[pure-silver] Re: Mixing My Own Fixer
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Sat, 16 Oct 2010 11:20:52 -0700
----- Original Message -----
From: "Gerald Koch" <gerald.koch@xxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Saturday, October 16, 2010 10:13 AM
Subject: [pure-silver] Re: Mixing My Own Fixer
You can buy sodium metaborate from www.thechemistrystore.com
They also sell
several other chemicals of photographic interest.
Jerry
Sodium metaborate (Kodalk) is easy to make in solution
from Borax and sodium hydroxide, both of which are easily
available.
There is no advantage whatever and some disadvantage to
making a fixing bath alkaline. The pH has no effect of the
fixing properties but has some on washing speed if no wash
aid is used. A neutral fixing bath will wash out as fast as
an alkaline one. The pH does have an effect on carried over
developer since it will continue to be active in the fixing
bath until exhausted. The higher the pH of the fixing bath
the more active carried over developer will be. A thorough
rinse is needed betweeen developer and fixer to minimise the
amount of carried over developer. Its important that it be
washed out rapidly because in the absense of sulfite or
other preservative the developer can cause staining. The
fixing bath should have enough sulfite in it to prevent
staining from carried over developer. A "plain" fixing bath
consists of the fixer and enough sulfite to protect it from
oxidation from the air, this is about 5 grams/liter. This is
probably _not_ enough to prevent staining. Acid fixing baths
have around 15 grams/liter and this is probably a better
amount to use for a plain bath even though its being used
for a different purpose, namely to prevent staining rather
than to prevent decomposition of the thiosulfate by the
acid.
The main objection to acid fixers seems to be the odor.
This is mostly from sulfur dioxide produced by the slow
decomposition of the thiosulfate by the acid. It is possible
to minimise the amount of the gas by making the bath less
acid and by the choice of the buffer components.
Fixing baths are made acid for two reasons: 1, to
prevent carried over developer from remaining active, and 2,
to allow the hardener to work. The common hardener used in
fixing baths is white alum which requires the pH to be acid.
If no hardener is used the fixer does not need to be acid
other than to stop the developer. That requires a lot less
acid than the hardener does.
The better control of development and elimination of
staining are probably good enough reasons to retain the
conventional acid stop bath and acid fixer in practice.
Because gelatin is amphoretic it takes on the pH of the
last bath it was in. The use of a buffered sulfite wash aid
establishes the pH of the emulsion at neutral where the
electrical charge conditions do not bind fixer or its
reaction products and the binding due to the mordanting
effect of white alum is eliminated without also eliminating
the hardening. Sulfite also has a specific ion-exchange
property for thiosulfate and its complexes so will have an
accelerating effect even when the fixer is neutral.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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