Dan: Am I wrong in thinking that today's film emulsions
increasingly use silver iodide? If so, does that mean
that sodium thiosulphate is a better fixer for
modern emulsions? Bob H
As for the amounts of silver iodide in today's emulsions over
years ago, I think Ron has answered that question. I seem to
recall some to-do over the Delta and T films when brought to
market around 20 years ago. Particular note was taken of
their iodide content. I'm quite sure there must be some
update versions of older emulsions which do not have
significantly increased iodide content. Certainly not
the increase seen in the T grain films.
Sodium has no affinity for silver so takes no part in the
removing of silver. Ammonia in solution does have some
affinity. I've read of it being used alone as a fixer in the
case of chlorided only silver emulsions. The chloride of
silver is the most soluble of the silver halides.
At the other extreme is silver iodide. It and silver sulfide
are two most insoluble salts of silver. The ammonium ion
has nearly no effect upon the iodide while the thiosulphate
ion with it's greater affinity for silver will, in some surplus,
clear the silver from an emulsion.
In all cases, chloride, bromide, or iodide, the ammonium
ion acts as a fixer; the more so with the chloride and least
so with the iodide. Temporary though is it's attachment to
silver because the thiosulfate ion, with it's much greater
affinity for silver, 'steals' the silver from the ammonium
ion. Both the ammonium and thiosulphate ions are
active in attaching silver where rapid fixers are
used. So Ron, on a technicality, is correct.
Rapid fixers are faster in all cases. Dan