> The formula contains a small amount of sodium bisulfite, and about that
> Dignan writes: "The Sodium Bisulfite is in the formula only to lower
> the pH to 7 - 7.5 to prevent softening the emulsion of film. If papers
> are to be used, leave out the Bisulfite to deliberately get more
> softening..."
>
> Now, my question is:
>
> 1) Is the above statement correct? (I've confirmed from other sources
> that sodium bisulfite does indeed lower the pH of a solution, but what
> about the "preventing softening"?)
I've been looking in some books for an answer to your question, and it
seems to me that more information about the formula in question is
needed. For instance, in the book: Neblette, C.B., Photography its
Materials and Processes, D. Van Nostrand Company Inc., 1962, pp. 250-252
is a description of the characteristics of fixer and hardening baths.
This book lists that a fixing bath usually contains in addition to a fixing
agent (thiosulfate):
" 1. An organic acid, usually acetic, to provide the necessary acidity to
stop development and create the proper pH for effective hardening.
2. Sodium sulfite, which prevents the decomposition of the thiosulfate
by the acid and forms colorless oxidation products of the developer thus
preventing staining.
3. Alum as a hardening agent."
In particular there is a discussion of hardening using alum as a hardening
agent that states: "The degree of hardening, other things (e.g.,
temperature, alkalinity of the film when placed in the fixing bath, etc.)
being equal, depends on the pH of the solution, which in turn depends on
the relative proportions of acid, sulfite, and alum.".
I'm wondering if maybe in your case the sodium bisulfite is intended as a
substitute for the sodium sulfite.
> 2) If the statement is correct, then how is my inference that sodium
> bisulfite is a hardener incorrect.
It might be that the amount of softening done by the bisulfite is
balanced by the other ingredients in your formula. Hence my original
remark about needing more information.
Hope this helps.
- Wayde Allen
(allen@boulder.nist.gov)