[pure-silver] Re: Chemical subsitution

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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