How does fixer deteriorate?
Does fixer deteriorate with use, or by oxidation - or do both factors apply?
The reason I ask is that I've begun mixing 4 litre fixer portions from Calbe A-300 powder (I bought that bag size by mistake).
Obviously, working through that much fixer will take some time, given my low DR productivity, and I wonder if the fixer will go weak by just sitting on a shelf, or if it is exhausted with use only.
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I know that fixer does deteriorate from use, being neutralized by contamination of residual developer remaining on the print/neg.
The thiosulfate in fixer does break down due to oxidation over time, but it does not happen nearly as quickly as it does to developers. When it does start breaking down, you will know it. I have a gallon of Kodak powdered hardening fixer mixed up and ready to go now for almost 6 months and it is still ok. The last batch was well over a year old and there was only about 1L left in a partially full container before it started to go bad. On another occassion, I had a small amount of rapid fixer concentrate in a bottle, again over 1 year opened, when it started to degrade. Sulfur precipitates out, making the mixture cloudy with coloidal sulfur in suspension and giving it a distinctly sulfurous smell. If the fixer is not too far gone, it will still clear a piece of undeveloped film and you can still use it. I would not use it for fixing film or any important prints. It will be fine for test strips, work prints, etc., in short anything where permanence is not required.
In use, fixer deteriorates because of several factors. Residual developer carried over into the fixing bath is one reason, but this effect is largely nullified with the use of an acid stop bath in the case of acidic fixers. I minimize the carry over by rinsing my films with clear water after the acid stop. Papers can be treated the same way if you have some running water in the darkroom. I don't, so a quick dip in a tray of water that gets changed every so often suffices. For the most part, the fixer simply gets used up while doing what it is supposed to do. Undeveloped silver halides are converted into water soluble compounds which are then carried away by the water in the fixer. It is precisely this reason why undiluted rapid fixer concentrates don't "fix" anything. There is simply not enough water to carry about the byproducts of the fixing process.
Last edited by fschifano; 03-28-2009 at 06:19 PM. Click to view previous post history.
Split it into a few glass bottles with good caps.
Originally Posted by Kvistgaard
When time to work open one bottle then dilute to
working strength. Keep batch sizes small but workable.
Plan ahead for a set amount of film or prints then
use that much fixer. At the end of the session
toss the fix. That is how I use fix. Developer
also for that matter. Dan
As I understand it, the biggest factor is the accumulation of silver thiosulfate complexes with use. That does two things: it depletes the thiosulfate needed for fixing and it shifts the chemical equilibrium to make the fixing process more difficult. The fixer also oxidizes over time, and developer carryover takes its toll.
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I have some old Kodak rapid fixer (2.5 gallons of concentrate) that has quite a bit of colloidal sulphur in it. I strain it through a coffee filter and use it at the recommended dilutions. It clears film in 15 sec. I have had no trouble with it at all. I picked up two containers from a school lab that was shutting down.
"I'm still developing"
Sulfur is soluble in sodium sulfite. The combination produces
Originally Posted by ricksplace
sodium thiosulfate. I don't know if the procedure is practical
on a small scale. Might give it a try; reconstitute. Dan