Rapid Fixer--is sodium sulfite necessary?

Discussion in 'B&W: Film, Paper, Chemistry' started by lensmagic, Jun 3, 2012.

  1. lensmagic

    lensmagic Member

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    Most formulae for rapid fixer call for the addition of sodium sulfite. Is sodium sulfite really necessary?
     
  2. michael_r

    michael_r Subscriber

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    It is a preservative. You don't have to use it but the fixer might not last as long. You can mix a "plain hypo" fixer using just water and sodium thiosulfate. Some people still use this type of fixer for film developed in certain staining developers, since the conventional wisdom is that sodium sulfite reduces image wise stain even after development. Other than that, I can't think of any good reason not to add sodium sulfite.

    If you are mixing either an acidic or alkaline rapid fixer using ammonium thiosulfate, then I'm not sure - the sodium sulfite may or may not be more necessary for the formula to work properly. Perhaps the chemists will comment further here.
     
  3. nworth

    nworth Subscriber

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    I've used a roughly 10 percent solution of ammonium thiosulfate as a one-shot rapid fixer many times. It is marginally faster than most rapid fixers, but it has less capacity and a much shorter life. As said above, sulfite is a preservative in fixers.
     
  4. Gerald C Koch

    Gerald C Koch Member

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    An acidified solution of either sodium or ammonium thiosulfate is unstable and will soon sulfurize. The sodium sulfite acts as a preservative by combining with the sulfur to form thiosulfate ion again. This happens before there is any visible precipitation of sulfur. Eventually all the sulfite is used up and the fixer is unprottected and will sulfurize.
     
  5. Doremus Scudder

    Doremus Scudder Member

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    Which begs the question, Jerry: Can I extend the life of, say, a rather old 5 liters of Ilford Rapid Fix by adding some more sodium sulfite? It would sure be nice not to have to toss the concentrate when the sulfurization starts.

    Best,

    Doremus

    www.DoremusScudder.com
     
  6. Gerald C Koch

    Gerald C Koch Member

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    Theoretically adding more sodium sulfite should help. The problem would be in getting the sulfite to dissolve in what is already a very concentrated solution. You can't warm the solution as this may start the sulfurization. I think the best solution would be to keep the concentrate cool and use it up as soon as possible.
     
  7. Doremus Scudder

    Doremus Scudder Member

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    Thanks for the response. I was more curious than stuck with large quantities of old fixer concentrate. I may try to dissolve a bit of sulfite in a small amount of the concentrate though, also just out of curiosity. I appreciate your taking time to address my question.

    Best,

    Doremus

    www.DoremusScudder.com
     
  8. Gerald C Koch

    Gerald C Koch Member

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    I would be interested in your experiment to see if sodium sulfite would dissove in the fixer concentrate. Whether it will dissolve or not is a complex question and an empirical answer is probably the easiest.
     
  9. Doremus Scudder

    Doremus Scudder Member

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    I'll definitely try it out, and post here with whatever results. But it will have to wait a few weeks till I get back to my darkroom in the States around July 1.

    Best,

    Doremus