[pure-silver] Re: stopbath kills fixer
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Mon, 24 Jan 2005 16:59:46 -0800
----- Original Message -----
From: "Garry Lewis" <garrylewis747@xxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, January 24, 2005 8:26 AM
Subject: [pure-silver] stopbath kills fixer
> The latest "anti-stopbath" reason doing the rounds is that
> adding small
> amounts ofacetic acid will eventually kill your alkaline
> fixer. Will it. The
> only reference I've found only warns against Potassium
> Metabisulfite based
> stopbaths. Enquiring minds want to know.
>
> Garry D. Lewis
> Maybe, as so many forums demonstrate, we should care less
> about numbers and
> more about results.
>
I find something like 18 responces (replys?) to this but
no answer.
Fixing baths work independantly of the pH, at least over
a reasonable range. The acid in acid fixing baths is there
mainly to allow the particular kind of hardener usually used
in hardening fixers to work.
Acid can cause decomposition of thiosulfate, that is why
acid fixing baths have sodium sulfite in them. The sulfite
prevents the decomposition, at least for a fairly long
period. Sulfite is necessary anyway to prevent staining from
the reaction products of any carried over developer. For an
acid fixing bath the usual amount of sulfite is about 15
grams per liter of working solution. For a non acid bath as
little as 5 grams per liter is enough for washed materials
but may not be enough to cope with much carried over acid.
There may be some confusion here with the effects of
citric acid stop baths on hardening fixing baths. Citric
acid is a sequestering agent for aluminum compounds. If much
citric acid gets into a hardening fixing bath using alum
hardener it can interfere with the hardening action. This is
irrevelevant for non-hardening baths.
There is no good reason for using an alkaline fixing
bath. If one is dealing with a process where acid may
destroy a dye or pigment, as in color processing, a neutral
fixing bath is sufficient. Supposedly, acid fixing bathes
and acid stop bath destroy part of the stain image from Pyro
developers. I haven't tried this nor have I seen any proper
densitometric data proving it, so I don't know if its true.
If it is then a plain water wash must be used in place of
the stop bath and a neutral pH fixing bath should be used.
Washing: The idea that alkaline or neutral fixers wash
out faster than acid ones is true but only if a sulfite wash
aid is not used. The gelatin of the emulsion takes on the pH
of the last bath its been in. If the pH is adjusted to a
point above (on the alkaline side) of the isoelectric point
of the gelatin (slightly acid for most photographic gelatin)
the electrical charges which tend to bind thiosulfate to the
gelatin when acid will be reversed so that the gelatin will
tend to repel these ions. Any alkali will suffice but
sulfite has the further property of being an ion exchanger
for thiosulfate so it accelerates washing much more than,
say, carbonate or borax.
If an acid hardening fixing bath is used followed by a
buffered sulfite wash aid, such as Kodak Hypo Clearing
Agent, the rate of washing will be independant of the pH of
the fixing bath used and will be about the same for a fixing
bath employing white alum as a hardener.
Purpose of Acid Stop Bath: The acid stop bath serves to
stop development very rapidly. Also, when an acid fixing
bath is used, the acid stop bath serves to prevent loss of
acidification of the bath by carried over alkali from the
developer. Most acid fixing baths are heavily buffered so
will not be affected by a reasonable amount of carried over
developer. The acid in both stop bath and fixing bath also
prevents any carried over developer from continuing to be
active in either bath. The allows more exact control of the
degree of development and also prevents the generation of
developer reaction products in an envirionment where there
is no sulfite to prevent them from causing stains. Also, the
presence of active developer in a fixing bath can cause
dichroic fog, which is a deposit of very fine metallic
silver on the surface of the film or paper.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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