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.