Peter Marshall (petermarshall@cix.co.uk)
Mon, 15 Feb 1999 21:44 +0000 (GMT)
Sil
I hope I can make a couple of small comments on your very sensible post:
> Its main use is as a preservative - it
> combines with any loose oxygen that is in or gets in the water, and
> prevents the developing agent from oxidizing. Only a little is used in
> color developers because oxidation of the developing agent is what
> combines
> with dye intermediates in the film to form the colors. It is also used
Of course it isn't the oxidation of the developing agent by oxygen that is
used to create the colours, but its chemical reaction with the silver
halide in the development process. This is also a redox
(oxidation/reduction) reaction, the silver halide being reduced to silver
and the developer oxidised to the oxidation product that then reacts with
the colour coupler present in the film to give the appropriate dye.
Oxidation of the developer by other means - such as reaction with oxygen
- would result in overall colour fogging and is thus undesirable.
> in
> washing aids because it's readily available; strangely, sodium chloride
> will do the same job, but is not used because it may react with some of
> the
> other ingredients. Sulfite is safe.)
Sea water was one of the first widely used washing aids. Of course if any
washing agent containing sodium chloride is used before all the silver
complexes have been washed out, solid silver chloride will be deposited,
which defeats the whole object of fixing.
Connecting with another thread (with apologies to Bob M), urine would
possibly be a reasonable washing agent. Chemists will also know that it
was also one of the major raw materials in the traditional production of
alum, at one time the major industry of the NE of England, for which
purpose it was collected on an industrial scale from the major cities of
that area. Alum is occasionally used in some alt processes.
Peter Marshall
On Fixing Shadows and elsewhere:
http://www.people.virginia.edu/~ds8s
Family Pictures, German Indications, London demonstrations &
The Buildings of London etc: http://www.spelthorne.ac.uk/pm/
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