Matt, Richard, Doremus, thank you for putting your time into sharing your thoughts with me.
Doremus, you make good points about your process, which clearly works well for you. I am inquiring specifically about making improvements to mine, which is, at the moment, very similar to your current process, yet which I wish to improve by taking advantage of the more recently available knowledge, and the collective wisdom of the community. I hope you will not be offended, however, if I politely question two points, which you have made, and which I think were the core of my inquiry, ie. re-evaluation of the use of sodium sulfite, and the presence of thiosulfate in KRST. I look forward to your, or other chemists' feedback, as I am not a chemist myself, although I do have an appreciation of the basic photochemical concepts.
A wash aid is not going to increase the capacity of any fixer. It will speed up washing but it won't chemically change any insoluble silver thiosulfate compounds. You need adequate fixing in fresh fixer to achieve complete fixing. If you want to use single bath fixing and still have optimum permanence, Richard's ten 8x10s per liter is the correct capacity (according to Ilford and others).
Ilford states:
"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
this document, page 3, right-hand column, point 3. This is in contrast to what they say is the capacity
without a wash aid:
"Fix only a few prints before replacing the fixing bath (approximately 10 8x10 inches prints)."
Similarly, you will find an extensive comparison of fixing and the fixer capacities, with and without the use of a washing aid, in
this publication of Digital Truth, which is very similar to Ryuji Suzuki's findings, and on the Pure Silver mailing list. That document states:
"Note that the processing capacity of fixer is considerably lower if fiber prints are processed without using a washing aid" and the capacity numbers show between
twice and
four times larger capacities of the fixer when a wash aid is part of the process. I think I saw a Kodak reference, similar to those two, but I cannot find it anymore.
So, why is the use of a wash aid, such as a 2% solution of sodium sulfite, able to increase the capacity of the fixer to the level almost identical to that achieved when using two-bath fixing? Based on what I have read it has to do with the removal of the silver thiosulfate compounds, as there is very little silver halide left in the emulsion of the paper when fixing even for a shorter time with a non-fresh fixer, but one that isn't exhausted, yet. I was re-reading Mees today (chapter 13, Fixation and Washing), while preparing to write this post, and I am always amazed by the complexity of the seemingly simple fixation process. However, I can surmise that two things are relevant to my question: as long as we are using fixer that is no longer fresh, but not exceeding silver levels of 2g/litre (so about the capacity of 40 8x10 sheets in ammonium thiosulfate 12% solution, such as a typical rapid fixer), there will be a build up of argentothiosulfates, or if you prefer silver thiosulfate compounds, including those that Mees refers to as salts of the argentothiosulfuric acids (page 512 edition 1 of The Theory of the Photographic Process), or what
Gudzinowicz calls monoargentomonothiosulate, monoargentodithiosulfate, and monoargentotrithiosulfate, while describing their increasing levels of solubility. From what I read, those compounds are likely to migrate from the emulsion to the solution only when a fresh fixer is used, less than 0.5g silver/l (up to about 10 sheet 8x10 per l, which you mention), or when the print is moved to the second, such fresh fix bath. A fixer than is more exhausted will cause some of those compounds,
including the more soluble ones, to adsorb to the silver grains, and also to the other layers in fibre paper, especially if fixation is anything but very brief.
However, what complicates this picture is the fact that it is even the more soluble argentothiosulfates will also form
anew on the surface of the emulsion, if the fixer is sufficiently unfresh, as mentioned in
this patent by Armstrong, in which he describes the efficacy of wash water being increased by the presence of certain levels of impurities, due to ionic exchange. This is also discussed in the 9th ed of "The Manual of Photography" by Jacobson, page 294:
"In the presence of a high concentration of soluble silver, or low concentration of free thiosulphate, when the fixing solution is nearing exhaustion, the complex sodium argentothiosulphates may become adsorbed,or "mordanted", to the image. These are difficult to remove by washing." Notice the comment "difficult" rather than "impossible".
Now we come to sodium sulfite. Not much I could find, but there are references to it acting this way:
"sulfite can desorb not only unreacted thiosulfate but also sparingly soluble and adsorbed argentomonothiosulfate complex" (arguably not the most reliable source,
Photography Wiki). I can see references in David Locker's
paper, but I do not have access to it, and Wiley quotes a low price of just $12,346 for a one-time access. However, since Ilford, and others, are quoting this action of sodium sulfite, there must be a reason, and I would love to know what it is, or if something else is at play, that increases the effective capacity of the fixer, while maintaining archival/optimum permanence.
The amount of thiosulfate in KRST does not make it a fixer, nor does the toning bath function as a fixer. Plus, if you try to use your toner as a second fixing bath, you'll have to toss it when the fixing capacity is reached, well before the capacity of the toner is reached. Uneconomical and environmentally irresponsible. (Do a search for some of my posts on replenishing and reusing selenium toner so you won't ever have to toss it.)
Doremus, I hope I did not make it sounds like I was ever suggesting that KRST is a fixer. If I did, please accept my deep apologies for not making myself clear: indeed, KRST is a well-known selenium toner, and
not a fixer. If one were to use it on a not fully-fixed sheet, staining would result, as selenium would react with silver halides.
I am merely curious, however, if the presence of 3-5% of fresh ammonium thiosulfate in a working strength KRST (it holds 25-30% in stock according to
Dead Link Removed), which is almost the same as a paper-strength rapid fixer, would have a positive effect on the process of desorbing of the complex argentothiosulfates. I hope you will agree this is a valid question to ask, and I would be surprised if no one has thought of it before. I realise, however, that the action of the fixer that is responsible for the conversion of the silver halides to argentothiosulfates is easily retarded by the presence of other ions, such as chloride, bromide, and especially iodide, but I have
no idea if selenium would have this impact on the desorbing function. Perhaps you could help me in this.
I very much appreciate your concerns for the environment, and I have, for many years, read your posts here and on the other forum. Perhaps you may have noticed my posts about recycling and environmentally sound removal of photographic chemistry, as it is a subject of great importance to me, and to the place where I live. I trust you see that there is an environmental aspect to my quest in process optimisation: removing a fixing stage and replacing it with something many of us do anyway (wash aid) will cut down on disposed fixer on every 2-stage final rotation, regardless of what is done with the toner.
I look forward to your comments, and I apologise to all chemists for any shortcomings in my post.