Sodium thiosulfate: anhydrous v crystalline
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 ...
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.
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.
-- Ole Tjugen, Luddite Elitist
Thanks for the help. Humbling to realize there's so much to learn ...
Hold on to your seat, there is even more to learn. Most of the
Originally Posted by couldabin
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
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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.
Photography has always fascinated me -- as a child, simply for the magic of capturing an image onto glossy paper with a little box, but as an adult because of the unique juxtaposition of science and art -- the physics of optics, the mechanics of the camera, the chemistry of film and developer, alongside the art in seeing, composing, exposing, processing and printing.
A good source for anhydrous Sodium Thiosufate is The Chemistry Store.
Their price for a 2 pound Pail of anhydrous Sodium Thiosufate is $3.94. A 10 pound Pail is $17.70.
Everything is analog - even digital :D
The penta is 33%, not 50%, water. Actually it's a bit more
Originally Posted by dancqu
than 33%; IIRC, the bit is in the order of 2 or 3 %. Dan
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.
Originally Posted by Tom Hoskinson
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.
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.