[pure-silver] Re: Chemical subsitution
----- 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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