I'm glad Marty pointed this out. There is more to fixing film than clearing it. The fixer eventually becomes saturated with silver, after which it can't hold any more. After fixing film, I pour a little bit of the fixer, say 1/2cc, into a small graduate cylinder. I add one drop of hypo check. If the fixer is saturated, some silver will precipitate out and form a white cloud. Then it's time to throw it away. This will happen even before the fixer is no longer clearing the film.
There is one extremely important thing to always remember: the fixer will keep fixing long after it is no longer fixing properly or in a way that is safe for the long-term storage of your film.
What fixer does is:
AgX + 2 S2O32− → [Ag(S2O3)2]3− + X−
AgX + 3 S2O32− → [Ag(S2O3)3]5− + X−
where X is a halide (bromide, chloride or iodide). The second reaction, which needs more thiosulfate, predominates in mono size flat emulsions like TMax films.
Having excess thiosulfate is vital for removing all the unexposed silver. Once you have finished fixing, you then need to be able to wash all the silver thiosulfate out of the film. If there is more than 6g/L of silver in the fixer enough will remain that your film is likely to deteriorate unduly over a timeframe where if you had fixed it properly it would not deteriorate.
This is the misleading part: fixer will keep clearing film and making it look like it is fixed after the thiosulfate concentration has dropped to a point where it is not fixing the silver halides efficiently and where it contains too much silver to be effectively washed out. So, either time, or measure. Dump when the clearing time doubles or the silver concentrations reaches 6 g/L.
Hmmm, I fixed a roll of expired T-max three weeks ago and the next time I used the fixer there was a lot of black sediment floating around in the fixer.
I thought the fixer might have oxidized because I didn't tighten the cap enough......might just be a coincidence.
The fixer was still good for fixing the next film within 5 minutes though.
If fixer goes off it is a reduction reaction, rather than oxidation. The reduction and sulfur precipitation can occur with age, irrespective of use. Thiosulfates are stable only in neutral or alkaline solutions, but not in acidic solutions, due to decomposition to sulfite and sulfur, the sulfite eventually being dehydrated to sulfur dioxide:
S2O32− (aq) + 2 H+ (aq) → SO2 (g) + S (s) + H2O
The sediment will be white or pale yellow - it is amorphous sulfur (S) and it usually smells a bit like rotten eggs because the process also produces sulfur dioxide (rotten egg gas). This occurs spontaneously over time, but is accelerated by use. Silver will never precipitate out of your fixer without a serious chemical nudge.
The black stuff in your fixer after you fixed TMax film is a mixture of the antihalation and anti friction/static layers. The pink stuff is one of the sensitization dyes, and if your film is properly fixed but still pink, can be removed with a little sunlight exposure. But don't overdo it, or the sun might damage your negatives.
Now, go take some photos. And later, when you develop the film, time clearing or measure the silver in the fixer, and forget all this. Kodak worked this out a long time ago. All you need to do is run with it.
On Plus-X (sniff) fixed with Fomafix liquid 1+5 (as recommended by Foma) for 5 minutes (2x the clearing time).
Marty
