a neutral one, which contains between 0.5-2g silver/l (ca 10-40 8x10 sheets), and that is primarily working on a chloride and/or bromide emulsion, with no significant amount of iodide.

    For those who wish to see the remarks by Ilford please look at this document (page 3, right-hand column, point 3), where it states that: "Use a single fixing bath plus a washing aid. The number of prints through the single fixing bath can be increased to approximately 40 8x10" prints per litre working solution." in contrast to the capacity without a wash aid: "Fix only a few prints before replacing the fixing bath (approximately 10 8x10 inches prints)." while discussing optimum/archival aims.
    Alternatively, this publication of Digital Truth, which is very similar to Ryuji Suzuki's findings mentioned on the Pure Silver mailing list, states: "Note that the processing capacity of fixer is considerably lower if fiber prints are processed without using a washing aid". The fixer capacity numbers shown in that document are twice to four times higher when a wash aid is part of the process, in line with Ilford comments, but with more certainty about this process being archival.

    Thank you for sharing your thoughts.
    Rafal Lukawiecki
    See rafal.net | Read rafal.net/articles

  2. #2

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    I'm not able to answer this, but I have used two-bath fixing since reading this post from Michael Gudzinowicz at the once wonderful rec.photo.darkroom, now a mess due to spam etc.



    https://groups.google.com/forum/?hl=en&fromgroups=#!searchin/rec.photo.darkroom/gudzinowicz$20fixer$20complexes/rec.photo.darkroom/m6T6NzxwPQo/yWteXry7PnUJ



    If the link doesn't work try going to

    http://groups.google.com.au/groups?h...photo.darkroom

    and search for "gudzinowicz fixer complex" and you should find it.

    He doesn't address using sulphite as an antidote for overused fixer, but he does describe the process of fixing in such a way that I was frightened to ever be tempted to use overworked fixer (for FB paper, anyway).

  3. #3

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    As I understand it:

    The object of washing is to remove the silver/hypo complex formed during fixing from the paper. When you start out with new fixer, there is no silver/hypo complex in the fixer; as it is used, the silver/hypo complex concentration builds up in the used fixer. This increases the level in your prints and makes it harder to wash the silver/hypo complex out of your prints. Using a hypo clearing bath makes it easier to remove the silver/hypo complex from the paper during washing, thus you can tolerate higher silver/hypo complex levels in your prints out of your fixer bath.

  4. #4
    Rudeofus's Avatar
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    It's all nice and dandy that we know that Sulfite does something useful during washing, but we could do a lot more if we had any idea what mechanism is at work. There are a few things I would like to bring to the table:
    • Sulfite forms complexes with Silver ion just like Thiosulfate does, just with much lower complex stability. It won't fix as much AgBr as there is in a roll of film, but it might dissolve as much Ag+ as there remains in fixed and somewhat washed film.
    • Complexes are often assumed to consist of one metal ion in the center and a number of one kind of molecule arranged around it. In reality more than one kind of ligand can be attached to the same metal ion, and we know little to nothing about these mixed complexes, much less about their stability and solubility. Is there a chance that the insoluble (Ag[S2O3])- complex can form a more soluble (Ag[SO3][S2O3])3- and that this new complex (or a similar one) is the key to wash aid?
    • Complex stability supplied as single number can be misleading, even if only one ligand is supplied. There are different numbers for (Ag[S2O3])-, (Ag[S2O3]2)3- and (Ag[S2O3]3)5-, and while it may be hard to break up (Ag[S2O3]3)5- into Ag+ and 3 S2O32- ions, splitting them into (Ag[S2O3]2)3- and one S2O32- ion is much easier. Note that the abundance of (Ag[S2O3]3)5- compared to the lower complexes may rely on the abundance of Thiosulfate ions, and that this concentration will inevitably go down as the film or paper enters the washing cycle. There's a chance that it is the washing which creates all these insoluble Ag[S2O3])- complexes to begin with, and it will create more if more Silver ions are in the fixer.
    Trying to be the best of whatever I am, even if what I am is no good.

  5. #5

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    The use of a washing aid was discovered during WWII. On shipboard fresh water is limited so the custom was first to wash in salt water and then a final short wash in fresh water. It was found that this system was more efficient than using just fresh water. Testing revealed that other salts could be used. Agfa at one time recommended using sodium carbonate for prints. However sodium sulfite is the best in terms of speed of removal of the fixer.
    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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  6. #6
    Ian Grant's Avatar
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    There's a missing link.

    It's the way the semi-soluble Silver complexes link and bond to the cellulose in Fibre based paper (the base). This is why films and RC papers can tolerate higher silver levels in working fixer than FB papers and suffer no long term ill effects.

    It's many years since I looked at all the equilibrium reactions taking place during fixing, and the complexes formed, I have a schematic diagram somewhere (I drew/typed it in 1976) not sure where at the moment. At that time few in the UK were using Sodium Slphite based wash aids, in fact some still used Peroxide or similar hypo eliminators, now long discredited.

    Essentially the use of Sulphite helps reverse some equlibrium reactions from less soluble to more soluble complexes. It's not a miracle cure for very poorly fixed prints though.

    Ian

  7. #7

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    I agree with Ian and would not personally try to extend the useful capacity of a fixing bath by any other means than a two bath system. Fixer is not that expensive and your images should be the primary concern.

    At one time Kodak recommended the use of a hypo eliminator solution which consisted of hydrogen peroxide and ammonia water. The ideo was to oxidize any thiosulfate ion still in the emulsion to sulfate. Paradoxically such treated film had less archival permanance than untreated film. It appears that some of the residual thiosulfate decomposes and coats the silver grains with silver sulfide. Sodium sulfide is used to increase the permanence of microfilm. Agfa Sisain did the same thing by using a thiocyanate.
    Last edited by Gerald C Koch; 03-18-2013 at 10:37 AM. Click to view previous post history.
    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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  8. #8
    Rudeofus's Avatar
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    Quote Originally Posted by Gerald C Koch View Post
    The use of a washing aid was discovered during WWII.
    Apparently that's a myth. See here for details.
    Trying to be the best of whatever I am, even if what I am is no good.

  9. #9

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    Quote Originally Posted by Rudeofus View Post
    Apparently that's a myth. See here for details.
    The quoted article cites two previous findings of the use of salt water. However there is the question as to whether this idea ever became common knowledge. The fact remains that film had to be processed on shipboard during WWII and the use of salt water was found to be beneficial. It is not uncommon in science to find that the same fact has been "discovered" more than once. Roger Bacon is credited with the discovery of gunpowder even though it had been known for some time in China.
    Last edited by Gerald C Koch; 03-18-2013 at 11:49 AM. Click to view previous post history.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

  10. #10

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    Further to what Gerald said in #5, I recently noticed that Adox recommend use of a 1% (I think, the docs are at home) sodium carbonate solution on the insert that comes with MCC110 paper. That could be a direct copy from Agfa, it's my understanding that MCC110 is a clone of an earlier Agfa product.

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