[pure-silver] Re: Chemical subsitution

Theoretically,

2 molecules of sodium bisulphite are needed to equal 1 molecule of sodium metabisulphite.

Theoretically,

208.12gram of sodium bisulphite equals 190.10gram metabisulfite. By "equals" I mean that in solution they are the same.

If you really are fussy, and if you really know that formulas are really making a distinction between pure bisulphite and pure metabisulphite (very unlikely) and if you know that your bisulphite is pure (very unlikely) then use 0.913gram of the metabisulphite to be equal to 1gram of bisulphite.

The difference is accounted for by the metabisulphite needing just a little more water to make up the required volume.

As has been pointed out, in photography the two terms are pretty much interchangeable, so making a correction is somewhat arbitrary.  In fact, if you look at the web pages of chemical suppliers to labs and industry, they often comment that their chemical is a mixture of the two.  And it is sensitive to water in the atmosphere which will alter the proportions, I suppose.

Sorry about the antipodean spelling, but "sulfite" just looks wrong to me, although I can understand the advantages.

John Stockdale
==========================
At 01:58 AM  28/04/2007, you wrote:
Thanks again. 
Back when I was at RIT, (when dinosours roamed the
earth), I did a project where I mixed D-72 5 times,
each mix missing a different one of the 5 components
to demonstrate the effect of each when printing.
Without sulfite the developer worked poorly and was
dark brown showing the importance of it preservative
aspects.

So here's a follow up question.  Does it matter which
one I buy now?  For formulas calling for bi or meta,
is it "better" to have these exact compounds?  Just
wondering if I should stock them both.
Eric
--- Richard Knoppow <dickburk@xxxxxxxxxxxxx> wrote:

>
> ----- Original Message -----
> From: "Eric Nelson" <emanmb@xxxxxxxxx>
> To: <pure-silver@xxxxxxxxxxxxx>
> Sent: Thursday, April 26, 2007 6:25 PM
> Subject: [pure-silver] Chemical subsitution
>
>
> >I have discovered that I am out of sodium bisulfite
> > and need to make some HCA tonight for prints I
> just
> > finished.
> > I have tons of sodium metabisulfite and have
> looked
> > through the archives to see if this compound is
> > specifically addressed as an AKA or substitute
> > compound for sodium bisulfite.
> > I've included some quotes below of Richard's that
> seem
> > to answer this question, so I'm wondering if there
> is
> > a definitive answer out there as to whether I can
> use
> > metabisulfite in place of bisulfite gram for gram
> in
> > the HCA formula or at what ratio it would need to
> be
> > at.
> >
> > I'm a cook not a chef so molar aspects of these
> > compounds and the Avogadro's number will go over
> my
> > head. ;-)
> > And what the heck is bisulfite lye?  Found that
> > mentioned in one of my search results.
> > Eric
>
>      I am not sure where I got this originally but I
> found
> this searching rec.photo.darkroom using Google
> groups
> search.
> Hypo clearing agent is mainly a mixture of sodium
> sulfite
> and sodium
> metabisulfite. The metabisulfite behaves very much
> like
> sodium bisulfite (as the
> Merck index says, "The bisulfite of commerce
> consists
> chiefly of sodium
> metabisulfite, Na2S2O5, and for all practical
> purposes
> possesses the same
> properties as the  true bisulfite.") (When sodium
> metabisulfite is hydrolyzed
> (reacts with water) it changes into bisulfite.)
>
>
> Anyway, sodium sulfite and bisulfite are related
> compounds
> with different
> characteristic pHs (add acid to sodium sulfite and
> you
> create the equivalent of
> a solution of sodium bisulfite.) They're put
> together to
> produce a buffered
> solution. (A buffer is a chemical that resists
> changes in pH
> when acids or
> alkalines are added. In the case of a
> sulfite/bisulfite
> buffer, adding acid just
> creates more bisulfite while adding alkaline creates
> more
> sulfite without
> appreciably changing the pH.) Sulfites, as you
> probably know
> from their use in
> developers as preservatives (not as silver
> solvents), tend
> to oxidize fairly
> easily to form sulfates. (This is also why they were
> used on
> lettuce in salad
> bars to prevent the lettuce from turning brown due
> to air
> oxidation.) So left
> exposed to air, the hypo clearing agent slowly
> changes from
> a solution of
> sulfites and bisulfites to a solution of sulfates
> and
> bisulfates.
>
>
> I should add a note in regard to tap water versus
> distilled
> water for making
> prepared chemicals. Prepared chemicals are intended
> to
> behave more or less the
> same way whether you mix them with New York City tap
> water
> or Tulsa water. They
> do this by adding pH buffers (of course hypo
> clearing agent
> is by nature a
> buffered solution anyway), ion strength buffers
> (when this
> is important), and
> chelating agents to bind-up unwanted metal ions such
> as
> calcium, magnesium (both
> found in hard water), iron, etc. In the case of Hypo
>
> Clearing Agent, Kodak has
> added a small amount of sodium citrate and
> tetra-sodium EDTA
> to bind-up unwanted
> metal ions effectively removing them from
> participating in
> the mixed solution.
> The original formulas for HCA used sodium
> hexametaphosphate
> instead (commonly
> called "Calgon"), but of course phosphates are a
> serious
> pollutant causing the
> death of bodies of water by promoting algae growth.
> You'll
> find similar
> differences between D-72 and Dektol. D-72 mixed from
> scratch
> needs to be mixed
> with "pure" water (in the research labs it was DI or
>
> distilled) wheras Dektol
> (same basic formulation with additions to compensate
> for tap
> water mixing.) F-5
> fixer and Kodak Fixer are a similar pair.
>
>
> -Doug
> Douglas Nishimura
> Research Scientist
> Image Permanence Institute
>
>
>
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk@xxxxxxxxxxxxx
>
>
>
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