[pure-silver] Re: Chemists: WHY Is It 28%
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Sun, 26 Oct 2014 10:32:43 -0700
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Sent: Sunday, October 26, 2014 8:57 AM
Subject: [pure-silver] Chemists: WHY Is It 28%
Since the beginning of time as a photographer, I've used
the advised 1.5oz of 28% Acetic Acid per
quart of working stop bath. Now, I realize this is not a
super critical thing, it just has to
stop any further development, but I am curious: Who came
up with this exact formulation, and more
importantly, *why* is this particular dilution right?
These are very good questions and I am not sure I've
ever seen an answer in the literature. At least up through
the 1940s it was common practice to use a plain water rinse.
The use of an acid stop bath became popular to stop
development at a predictable point and also to deliver the
film or paper to the fixing bath in an acid condition. Now,
most fixing bath formulas are compounded to compensate for
some alkaline carryover from the developer. The standard
Kodak formula contains a buffer to maintain the acidity. Of
course, the acid in the fixer also acts to stop development
but there is a limited reserve so its good practice to
neutralize the alkaline from the developer before going to
the fixing bath. The acid in the fixing bath is needed only
when a non-organic hardener is used. Both white alum and
chrome alum (which has no aluminum in it) require an acid
environment to work. If the hardening action is not needed
or desired no acid is needed in the fixing bath. If there is
a reasonable amount of sulfite in the fixing bath it will
prevent staining from carried over developer that continues
to develop for a time.
I think the concentration of acid in the fixing bath is
determined mainly by the expected alkalinity of the
developers in general use and to limit the amount of
swelling of the emulsion. Gelatin has no characteristic pH,
rather it has an isoelectric point. This is defined as the
pH at which the gelatin has the minimum of swelling. For
most photographic gelatin its slightly on the acid side of
neutral. When made more alkaline or more acid than the
isoelectric point the gelatin swells. The main purpose of
hardener is to minimize the amount of swelling.
Note that acids other than acetic acid may be used as
stop baths. It is common now to use citric acid because it
does not produce the odor associated with acetic acid.
However, if much citric is carried over into a conventional
fixing bath it will interfere with the hardener since citric
acid is a sequestering agent for the aluminum compound used
for hardening. This is of no consequence when hardening is
not used. Also, some non-organic acids such as sodium
metabisulfite can be used and work fine. During World War
Two there were many formulas published for using substitutes
for acetic acid was a strategic material.
FWIW, Kodak and others recommended stronger stop baths
for use with process materials that were developed in
solutions with much higher pH that the usual pictorial
developer.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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