[pure-silver] Fixer Hydration
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Tue, 25 Jan 2005 17:41:14 -0800
Sodium Thiosulfite comes as crystals or in anhydrous form
(sometimes called dessicated). Older formulas generally
specify crystaline thiosulfate but the anhydrous form is
used for packaged fixers because of its light weight and
because it will dissolve in luke-warm water. Crystaline hypo
is strongly endothermic so one must start with very hot
water to get it into solution. In fact, crystaline hypo
going into solution was a common chem lab trick to form a
cooling bath when ice was not avialable. Anhydrous
thiosulfate is exothermic but has little heat of solution.
It will dissolve readily in water at around 80F.
At one time (perhaps still) there was a "Photo" grade of
chemicals. These were not necessarily exceptionally pure but
were guarateed not to contain impurities known to affect
photographic processing. Photography does not always require
very pure chemicals but does require that certain
contaminents not be present. Often commercial grade
chemicals will do quite well and are cheap. Reagent grade
(called something else now I think) are always pure enough
for photo work but they come with an assay of impurities
which makes them expensive. Note that they may not be
outstandingly pure, its that the impurities are known and
listed.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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