How Does Sodium Sulfite Wash-Aid Increase Fixer Capacity?

Discussion in 'B&W: Film, Paper, Chemistry' started by Rafal Lukawiecki, Mar 17, 2013.

  1. Rafal Lukawiecki

    Rafal Lukawiecki Subscriber

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    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.
     
  2. john_s

    john_s Subscriber

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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?hl=en&lr=&group=rec.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. Prof_Pixel

    Prof_Pixel Member

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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. Rudeofus

    Rudeofus Subscriber

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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.
     
  5. Gerald C Koch

    Gerald C Koch Member

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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.
     
  6. Ian Grant

    Ian Grant Subscriber

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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. Gerald C Koch

    Gerald C Koch Member

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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.
     
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  8. Rudeofus

    Rudeofus Subscriber

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    Apparently that's a myth. See here for details.
     
  9. Gerald C Koch

    Gerald C Koch Member

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    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.
     
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  10. Steve Goldstein

    Steve Goldstein Subscriber

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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.
     
  11. Ian Grant

    Ian Grant Subscriber

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    I've used Sodium Carbonate as Agfa recommended and it works well, you are right the Adox MCC datasheet is a copy from the last Agfa one. I always found that Agfa published particularly good datasheets and accurate data/recommendations.

    Ian
     
  12. Rudeofus

    Rudeofus Subscriber

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    IIRC there were published tests of the individual hypo clearing agents, and that Sodium Sulfite was deemed the best. What one will eventually use probably depends on availability and cost as much as on actual performance.
     
  13. john_s

    john_s Subscriber

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    I have found in my archives a copy of an article that used to be on Ryuji Suzuki's web site. In it he says, among other things, that the carbonate bath was more relevant in the days of handening fixers, and that sulphite can assist in washing out a complex formed in fixing. I have not edited the article except to remove some heading links.
    Sorry about the formatting of the table. Extra spaces are eliminated by the system.
    <start of quote>

    Washing aid
    From Silvergrain Labs

    Washing aid is used most commonly in archival processing of fiber based paper, after fixing to reduce washing time and save washing water. It is not necessary for film or RC paper.

    Although washing aid is used in archival processing, there are more requirements than proper washing process to make archival images. The use of washing aid does not mean that the resulting material is archival. See archival process for guideline.

    Both washing aid and hypo clearing agent refer to the same chemical process described in this article. However, hypo eliminator is a very different chemical process although they are both designed to accelerate washing of thiosulfate from fixed materials. The use of the latter, hypo eliminator, is discouraged. For hypo eliminator, see the section at the end of this article.

    About the methods of testing for residual thiosulfate (hypo), refer to residual thiosulfate tests. About the washing process, refer to wash.
    Contents

    How it's used

    If the print is not toned, follow the procedure after fixing. If the print is to be toned, fixed print should be washed for 2 minutes (RC) or 5 minutes (baryta) in running water. The washed print should be toned and rinsed, followed by the procedure below.


    Fixed print is rinsed in water, and immersed in washing aid with intermittent agitation. The treated print is washed in running water.
    Washing time (with non-hardening fixer)
    material_____________ water rinse_________wash aid________ final wash

    film ___________________ - _____________ - _________ 5 min
    RC print __________________- ______________ - ______________ 5 min
    Baryta (single weight) 1 min ____________ 2 min ________ 10 min
    Baryta (double weight) 1 min ____________ 3 min_________ 20 min

    The temperature of the final washing water can be between 13C to 30C, or however warm, as long as the emulsion can sustain.

    Without using a washing aid, the washing time required would be at least twice as long, and the washing water would have to be at least 18C. That is, if the temperature of the washing water is low, use wash aid.
    Formula

    A standard washing aid is:
    Washing Aid
    sodium sulfite 15g
    sodium metabisulfite 5g
    water to make 1.0 l
    target pH 6.5 to 8

    When scum is formed on the solution or processed material, addition of about 0.5g/L of disodium or tetrasodium EDTA with about 1.0g/L of sodium citate is suggested. Stock solution including the EDTA and citrate tends to keep longer.

    Kodak Hypo Clearing Agent contains EDTA and sodium citrate to reduce the calcium scum. With hard water, sulfite in the wash aid forms insoluble calcium scum unless EDTA is added. These are also useful in preventing precipitation of aluminium hardener that may be carried over from the fixer (if acid hardening fixer is used).

    Similar product, FUJI QW is sold by Fujifilm. The MSDS indicates that the product is similar to the above: 60-80% sodium sulfite, 16-26% sodium metabisulfite, 1-5% sodium citrate, 2.3% metaboric acid, 1-5% tetrasodium EDTA, 1-5% water.

    (As an environmentally safer alternative, there is Silvergrain Clearwash sold from Digitaltruth. This product is free of EDTA and borates, but it uses other safer agents that are equally effective in producing high quality results without scum formation.)

    Negatives that already have white stain that cannot be removed by water wash may be treated in a solution containing disodium EDTA and sodium citrate for 5 minues and washed in water.

    A simplified version of washing aid is simply 2% solution of sodium sulfite, which has a bit too high on the pH, but works quite effectively, if prepared fresh every day. If the scum problem occurs, it is best to use the above formula with EDTA and citrate added.
    How it works

    Washing aid is usually a 2% sulfite bath, preferrably with the pH buffered to 7 to 8 region. The reason why thiosulfate from fixer takes a long time to wash out is because thiosulfate is tightly adsorbed to silver surface, gelatin matrix, alum hardener (if used), sizing material and the cellulose fiber. The sulfite anion in the washing aid can displace the adsorbed thiosulfate, and the wash water can more effectively wash out adsorbed sulfite as well as desorbed thiosulfate.

    Without washing aid treatment, desorption of thiosulfate is a very slow process, and it also requires certain energy. That is, washing is very inefficient at temperature lower than 18C. However, once treated in washing aid, cold washing water can now wash out sulfite and desorbed thiosulfate as effectively.

    Washing aid has quite large processing capacity. The limiting factor is dilution by water carryover, and contamination by thiosulfate. A quick rinse of prints before washing aid treatment is very helpful in expanding the working capacity.

    Washing aid has shelf life similar to fine grain developers. Sulfite in the solution can be oxidized by air. The oxidized washing aid is better than no treatment, but it is not very effective.
    Discussion
    Washing property of fixer

    Without washing aid, the kind of fixer that is fastest to wash out is:

    * ammonium thiosulfate fixer
    * unexhausted
    * nonhardening
    * absence of acetic acid
    * contains sulfite
    * preferrably pH buffered to 6.5 to 8.5 range

    Why sulfite?

    Besides sulfite, sulfate, bicarbonate were commonly seen in literature reporting on the subject. Bicarbonate is a reasonably effective washing aid. A 2% solution of sodium bicarbonate (baking soda) may be used, although, sulfite bath is preferred whenever possible. Sulfite is superior in that sulfite can desorb not only unreacted thiosulfate but also sparingly soluble and adsorbed argentomonothiosulfate complex.

    Sea water or solution of common salt is occasionally said to have the same effect as washing aid. For practical use, it is advised to ignore this information. While sodium chloride may have very small enhancement in washing compared to distilled water, it is vastly inferior to sulfites. Also, residual chloride salts can be harmful to the image; a very thorough water washing process is required following the sea water bath, negating the purpose of using a washing aid bath.

    Alkaline baths

    There are several studies that investigated the washing accelerating properties of alkaline bath. For example, Crabtree compared sodium hydroxide, ammonium hydroxide, sodium carbonate, Kodalk (sodium metaborate), and distilled water. This result was probably valid at that time, but the factors involved in washing property of fixed film and paper are somewhat different nowadays, because today's emulsions are highly hardened during manufacturing and use of hardener is becoming uncommon in manual processing. Therefore, it's probably not very meaningful to discuss those methods in detail. If there is anything worthy of noting, it is probably that Crabtree found that ammonium hydroxide was most effective of those.

    This may be related to ammonium ion's affinity to silver ion. However, ammonia vapor is quite unplesant to use as a washing aid in practice. It may be interesting to investigate efficacy of organic amine compounds as an additive to the standard sulfite wash aid to see whether such an additive can farther expedite the washing process. However, the magnitude of the efficacy is somewhat questionable when rapid fixer is used, because ammonium ion is carried over from rapid fixer, which is known to wash faster than non-rapid fixer.

    Benefits of residual thiosulfate

    It is known that a trace amount of residual thiosulfate may indeed protect the silver image. When this issue is discussed, the level being talked about is about 10-20 mg per square meter, right about the borderline for archival standard. However, this should not be taken to mean that importance of thorough washing is now dismissed. Excess thiosulfate is known to cause image fading, staining or browning problem. It is best to treat the image in archival toner, and remove residual thiosulfate to the archival standard.

    However, untoned RC prints are easy to overwash if washing aid is used. This author recommends that RC paper should be fixed in non-hardening fixer and washed for 5 minutes in running water, without using washing aid.

    Hypo eliminator

    Hypo eliminator usually refers to a solution containing hydrogen peroxide or peroxide-releasing compound such as percarbonate, perborate or persulfate. These solutions may also contain ammonia, other alkaline agents, bromide, iodide, and other additives.

    The idea behind hypo eliminator is to oxidize thiosulfate to harmless compounds that are not adsorbed by silver surface. However, it was later found that peroxide solutions damage and undermine image-forming metallic silver. Today, the use of hypo eliminators are discouraged.

    Indeed, peroxide is a leading environmental oxidant that attacks image, and because it's a very common cause of image degradation, its fume is used for accelerated testing of image permanence. It's called peroxide fuming test.
     
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  15. Rafal Lukawiecki

    Rafal Lukawiecki Subscriber

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    Thank you, everyone, for your ideas.

    Rudi, it sounds like sodium sulfite is doing more work than just being a "simple" wash-aid which helps to liberate thiosulfate from paper fibres, sizing etc. I have always thought it had to be more active, after all, it is a known grain solvent when used as part of some film developers. Thank you for sharing the suggestions, I share your curiosity as to that it may convert insoluble complexes to soluble ones, or that it may desorb them from the grain crystal. I would love to know if others support this line of thinking.

    Ian, if there is a chance you find that diagram, I'd be interested to learn from it. I am curious about the equilibrium process when sodium sulfite is part of it. Thank you for sharing your other thoughts, too.

    Gerald, this is a very reasonable suggestion, and one which seems to agree with the common sense of today, and one which I follow in my process, wholeheartedly. May I, however, politely ask you, as a chemist, if you have a justification for this otherwise very sensible suggestion, other than based on your feelings and the reason of fixer being not that expensive? For example, would you say that a two-bath fixer regime offered significant advantages to a regime that used only 1 fixer, and which showed through test results that there are no residual thio (silver nitrate test) and no residual silver halides (sodium sulfide test)? For argument's sake, assuming the paper was fixed in a single bath of a neutral ammonium thiosulfate fixer containing no more than between 0.5–2g silver/l, with no significant amount of iodide, for 1 minute, followed by a brief water rinse, and a 3 min 2% fresh sodium sulfite bath, plus a wash of 1 hour, all at 20C (68F)? I can certainly see the safety net that a 2nd fix can provide in case the first one goes significantly over 2g/l, but what other reasons would be there? I am simply trying to find out if the action of sodium sulfite is far more important than we have been assuming it to be.

    John_S, thanks for posting the comments by Ryuji, but I am concerned that his article has been posted here with his permission. If not, would it be better, perhaps, if you posted only a relevant extract, rather than the entire article? Many thanks for your kind help, I do not want to upset anyone.
     
  16. Gerald C Koch

    Gerald C Koch Member

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    Companies like Kodak and Ilford and independant groups have published values for the recommended capacity of their fixers. I leave the testing to them.

    Once again testing has shown that in terms of residual silver a two bath system is better and more economical than a single bath one. I remember an article some years ago in either Modern or Popular Photography which showed this to be true.

    A hypo clearing agent is important only if one is concerned with reducing wash times and water usage. Without one the amount of silver and thiosulfate can still be reduced to a desirable level but at the cost of time and water. The fiixing process is a diffusion controlled process. So the role of sulfite is to cause the paper fibers to release the silver-thiosulfate complex more readily.
     
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  17. Rudeofus

    Rudeofus Subscriber

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    Since I keep seeing numbers for maximum Silver ion content in fixer, is there a simple way of measuring it? Would adding Potassium Iodide to a fixer sample plus filtering plus drying the filter plus weighing it give us a reasonable estimate?

    Unless we have a reasonably easy way of quantifying Silver ion content, any limit numbers are useless.
     
  18. Rafal Lukawiecki

    Rafal Lukawiecki Subscriber

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    Rudi, I have been using silver estimator test strips for many years now. I am being told that they are not very precise, but they have been correlating quite well with sheets or films fixed numbers for me. I can read from 0.5 till 3g relatively easily. They used to be branded by Tetenal, and you can get them from Macherey-Nagel as Ag-Fix test strips, part 907 41. Silverprint sells them, too.
     
  19. Rafal Lukawiecki

    Rafal Lukawiecki Subscriber

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    Thank you, Gerald. May I ask you to clarify, therefore, if this also means that, in your opinion, sodium sulfite does not fulfil any roles in desorbing the silver-thiosulfate complexes from the emulsion, especially those adsorbed to the silver crystal grains (as opposed to just the paper fibres), nor can it aid in converting the less soluble complexes to the more soluble ones, as Doremus stated earlier? I appreciate your opinion on this, very much.
     
  20. Gerald C Koch

    Gerald C Koch Member

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    The silver-thiosulfate complexes are absorbed into the paper fibers and not the silver grains. The washing problem is not associated with the emulsion but with the paper backing. This is why film and RC papers require less washing time than FB papers.

    As was mentioned previously many salts can be used as hypo clearing agents. What is involved is a compeetition for adsoption in the paper between the clearing agent and the thiosulfate ions. By making the clearing agent ions more populous than the thiosulfate ones insures that the thiosulfate ions will be desorbed. There is nothing particularly special about sodium sulfite other than it is somewhat more effective.

    Determining the exact amount of silver in a print would be very expensive. One method would be neutron activation analysis. Classical quantitative methods would not be accurate because of very small amount of silver present.

    There seems to be some confusion as to what acceptable permenance involves. There are two considerations. First is the reduction of silver and second is the reduction of thiosulfate to an acceptable level. The amount of residual silver in a print is determined by the amount of silver in the last fixing bath and the effectiveness of the wash. The amount of thiosulfate is determined only by the effectiveness of the washing of the print.

    The amount of thiosulfate present in a print can be roughly determined by using Kodak HT-1a solution. This involves the amount of decolorization of a potassium permanganate solution. The more thiosulfate the more the test solution is decolorized.
     
  21. Rafal Lukawiecki

    Rafal Lukawiecki Subscriber

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    Gerald, I think I now know why I am confused. Bear with me, please, as I am not a chemist, and I greatly respect your knowledge. You say that silver-thiosulfate complexes are absorbed into the paper fibres and not to the silver grains. However, I was referring to those complexes being adsorbed to the silver grains, based on what Mees writes in "The Theory of the Photographic Process", ed 1, page 515 (my underscore): "In practice, the life of a fixing bath is definitely limited to a silver halide concentration considerably below that required for saturation because of the tendency of the complex silver salts to become adsorbed to the gelatin or silver grains of the image. These adsorbed complexes are not readily washed out and so remain in the film, where they later decompose and cause stain. The tendency for the complex salts to be adsorbed seems to be greater in acid than in neutral baths so that in fixing power acid baths are less efficient than plain solutions of thiosulfate."

    I have found other references mentioning that phenomenon of adsorption to the silver crystal grains. Gudzinowicz says the following in his 1998 "Post Development Processing": "In either case, the first complex (I) is >very insoluble< and remains tightly adsorbed to the surface of the solid silver halide." I have also read about the complexes reforming from the fixer solution back again on the surface of the emulsion (not paper) in this, perhaps a bit tangential, patent by Armstrong: "As the fixing solution becomes older, heavier or more complex, argento salts are formed on the emulsion."

    Based on what I have read, I strongly believe that the silver-thiosulfate complexes, and even the unreacted thiosulfate itself, do bind with the paper sizing, baryta, and even the fibres, especially during a lengthier fixing, in agreement with your statement, even if I may be a bit unclear as to which form of sorption this represents. In that respect, however, I fully follow your line of thinking regarding the role of sodium sulfite as a wash-aid. Nonetheless, I am confused regarding its possible action regarding the desorption of the complexes adsorbed to the silver crystal grains, because the references, which I have quoted, state this phenomenon takes place, however, you say that it does not happen. Is this simply a matter of a difference of learned people's opinions, or could you help me understand and reconcile the apparent contradiction? I realise that I have already taken up a lot of your time, and if this is too much to ask of your patience, then please accept my sincere gratitude for all of your helpful suggestions thus far.
     
  22. john_s

    john_s Subscriber

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    In the pasted article from Ryuji Suzuki's web site a few posts above, he does say "Sulfite is superior in that sulfite can desorb not only unreacted thiosulfate but also sparingly soluble and adsorbed argentomonothiosulfate complex."

    I understand that the complexes are less soluble than thiosulphate and are thus harder to wash out. So if Suzuki is right, the sulphite does in fact help to wash out the complex (at least the mono one), if not "convert" it. I don't know where Suzuki got that information. Maybe it's in one of the research papers from Kodak in the golden age of black and white.

    The other thing that I find interesting is that once the sulphite bath is used, washing can proceed in colder water than would be needed for timely washing without the sulphite bath. This has a practical application for many people whose tap water is rather cool and who don't have a mixer.
     
  23. Gerald C Koch

    Gerald C Koch Member

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    Whether the silver complexes are adsorbed in the emulsion or trapped in the the paper fibers is really immaterial in a practical sense. What is important is the kinetics of the process. The silver complexes in film and RC papers are removed much more quickly with washing than those in FB papers. So what controls the rate of silver removal in FB papers is absorption on the paper base. In any set of kinetic reactions it is the slowest reaction which determines the overall speed.
     
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  24. Bob Carnie

    Bob Carnie Subscriber

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    Here is a question for you gurus.

    Last year I was having problem with my clearing agent and water, I was getting scum (white deposits) on final prints even after proper wash.

    It was suggested here ( thank you) to add Sodium Hexametaphosphate to my Sodium Sulfite.

    the scum problem went away immediately.

    What I notice is when mixing Sodium Hexametaphosphate it hardens like toffee and takes quite awhile to melt away.
    I am using 2 litres of hot water , 10 grams SH and 200 grams Sodium sulfate .

    Then I am adding 8 litres of 70 degree of water to make my working solution.

    Why is the SH Clumping and going to this toffee like state. Am I mixing it too hot?? 120 degrees.
     
  25. Gerald C Koch

    Gerald C Koch Member

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    What you were experiencing was calcium sulfite being precipitated from hard water. Your solution was a correct one and any calcium chelating agent could be used. I would suggest dissolving the sodium hexametaphospahe first in a small amount of hot water and then adding it to the rest of the solution.

    I would also suggest acidifying your clearing bath with sodium bisulfite as in the formula given below.

    You could also use the following formula which is very close to Kodak's Hypo Clearing agent.

    Distilled water (50°C) …………………………………………… 750 ml
    Sodium sulfite (anhy) ……………………………………………… 100 g
    Sodium bisulfite …………………………………………………………… 25.0 g
    EDTA, Na4 ……………………………………………………………………………… 2.0 g
    Sodium citrate ………………………………………………………………… 5.0 g
    Distilled water to make ………………………………………… 1.0 l
     
  26. Rudeofus

    Rudeofus Subscriber

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    Instead of Sodium Metabisulfite and Hexametaphosphate one can simply add Citric Acid to the milky clearing bath. Not only does it form a nice buffer with Sulfite between pH 6 and 8, the Citric Acid also works as chelating agent for all these ions which tend to form white scum with Sulfite or Carbonate, and it dissolves easily in water.

    Instructions for making such a mix are very simple:
    • Prepare working solution of hypo clearing agent with Sodium Sulfite, Sodium Carbonate (or whatever your recipe says) and tap water.
    • The mix will be milky depending on your water hardness. Now add Citric Acid (anhydrous, monohydrate, it doesn't matter) until the working solution clears. Don't add too much Citric Acid or your bath will smell from SO2. If it does, add Sulfite or Carbonate to raise pH back to where it doesn't smell.
    • Done!