Sodium Thiosulfate question

Discussion in 'B&W: Film, Paper, Chemistry' started by StigHagen, Jan 9, 2013.

  1. StigHagen

    StigHagen Member

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    This is for the chemist expert. Im going to buy some Sodium Thiosulfate and found two providers with a slightly different formula:

    Na2O3S2x 5H2O
    and
    Na2S2O3 6H2O

    They both say it is good for fixing papers. What is the difference between those? And does it matter?

    Thank you
     
  2. andrew.roos

    andrew.roos Member

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    The pentahydrate (5 H2O) is the familiar one ("hypo") so I would get this; I'm not sure whether the 6 H2O is genuinely different or just a typo by the seller.

    Edit: I see they both state they are pentahydrates, so presumably it's a typo. But I'd still be safe and get the one that is correctly identified (5 H2O).

    Andrew
     
    Last edited by a moderator: Jan 9, 2013
  3. StigHagen

    StigHagen Member

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    I have contacted the provider for details, the 6 H2O is much cheaper :smile: Thank you!
     
  4. jochen

    jochen Member

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    Hello,
    definitely there does not exist a sodiumthiosulfate-hexahydrate ( x 6 H2O), only a pentahydrate (x 5 H2O) and the anhydrous form. I'm a chemist and I have read many old and new books about photographic formulations, I never found a hexahydrate. Also in books about inorganic chemistry and in chemical encyclopedias you'll only find the pentahydrate and the anhydrous hypo. This is an error.
     
  5. StigHagen

    StigHagen Member

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    You are right, it was a typing error. The provider have fixed the error :smile: I'm pretty impressed by the level of knowledge on this forum. Thank you!
     
  6. paul_c5x4

    paul_c5x4 Subscriber

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    Didn't see the listings before the info was updated, but do pay attention to the quantities shipped. One is for 25Kg, the other, although cheaper, is only 5Kg - In terms of Kilo per Euro, the 25Kg listing is better value.
     
  7. StigHagen

    StigHagen Member

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    I will buy the 25kg....that is cheap fixer for sure :smile: I will add some Sodium Bisulfite to the first fixer tray (2 bath system).
     
  8. aldobranti

    aldobranti Member

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    there is slightly more water of crystallisation in the second product and this will affect a calculation of the weight of product to add to a solution -- I think the x in the first product formulation is just ignorance, throw away the x and reorder the Sulphurs and the Oxygen content. Conventionally the product would be written Na 2 S2 O3 and suitably subscripted...

    you make up solutions to add a fixed weight of the thio to a fixed quantity of water
    the following solutions are chemically identical
    158 g of anhydrous thio sulphate in 1 litre water
    248 g of Na2 O3 S2 5H2O
    266 g of Na2S2O3 6H2O

    having said all that , one of those hydrations is unstable and the product will absorb or lose water to the environment.