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.