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  1. #1

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    Sodium Thiosulfate question

    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. #2

  3. #3
    andrew.roos's Avatar
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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 andrew.roos; 01-09-2013 at 05:12 AM. Click to view previous post history.

  4. #4

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    Quote Originally Posted by andrew.roos View Post
    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
    I have contacted the provider for details, the 6 H2O is much cheaper :-) Thank you!

  5. #5

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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.

  6. #6

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    Quote Originally Posted by jochen View Post
    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.
    You are right, it was a typing error. The provider have fixed the error :-) I'm pretty impressed by the level of knowledge on this forum. Thank you!

  7. #7
    paul_c5x4's Avatar
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    Quote Originally Posted by StigHagen View Post
    I have contacted the provider for details, the 6 H2O is much cheaper :-) Thank you!
    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.

  8. #8

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

  9. #9

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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.



 

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