Kodak Hypo Clearing Agent is mostly buffered sodium sulfite
(sulfite/bisulfite), with EDTA added to act as a sequestering agent for
hardness in tap water.
Since almost any electrolyte will act as a "hypo clearing agent", it's
interesting to note that original research resulted from the use of sea
water (mostly sodium chloride solution) for this use. Sulfite is better, but
salt water will work. The idea is to displace the sodium thiosulfate (hypo)
from the interstices of the gelatin with a non-interactive, very soluble
compound which can be more readily washed out of the spaces. (Oh? You didn't
know gelatin is a colloid?)
While I have your attention (and it has been bandied about in this forum)
keep in mind always that the ammonium radical (unlike sodium and potassium)
is not a metal, but is a combination of one part nitrogen and four parts
hydrogen. It is completely unstable in alkaline solutions (unless a suitable
buffer is present), breaking down into ammonia (NH3) and hydrogen (H). This
is not good. Also, most ammonium compounds are unstable in the presence of
heat (or even compression), at which time the nitrogen and hydrogen
dissociate, resulting in some quite explosive compounds (as witness the use
of ammonium nitrate in some of the recent bombing incidents). Ammonium
dichromate was used at one time in pyrotechnical "snakes" due to its ability
to expand upon the application of heat (I won't go into this further). So my
personal preference is to steer away from ammonium compounds wherever
possible, and never (if I can help it) in an alkaline or hot environment.
(Not pertinent to this discussion, but an explosive that can be detonated -
when dry - with a feather is made of ammonium hydroxide and iodine,
resulting in nitrogen iodide, a very unstable substance. So you see, the
nitrogen and hydrogen can react independently in many situations. That's
probably why the "dangerous chemical" lists show the ammoniums as a bit more
dangerous than the similar sodium and potassium compounds.) Class dismissed.
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net