U of S | Mailing List Archive | alt-photo-process-l | Re: Dichromate Shelf Life Mixed

Re: Dichromate Shelf Life Mixed

Quite surprising to see unnecessary generalization here... where I
remember seeing strong cautions against generalizations.

When preparing solution of a chemical X, the shelf life of the solution
may be limited by a different factor depending on how X is deteriorated.
If X deteriorate due to external factor, such as air, that
generalization may very well apply most of the time. However, some
chemicals degrade by self-decomposition of X (possibly involving water),
or reaction among molecules of X. When multiple compounds (X, Y, Z, ...)
are dissolved in a single solution, reactions among the solutes (X, Y,
Z, ...) are also possible. For example, sulfamic acid sometimes used in
alt-process decompose in solution, and this decomposition is faster in
stronger solution (and at higher temp). Compounds that polymerize are
another case. Acid fixers decompose due to interaction of acid and
thiosulfate and this is also faster when the solution is more
concentrated. (Although in the last case presence of bisulfite ion
prevents the decomposition.)

On Wed, 06 Jun 2007 14:29:08 -0400 (EDT), "Judy Seigel"
<jseigel@panix.com> said:
> But general rule of thumb for all solutions:  a saturated solution, or 
> just relatively strong, can be expected to keep better than a much
> diluted 
> solution.
Ryuji Suzuki