[pure-silver] Re: Chemical subsitution
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Fri, 27 Apr 2007 03:20:33 -0700
----- 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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