Does stop and fixer oxidize like developer?

[rpavich]

I was just replacing my paper fixer and stop bath and was wondering: is the exhaustion of the fixer only due to silver saturation or does it go bad just sitting in the bottle or tray?
And then I started wondering about the stop bath also.

It's not a big deal, just wondering.

 

[Anon Ymous]

Fixer can go bad, but stop bath doesn't. Depending on the storage conditions, the concentrate can go bad fairly early. If you notice a yellowish precipitate on the walls or the bottom of the container, then it has sulfurised. It may still clear film, but it will ruin your film, so don't use it. I've noticed that such fixer gives cloudy solutions.

The manufacturer of the fixer gives the storage life of the solution and the concentrate at the datasheet, along with the capacities.

 

[rpavich]

Thank you Anon Ymous.

 

[Halford]

As for stop bath, it won't oxidize as such -- but there have been experiences reported here of microbial in citric acid based stop baths (i.e. 'odorless' stop baths). In theory it could also happen to acetic acid stops (but not to pure glacial acetic acid) though this doesn't seem to be an issue in practice.

 

[Steve Goldstein]

As for stop bath, it won't oxidize as such -- but there have been experiences reported here of microbial in citric acid based stop baths (i.e. 'odorless' stop baths). In theory it could also happen to acetic acid stops (but not to pure glacial acetic acid) though this doesn't seem to be an issue in practice.

I actually did have this happen with some Kodak indicator stop bath that I'd mixed to half working strength, the way I always use it. It was a bit used and then sat on the shelf for some months - when I went to print again I noticed it had some slimy "stuff" inside, so I dumped it and mixed a fresh batch. It's only happened that once.

PE suggests 1 gm/liter of Benzoic acid will deter this.

 

[Mr Bill]

. . . is the exhaustion of the fixer only due to silver saturation or does it go bad just sitting in the bottle or tray?

Hi, the typical fixing agent, thiosulfate goes through some sort of breakdown process with free sulfur as one of the breakdown products. In a "healthy" fixer with surplus sulfite ion, there is some sort of regeneration cycle where it is recycled back into thiosulfate again. I'm not a chemist, speaking from an older 1980s (?) SPSE paper. But this bears with my experience in a large lab where we used literally thousands of gallons of replenished chemicals daily. (We regenerated bleach-fix, which means that a certain proportion of the thiosulfate stays in use virtually "forever," so for the naysayers why doesn't THIS sulferize in the system?)

As a note, the "protective" sulfite (SO3) is oxidized by air to sulfate (SO4) and is no longer helpful. After most of the sulfite is gone, then the thiosulfate can go bad in short order, especially if the pH is on the low side.

One further note on fixer "going bad;" when the C-41 (color neg) process was new, we experimented with regenerating the fixer. (You have to collect the surplus fixer - processor overflow due to the addition of replenisher; this overflow is electrolytically desilvered to an acceptable level, then the missing chemicals, mainly thiosulfate and sulfite are replaced.) What we found was that the rate, or speed of fixing film fell off significantly in short order. We concluded that this was due to a build-up of iodide ion in the fixer - relatively small amounts just kill the rate of fixing (the iodide comes from modern films). As a result we would only be able to reuse something like 50% of the used fixer, and we decided that the savings were not worthwhile.

In summary, the main way your conventional fixer will go bad is either by the build-up of silver OR sulferization due to loss of free sulfite ion. OR, if you tried to reuse FILM fixer by desilvering and regenerating, then a build-up of iodide from modern films would eventually "kill" the fixer.

 

[darkroommike]

I mix my stop bath from concentrate and discard after a single use, it's that cheap. When I was printing more I would use indicator stop to exhaustion then replace it. My practice was to float another tray on top of my stop bath and fixer trays between sessions to keep the air and crud out of my solutions.

 

[R.Gould]

I have used Fixer that has been stored for up to 2 years in concentrate form still clear and working fine, I have used dilute fixer for printing after storing for 3 or 4 months, never a problem, working fine and still clear, film fixer I mix 600ml and use for 8 films, then discard. As regards stop, I use the change color when exhausted, when it changes color I discard and mix fresh, so no, personally fixer and stop does not go off, oxidise or cause problems, at least not for me, I use Champion fixer which I buy in 2 5 litre containers per purchase,so 10 litres last a while, and over 50 years of using I have yet to throw away a drop of the stuff, SWith my dilute fixer/stop for printing I always pour back into a storage bottle after use, same for film dilution
Richard

 

[Wayne]

You will smell it if fix goes bad.

 

[Rudeofus]

(We regenerated bleach-fix, which means that a certain proportion of the thiosulfate stays in use virtually "forever," so for the naysayers why doesn't THIS sulferize in the system?)

Sulfurization of Thiosulfate happens not directly because of oxidation, but because pH drops below a certain level around pH 4. This pH drop can happen if Sulfite or Thiosulfate is oxidized, or because some other source of acid enters the fixer.

As a note, the "protective" sulfite (SO3) is oxidized by air to sulfate (SO4) and is no longer helpful. After most of the sulfite is gone, then the thiosulfate can go bad in short order, especially if the pH is on the low side.

That's pretty much what happens. As Oxygen enters fixer, Sulfite is converted to Sulfate, which already lowers pH. Once all Sulfite is used up, Oxygen starts to attack the Thiosulfate, which causes pH to drop like a rock - and the result is that yellow Sulfur precipitate.

There are some factors which determine how quickly this happens:

1. pH of the fixer: acidic fixer starts at a pH closer to Sulfur precipitation, and it oxidizes quicklier. I get incredible shelf life with neutral fixer.
2. Amount of Sulfite in fixer. Add too much, and fixing speed goes down. Add too little, and it will be gone too soon.
3. Concentration. Concentrated aqueous solutions dissolve a lot less oxygen than diluted ones. Fixer concentrate will last longer than working solution

 

[Mr Bill]

Sulfurization of Thiosulfate happens not directly because of oxidation, but because pH drops below a certain level around pH 4. This pH drop can happen if Sulfite or Thiosulfate is oxidized, or because some other source of acid enters the fixer.

As I said, I am not a chemist, but my understanding is that there is always some free hydrogen ion in aqueous solutions as a result of random (?) association and dissociation of water into hydrogen and hydroxyl ions (this is the basis of pH measurement). Perhaps this is an adequate "other source of acid?"

At any rate, my limited experience with (presumed) "beginning to sulfurize" thiosulfate is that 1) analysis for sulfite shows virtually no sulfite left, and 2) addition of a couple g/L sulfite eliminates the characteristic smell/cloudiness. (These were cases with pH values well above ~4.)

As I recall, the pertinent technical papers were by either Hendrickson of CPAC or Cooley of Kodak, circa 1980. They were related either to the general use of sulfite in photographic solutions or the reactions during electrolytic desilvering of fixer (Austin Cooley was our hero in this respect).

 

[Rudeofus]

As I said, I am not a chemist, but my understanding is that there is always some free hydrogen ion in aqueous solutions as a result of random (?) association and dissociation of water into hydrogen and hydroxyl ions (this is the basis of pH measurement). Perhaps this is an adequate "other source of acid?"

What I wanted to express with my statement was "it doesn't matter whether pH drops down because of oxidation, or because of carryover of a concentrated, buffered stop bath, once pH is below a certain level, Sulfur precipitate will form".

The Hydronium ions present in water will only matter, if there is more than 10-4 mol per liter of them. They are no "source of acid", they are what is there in quantity when an acid is present.

 

[Doremus Scudder]

I was just replacing my paper fixer and stop bath and was wondering: is the exhaustion of the fixer only due to silver saturation or does it go bad just sitting in the bottle or tray?
And then I started wondering about the stop bath also.

It's not a big deal, just wondering.

Back to a simpler answer for the OP , because it is a big deal and there seems to be a lot of misunderstanding and misuse of fixer reported on the fora lately.

Stop baths are commonly either acetic acid or citric acid. The working solutions of these don't oxidize, but, as mentioned above, citric-acid stop baths will grow bacterial slime rather quickly and should be discarded after a short time. I mix mine "one session." Acetic-acid stops, like Kodak Indicator Stop Bath will last much longer. I've kept working strength KISB for some months with no ill-effects. Do, however, keep an eye out for bacterial growth, since, as reported, acetic-acid stops can grow this as well.

Fixers will go bad with time through exposure to air. A simple way to understand what happens is this: Fixer contains sodium sulfite, which acts as a preservative and keeps the pH correct. The sulfite oxidizes gradually. When enough has oxidized, the preservative effect goes away and the fixer "sulfurs out," i.e., begins to precipitate out sulfur in the form of flakes or granules. There is an accompanying rotten-egg smell. When this happens, regardless of how much the fixer has been used, you should discard it and mix new. (Regenerating fixer like Mr. Bill mentions above is not practical for low-volume users.)

So, we should not only keep track of throughput to determine fixer exhaustion, but also how old the fixer is and how it has been stored.

The lifespan of a fixer in this latter regard depends on how long and how much it has been exposed to air. Ilford's recommendations are in their tech sheet on Rapid Fixer here: http://www.ilfordphoto.com/Webfiles/2006130218312091.pdf . The tech pub has a lot of useful info about fixers in general. I suggest reading it carefully. The relelvant info is below:

"Unreplenished ILFORD RAPID FIXER working
strength solutions should last for up to:-
6 months in full tightly capped bottles
2 months in a tank or dish/tray with a floating lid
1 month in a half full tightly capped bottle.
7 days in an open dish/tray."

Best,

Doremus

 

[rpavich]

Thanks Doremus, that was simple and to the point.