Re: Sodium Bisulfite question
Carson Graves x4692 3NE (carson@zama.hq.ileaf.com)
Wed, 24 Jan 96 14:45:22 EST
> From: "J. Wayde Allen 303-497-5871" <allen@boulder.nist.gov>
>
> On Wed, 17 Jan 1996, Carson Graves x4692 3NE wrote:
>
> > 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.".
Thank you for your research. I actually had looked at Neblette, but
apparently not hard enough to pick up on this tidbit. It is starting
to make a little sense to me.
>
> I'm wondering if maybe in your case the sodium bisulfite is intended as a
> substitute for the sodium sulfite.
Actually not. The formula I was referring to is a washing aid that is
composed almost entirely of sodium sulfite (a 20% solution) with a
small amount of sodium bisulfite added (in Dignan's words) "to prevent
softening the emulsion."
What I gather from the Neblette excerpt you quote is that the increased
acidity caused by the sodium bisulfite inhibits the softening effect of
the sodium sulfite. The question this raises for me is to what degree
does this make sodium bisulfite a hardener? Apparently, it doesn't -
(for example, as Sandy pointed out, it is a component in Kodak's
F-24 non-hardening fix formula), but it does seem to indicate that it
is some sort of "anti-softener" due to its ability to lower the pH of
a solution.
So, any chemistry guru's out there want to comment? The fate of a footnote
in an appendix of the forthcoming 2nd edition of my B&W printng book hangs
in the balance :-)
>
> > 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.
Absolutely. Thanks for taking the time.
Carson Graves
carson@ileaf.com