Sodium thiosulfate: anhydrous v crystalline

Discussion in 'B&W: Film, Paper, Chemistry' started by couldabin, Jun 21, 2005.

  1. couldabin

    couldabin Member

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    Help! I started to mix Kodak F-24 non-hardening fix as found in Anchell/Troop's Darkroom Cookbook, using sodium thiosulfate ("hypo, penta" the label says) from Photographer's Formulary, and I realized I don't know a) whether it's cystalline or anhydrous; or b) which of those forms the formula calls for (240g to make 1 liter).

    I'm guessing I have crystalline, because it looks like rock crystals. But what do I know? And which does the formula call for? I see the book has conversions from one to the other; I just need to know which the formula is specifying ...

    TIA!

    duane
     
  2. srs5694

    srs5694 Member

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    First, I'm not a chemist or an expert on this; however, some key points, from Anchell's The Darkroom Cookbook, 2nd Edition (note that Troop wasn't a co-author on that, but he was a co-author on The Film Developing Cookbook, so it's not clear to me which you've got):

    • On p. 10, "anhydrous" is defined as a "chemical with all water removed."
    • On p. 11, "pentahydrate" is defined as a chemical "having five molecules of water."
    • On p. 210, near the top, there's a description of crystalline vs. anhydrous hypo that seems to imply the following formulas specify the crystalline form.
    • On p. 214, the formula for F-24 specifies 240g of sodium thiosulfate, just as you report.

    Thus, since your jar is labelled "penta," it appears to not be anhydrous and I'd infer that pentahydrate is synonymous with crystalline in this case. In other words, it appears that you've got the crystalline form. Based on the other comments in the book, it seems that the formula calls for the crystalline form. As another data point, this site provides the formula for F-24 and is more explicit about the anhydrous/crystalline form. It says to use 240g of crystalline or 152g of anhydrous, which tracks with my inference that Anchell is using crystalline measurements.
     
  3. Ole

    Ole Moderator Staff Member Moderator

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    The person with the number at the end of the handle is correct - "hypo penta" would be sodium thiosulfate pentahydrate. It often looks like small clear rice grains, while anhydrous sodium thiosulfate is a fine yellowish-white powder.

    240 g/liter is about standard for the pentahydrate in fixers.
     
  4. couldabin

    couldabin Member

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    Thanks!

    Thanks for the help. Humbling to realize there's so much to learn ...

    duane
     
  5. dancqu

    dancqu Member

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    Hold on to your seat, there is even more to learn. Most of the
    formulas from A&T and many other sources are old, old, old. I'd
    not be surprised if that fixer formula you have is but the
    equivalent to one recommended in 1865.

    About that time silver-GELATIN emulsions started to appear.
    Perhaps the penta was all they had or could produce in
    quantity, one hundred and forty years ago.

    The penta is 50% water. It will chill very measurably on
    dissolution. Save on shipping, go anhydrous. Dan
     
  6. Donald Qualls

    Donald Qualls Member

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    Well, you'll save some on shipping, but you'll pay for it on the chemical. You can get "sodium thiosulphate rice crystals" (which are penta) at the local pool & spa store for a lot less than either anhydrous or penta from Formulary or DigitalTruth, and you won't have to pay for shipping, just the gas to get to the shop.
     
  7. Tom Hoskinson

    Tom Hoskinson Member

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  8. dancqu

    dancqu Member

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    The penta is 33%, not 50%, water. Actually it's a bit more
    than 33%; IIRC, the bit is in the order of 2 or 3 %. Dan
     
  9. srs5694

    srs5694 Member

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    Have you ever bought this item from them? I have, and although the description mentions "sodium thiosulfate anhydrous" as a synonym for what they're selling, what I got matches the description for the crystalline form earlier in the thread. The jar isn't clearly labelled one way or the other, and I have yet to use it in anything.

    That said, TCS's price is still the lowest I've found. I must point out that they hide shipping costs until late in the ordering process, and then they tack on an extra $3.50 "handling" charge. In the end, you're likely to pay as much for shipping as for the product. I'll have to check some local pool supply stores, as Donald suggests.
     
  10. Claire Senft

    Claire Senft Member

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    I just bought a 30 pound pail of hypo from The Chemistry Store. The total cost including shipping was 69.00 USD and change. This is my second order from them and each order was handled very nicely. If there are two items a darkroom user is likely to use quite a bit of it would be sodium sulphite and fixer. I believe it makes economic sense to but these items in good size quantities.

    I checked my local pool supply store before ordering from TCS and they were not particularly competitive and the product included approximately 25% of other ingredients.
     
  11. Ole

    Ole Moderator Staff Member Moderator

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    A quick check is to heat the substance - nothing much happens to the anhydrous, while the pentahydrate will melt (actually dissolve in its own crystal water) at around 80 C.
     
  12. avandesande

    avandesande Member

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    I had a good experience with them as well. The nice thing about anhydrous is you are not paying to ship water. No problems dissolving to make my solution.

     
  13. srs5694

    srs5694 Member

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    I just tested. I put a couple teaspoons of the sodium thiosulfate from The Chemistry Store in a microwave oven and heated it for close to a minute. The result was that part of it turned into a slushy mess and another part started looking awfully powdery. So, despite listing "sodium thiosulfate, anhydrous" as a synonym for what they're selling, I don't think TCS is really selling that variant -- or at least, what I got was pretty clearly the crystalline form. They've still got the best price on it I've seen on the Web, at least for the sorts of quantities amateur photographers are likely to need. If somebody else has definitely gotten the anhydrous form from them, then it seems that they're not being very consistent in what they ship.
     
  14. avandesande

    avandesande Member

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    You can melt metal in a microwave. Not a good test for melting point IMHO.
     
  15. Ole

    Ole Moderator Staff Member Moderator

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    That's what I was going to say - "testing" something in a microwave is close to worthless. You have absolutely no control over the temperature, even heating it over a candle is more predictable!

    You are not trying to melt it, but to allow it to dissolve in the water that is already in the crystal. So it's important that you don't heat it too much or you will boil off the water!

    Another way is to grind it in a mortar, or between two spoons, or similar. Sodium thiosulfate pentahydrate won't turn to powder, but to liquid!
     
  16. dancqu

    dancqu Member

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    Cool it! Cool a spoon full in a small glass of room
    temperature water. Real Cool!
    While the penta will chill the solution the anhydrous
    will not. Dan
     
  17. Ole

    Ole Moderator Staff Member Moderator

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    No boiling, not room temp...
     
  18. srs5694

    srs5694 Member

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    FWIW, I just made some fixer (the citric acid variant of F24) with the sodium thiosulfate I got from The Chemistry Store. It chilled the water quite substantially when I added it, to the point that the originally room-temperature mixing vessel, holding what had been room-temperature water, ended up with condensation forming on it. Thus, I'm quite certain it's the crystalline form, although "anhydrous" is mentioned as a synonym on the TCS Web page.
     
  19. Gerald Koch

    Gerald Koch Member

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    What TCS sells is the pentahydrate. They are not unique in supplying a MSDS for the anhydrous form of a chemical while they sell a hydrated form. I have found that other suppliers do the same. In fact some suppliers of chemicals aren't sure just what they are selling. Part of the general dumbing down of the public as far as I am concerned.
     
  20. dancqu

    dancqu Member

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    That's easy to believe. I do feel safe buying from
    Photographer's Formulary. They are many years in the
    business of being a specialty supplier to darkroom workers.

    They have the anhydrous and the penta thiosulfates.
    Pay for three pounds of the penta or two pounds of the
    anhydrous. With the penta a pound of water is
    included at no extra charge. Dan
     
  21. srs5694

    srs5694 Member

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    Actually, the "no extra charge" water actually costs a negative amount. Because PF doesn't sell 2 and 3 pound quantities, I can't compare those; but for 10-pound quantities the costs are $55.50 for anhydrous vs. $29.95 for pentahydrate (crystalline). That's $5.50/pound for anhydrous vs. $3.00/pound for pentahydrate; or adjusting for the "no extra charge" water, the 10 pounds of pentahydrate is equivalent to just 6.4 pounds of anhydrous, so the cost is $4.68/pound equivalent -- $0.82/pound less for the pentahydrate form. That doesn't consider shipping costs, though, which will eat into the cost advantage for the pentahydrate. I've also not checked the equivalences for most other sizes.

    FWIW, although the Chemistry Store Web page is unclear at best about the form of their sodium thiosulfate, their cost is $17.70 for a 10-pound pail of the pentahydrate form, or $2.77/pound for anhydrous-equivalent. Again, this doesn't consider shipping.

    Also FWIW, by my calculations, typical sodium thiosulfate fixers cost about $2.50-$3.50 per liter using Chemistry Store chemicals, including shipping. This contrasts to about $2.00-$3.00 per liter for similar commercial fixers (again, including shipping), so mixing your own sodium thiosulfate fixer doesn't make financial sense unless you can find sodium thiosulfate for much less than even the Chemistry Store price.
     
  22. lee

    lee Member

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    have yall considered Mike at Artcraft for say like 50 lbs?


    lee\c