Sodium Thiosulfate in Fixer.

Discussion in 'B&W: Film, Paper, Chemistry' started by Bruce Osgood, Mar 5, 2008.

  1. Bruce Osgood

    Bruce Osgood Membership Council

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    For the purposes of photographic chemistry is there a difference in these two products in use:

    The Chemistry Store
    Sodium Thiosulfate 20 lb pail $1.47/lb $29.40 per pail

    Art Craft
    Sodium Thiosulfate penta 20 lb. $44.00

    Does the term "penta" in the Art Craft description warrant the additional cost for use in a non-hardening fixer?
     
  2. ann

    ann Subscriber

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    there are two forms of sodim thisulfate, anhyrous and the penta version.

    i have used both from both places, with the most recent being from the chemisty store.

    the "rice" is clearer than that from art craft but works the same as far as i am concerned
     
  3. Ole

    Ole Moderator Staff Member

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    "Penta" probably means "pentahydrate" (five waters per sodium thiosulfate molecule). That's the normal for of sodium thiosulfate, the "rice grain" crystals.

    The only other form I know of is anhydrous (no water), which tends to be a lot more expensive.

    The purity may be different, but I doubt it will make any practical difference.
     
  4. Snapshot

    Snapshot Member

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    As "penta" would likely indicate a pentahydrate form of sodium thiosulfate (having 5 water molecules), you would need more of it than the anhydrous or dessicated form of sodium thiosulfate. Other than that chemical difference, they should work the same.
     
  5. dancqu

    dancqu Member

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    Very nearly 50% more of the penta; all that H2O.
    Dissolving the penta causes the solution to chill.
    That is one way to tell the two apart.

    I add a measure at processing time to the needed
    volume of water. Easy fresh fix. Dan
     
  6. dancqu

    dancqu Member

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    PS: The Store may not say so but they may
    have the penta. The anhydrous is what you
    want; more fix per pound and no chill.

    A lot of formulas are from the early years of
    the last century. Why else all that penta still
    hanging around, still being stocked. Can any
    one name a single photographic application
    where the anhydrous won't do? Dan
     
  7. srs5694

    srs5694 Member

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    This thread could easily turn into another chemical purity debate. That's not entirely unjustified, since that's the only likely difference between the two, from a chemical perspective. My notes indicate that The Chemistry Store sells the pentahydrate form -- but that's from a few years ago. I've used their sodium thiosulfate with no obvious ill effects, but lately I've been using ammonium thiosulfate-based fixers instead of those based on sodium thiosulfate. Ammonium thiosulfate fixers are less expensive, on average, and they work faster.
     
  8. Photo Engineer

    Photo Engineer Subscriber

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    Yes Dan. Stored in cardboard drums on a dock in south east asia. It melts. :sad:

    I told them to order the tropical pack in plastic bags but the USAF wanted to save money.

    Actually, no kind of hypo would stand those conditions. But I do believe that both the penta and anhydrous chill out when added to water. I'm not sure anymore, as it has been years since I've used the anhydrous.

    PE
     
  9. PhotoJim

    PhotoJim Member

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    Isn't the pentahydrate more stable? Anhydrous gradually absorbs water.

    Pentahydrate will stay as pentahydrate, so you aren't chasing a moving target.

    Anhydrous is fine if you can keep it dry, but it is hygroscopic (absorbs water) so it will gradually get heavier as more and more of it becomes pentahydrate. This means that using the same weight of crystals in your solutions will result in weaker and weaker solutions over time.
     
  10. Paul Verizzo

    Paul Verizzo Member

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    It's Hypo!

    Surprised that no one has mentioned that the pentahydrate is what used to be called "hypo." Which is why we still use the term; a long, long tradition.

    No need for the anhydrous unless you are going backpacking........ :D
     
  11. Ole

    Ole Moderator Staff Member

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    Eh - no.

    "Thiosulfate" used to be called "hyposulfite". It doesn't have to mean sodium thiosulfate pentahydrate - it can mean any thiosulfate in any hydration state. But analog photographers tend to be a conservative bunch; and the little detail that all chemists in the world are using a different word carries little weight. :tongue:
     
  12. Paul Verizzo

    Paul Verizzo Member

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    Yes.

    Quite right, of course. I was limiting my comments to the modern offerings.

    Believe it or not, in my father's garage is a yellow box from Kodak that says "Sodium Thiosulfate, Hypo-Prismatic Type (Pentahydrated)". My father, ever the photographer had written on the side, "Hypo" many years ago. I have no idea why he kept this of all things.

    There is a pencil marking which I presume is the price: 45 cents for one pound. On the back are recipes for various fixers. "Plain Hypo" is 1 pound in two quarts of water; F-24 acid fixer, and than an acid hardening one. The inner wrap is paper, not plastic, and the hypo itself is chunky after decades in the Florida humidity. But it looks to be just fine.
     
  13. dancqu

    dancqu Member

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    Or don't mind the extra postage.
    Or short on H2O. Dan