Thank you, everyone, for your ideas.

Quote Originally Posted by Rudeofus View Post
  • 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.
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.

Quote Originally Posted by Ian Grant View Post
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.
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.

Quote Originally Posted by Gerald C Koch View Post
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.
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 0.52g 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.