Re: Sodium Bisulfite

Here's the math:

Formula weights are potassium metabisulfite=222, sodium metabisulfite=190,
sodium bisulfite=104. Remember that you get two bisulfites from every
metabisulfite. There was no listing for potassium bisulfite.

A 5% solution of sodium bisulfite=0.48M The equivalent molar concentrations
are 5.33% potassium metabisulfite and 4.56% bisulfite. All are close enough
for 5% to work in clearing baths for most situations.

The potassium salts are listed as pharmaceutical antioxidants, and the
potassium salt as the food additive, and wine preservative (there may be a
cost-effective opportunity for bulk purchases here). The metabisulfites are
listed as "freely soluble" in water. The sodium bisulfite is listed as
soluble in 3.5 parts water (~30% solution). pH may affect the ability to
dissolve.

All are listed as giving off a sulfur dioxide odor. They pick up water from
the air, and oxidize to sulfate (and are then ineffective). Keep them dry
and tightly closed. As far as the smell of a solution goes, check the pH.
A solution is acidic. The acid form is volatile, and therefore smelly. Get
some test strips or an inexpensive meter. I don't know the ideal pH. With
bulk and technical grades, this may be the source for differences from type
and batch differences. Remember, the difference between a bisulfite
solution, and a sulfite solution (as in developer) is just the pH.

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Wednesday, August 25, 2004 1:51 PM
Subject: Re: Sodium Bisulfite

> Katharine Thayer wrote:
> >
>
> >
> > But the problem is that it's sodium bisulfite and *potassium*
> > metabisulfite that are at issue in this discussion, so how similar are
> > they? My chemist consultant says that he doesn't see a role for the
> > potassium and sodium, so as far as he's concerned they are spectator
> > ions in the reduction. If this is correct, then it's only the sulfite
> > that matters, and metabisulfite should perform about the same regardless
> > of whether it came into solution along with potassium ions or sodium
> > ions. If that's so, then why would we expect big differences in how they
> > perform in the clearing and washing process?
> >
>
> I just re-read my chemist's message, and found that the conclusion I
> came to by reasoning my way through a logical sequence, he actually
> said straight out to start with: "I wouldn't think there would be any
> difference between sodium and potassium
> metabisulfites at the same molar concentration." So maybe I should just
> read my mail more carefully and give my brain a rest. Any chemists
> disagree with this, please say so, and support your answer. Thanks,
>
> kt
>
Received on Wed Aug 25 22:38:11 2004