Re: Sodium Bisulfite question

michael gudzinowicz (ab366@osfn.rhilinet.gov)
Thu, 25 Jan 1996 13:52:43 -0500

Carson Graves <carson@zama.hq.ileaf.com> writes:

> > From: "J. Wayde Allen 303-497-5871" <allen@boulder.nist.gov>
[cut]
> > 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 fixin
> > 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."
[cut]
> 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 :-)

I missed the original thread, but I'll comment anyway. The
common alum hardeners are most effective in acidic conditions
around pH 5. They form coordination complexes crosslinking the
gelatin proteins - if too acidic, the carboxyl groups on the
protein functional groups aren't ionized and the complex
doesn't form; if too basic, the reaction rate is very slow; if
very basic (pH 8+), hardeners are removed. To remove them, a
soak in 0.3% ammonium hydroxide or 2% metaborate or sodium
carbonate for 5 minutes works well.

Sulfite and bisulfite (sulfite at a lower pH in solution) don't
form crosslinks and aren't considered to be hardeners, unlike
chrome and potassium alum, chrome compounds, aldehydes, tanning
developers, etc. The rationale for using bisulfite rather than
2% sodium sulfite in a wash aid is to keep the pH in the 6 to 7
range to avoid removing hardener and to prevent excessive
swelling. One recipe is 26 g sodium bisulfite, 5 g sodium
sulfite and 1 g Calgon per liter (probably from Haist's book,
below.)

The primary reason bisulfite is added to acid fixers is to
prevent the "decomposition" of hypo and formation of sulfur
precipitates. Thiosulfate demonstrates the following
equilibrium which is pH dependent:

S + SO3-- <-> SSO3--
sulfur sulfite thiosulfate

In basic solutions the equilibrium is far to the right
(thiosulfate), but even carbon dioxide absorbtion generates
enough carbonic acid to shift the equilibium to the left,
forming sulfur, which then polymerizes and precipitates.
Excess sulfite is added to the fixer to drive that reaction to
the left via mass action which keeps sulfur levels very low,
even in acidic conditions, and prevents polymerization and
precipitation. For "plain" basic fixers, sodium sulfite is
used; for acid fixers, bisulfite is used to reduce the amounts
of other acids which might need to be added.

I think most of the chemistry is covered in Mees & James'
"Theory of the Photographic Process" and Grant Haist's "Modern
Photographic Processing, Vol. I & II".