Quote Originally Posted by AgX View Post
A series of complexes is formed, only a later one will be watersoluable. That is why time is of importance.
Yest, the fixing time should be as short as possible and there is also the need for a second fixing bath so that Ag(S2O3)- which is insoluble turns into Ag(S2O3)2- and Ag(S2O3)3- which are soluble.

I also found an answer by Richard Knoppow that I would like to share here (it tells what Ian and PE said already):

Ammonium thiosulfate fixers are less sensitive to soluble iodide
from fast emulsions. Sodium thiosulfate fixing baths are slowed down
by accumulated iodide but continue to fix the emulsion, they simply
need more time.
The use of a two bath fixing system eliminates to a great extent the
problem of accumulated iodide since nearly all of it comes out in the
first bath, leaving the second bath working at full speed.
Two bath fixing has the same advantage for rapid fixers that it does
for sodium fixers, namely the capacity for archival fixing is extended
by four to ten times and the relatively fresh second bath assures
complete solublization of the silver complexes.

The rapid fixers made by adding ammonia salt to a sodium fixer are
not quite the same as rapid fixers made with ammonium thiosulfate.
They are not quite as rapid.

As far as what types of films and paper have silver iodide in them,
all reasonably fast films are made with silver iodide. That means even
slow pictorial films like the late, lamented Agfa Agfapan-25. Faster
films, like Tri-X have an abundance of iodide, and the newer tabular
grain films also have a lot. Kodak recommends longer fixing times for
Tri-X as they do for T-Max films. Both can be fixed in sodium
thiosulfate fixers, provided enough time is alloted for complete
fixing.
The old rule of thumb is to allow double the clearing time for
complete fixing. You will find the clearing time for T-Max nearly
twice that for a film like Plus-X or Verichrome Pan. Some recommend
fixing T-Max for three times the clearing time. I think this is
unnecessary when a two bath fixer is used.

Its possible that some paper has silver iodide in it but I don't
know specifically which ones. Traditionally, paper is made with silver
chloride, silver bromide, or a mixture of the two. For the most part
the extra sensitivity given by siler iodide is not necessary for
paper. Most paper emulsions will fix out in a minute in fresh sodium
thiosulfate fixer when the print is fixed alone. Much longer fixing
times have traditionally been recommended to compensate for the use of
partially exhausted fixer and the fixing of many prints together,
preventing adequate access of fresh fixer solution to the surface.
Fresh film strength rapid fixer will fix out most paper emulsions in
30 seconds. This is the basis of the Ilford archival processing
method. Its purpose is to fix out the paper quickly enough to prevent
substantial adsorption of the fixer to the paper fibers.

Clearing time for film can be observed directly. A clip of the
unprocessed film is soaked in water for a few minutes and placed in
the fixer. Agitate it gently and measure the time until it becomes
transparent. Fixing time is generally twice the clearing time. The
reason for soaking the film first is that wet film fixes at a slightly
different rate than dry film. When dry the emulsion must swell first
for the solution to fully penetrate it. In practice, the film is
wetted and swollen when introduced into the fixer.
Please not that partially exhaused fixer can still clear the film,
but, if there are not enough thiosulfate ions left, it can not
complete the somewhat complex reaction that results in the reaction
products being soluble in water so they will wash out.
The two bath system insures these reactions will be completed and
the reaction products will wash out.
Any silver complex left in the emulsion will eventually decompose
and attack the image. Once the film has dried these reaction products
begin to change, so that even subsequent fixing in fresh fixer may not
make incompletely fixed film or paper permanent.

This is much more than I intended to write. I hope it is helpful
rather than confusing.
To summarize:
Conventional sodium thiosulfate fixer is suitable for high iodide
films, providing a two bath fixing system is used.
All pictorial films have some silver iodide, but fast films have
more.
Some paper may have silver iodide but it is not traditionally used in
paper emulsions.
A kind of rapid fixer can be made without ammonium thiosulfate by
adding ammonium salts to conventional fixer.
Even rapid fixer is not a gurantee of complete, archival, fixing. A
two bath system with either type of fixer is a large help.
I will add finally that the use of a fixing bath test solution is
helpful in keeping track of fixing bath condition. The formula and
instructions for Kodak FT-1 is given in the _Kodak Black-and -White
Darkroom Dataguide_. I prefer this to the prepared Edwal Hypo Check
because the Kodak formula is of known strength and specific dilution
is given for testing various types of baths.
The same book has the formula for the residual silver test ST-1,
whichis a 2% solution of Sodium Sulfide. A 1:9 dilution of Kodak Rapid
Selenium Toner can also be used to test for residual halide. A few
drops are applied to the wet emulsion and allowed to work for about
two minutes. The emulsion is then blotted and examined for any stain.
Either solution will stain silver halide. There should be NO stain if
the film or paper is completely fixed.

Enough !
---
Richard Knoppow
Los Angeles, Ca.