Re: Sodium Bisulfite

You're right

Bisulfite, metabisulfite, and sulfite in solution are essentially
equivalent, except for the pH of the solution, which can be controlled.
sodium or potassium forms should not have an impact on colloid processes.

As far as hardening goes, pH may have a impact, or it may not. Gum Arabic
is a long complex sugar, whereas gelatin is a protein. With proteins, acids
harden and alkalis reverse the process. Think stop bath, and a carbonate
"dehardener." Carbohydrates sometimes follow this, but not all the time.

It does sound like you got some bad or old stuff.

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Tuesday, October 05, 2004 5:15 AM
Subject: Re: Sodium Bisulfite

> This goes clear back to a discussion we were having in August about
> whether there is a difference between sodium bisulfite and potassium
> metabisulfite. I thought I had seen a difference between sodium
> bisulfite and sodium metabisulfite, but later came to the conclusion,
> (quoted below) that the batch of sodium metabisulfite I had made my
> observations on may not have not a good batch, it probably had got some
> water in it.
>
> More recently, I needed more sodium bisulfite and while ordering it, I
> ordered some potassium metabisulfite, just for a comparison. I could see
> no difference between the potassium metabisulfite and the sodium
> bisulfite, either in strength of odor or in clearing effectiveness at
> the same concentration. This seems to support my earlier belief that
> there was something wrong with the sodium metabisulfite I had used
> before, (Na or K is irrelevant) and also supports the statements from
> chemists in that thread saying that the different forms of bisulfite are
> functionally equivalent. I didn't test whether there was a difference in
> how much they softened the gum, but given that chemists seem to agree
> that all the bisulfites are essentially the same functionally, I would
> suspect that the belief that sodium bisulfite softens the gum more than
> potassium metabisulfite is just another of those myths.
>
> Katharine Thayer
>
>
>
> Katharine Thayer wrote:
> >
> > So here's what I understand from the chemists:
> >
> > All the sulfites are basically the same functionally, and should work
> > the same at roughly the same concentration, and should smell the same,
> > unless the pH is different.
> >
> > My experience with sodium metabisulfite is based on one batch only; I
> > found it more difficult to dissolve than sodium bisulfite because of the
> > "plates" or wafers that it formed, and it was ineffective at 5%; I had
> > to mix it at many times 5% to get any clearing effect. (And "freely
> > soluble" seems right; I put the entire 100 g into maybe 500 ml water.)
> > And while its smell was similar in quality to that of sodium bisulfite,
> > it differed in intensity; the fumes didn't knock me over and make me
> > cough like the fumes of sodium bisulfite. So what is the meaning of all
> > that, if the two things should be equivalent? The dry material came in a
> > sealed plastic bag and I kept it sealed until I mixed it, but I'm
> > beginning to think, on the basis of what I'm hearing, that there was
> > something wrong with my batch.
> >
> > Specifically, I'm thinking that perhaps some water got into the material
> > (the way the stuff was in wafers instead of powder sort of suggests
> > that) and rendered it less effective by oxidizing it to sulfate. (I
> > haven't had that happen with sodium bisulfite). So I'm retracting any
> > statement I've made here about the difference I've observed between the
> > sodium bisulfite and metabisulfite; I think that observed difference was
> > probably spurious and not useful to the discussion.
> >
>
Received on Tue Oct 5 21:42:06 2004