>My last organic chemistry course was over 30 years ago so maybe the
>chemist in the group should jump in here but I understand that EDTA is a
>sequestering agent which I assume retards the oxidizing of the developer.
>Some of the premixed PMKs come with EDTA already in the developer
EDTA is a chelating agent; i.e., it minimizes reaction between ions. The
major need for it in developers is to prevent interaction of calcium (and
heavy metal) salts with sulfite (and other ions which form insoluble
precipitates with calcium and heavy metals) so it is actually not needed
(nor desireable) when truly distilled water is used. (I say "truly" because
commercial distilled water available in grocery stores can have traces of
undesireable ions such as copper.) Though normally it doesn't hurt anything
if you use EDTA, the most common compound (Tetrasodium EDTA) is alkaline
and this must be compensated for in any pH sensitive solution. It is
definitely not an anti-oxidant.
Premixed developers contain EDTA because the mfrs consider you will be
using tap water and that you will be unhappy if you see a precipitate at
the bottom of the bottle. As far as I know, it has no effect on the
developing action (other than the small amount of alkali it introduces).
<<sil>>
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<webmaster@psa-photo.org>
Sil Horwitz, FPSA
Technical Editor, PSA Journal
check out <http://www.psa-photo.org>