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  #1  
Old 30th May 2021, 06:47 PM
Nat Polton Nat Polton is offline
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Default Sodium Thiosulphate identification.

I have some Sodium thiosulphate labelled anhydrous.
It looks like 8mm x 5mm translucent white crystals.

The last time that I used the anhydrous version it was in the form of a white powder.

The only test that I can find to identify anhydrous from penta says to mix a spoon full of the unidentified chemical in a small glass of room temperature water, and if it is penta it will chill the water and anhydrous wont.

My tub is labelled anhydrous and chills the water by 5c. in 1.5 minutes.
So this should tell me it is labelled incorrectly, or is it?

Can anyone confirm this test or recommend another simple test to identify penta. from anhydrous?


Cheers.
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Old 30th May 2021, 08:19 PM
Mike O'Pray Mike O'Pray is offline
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No actual knowledge of either, Nat, but yes from research you are right that only the penta will cool water down and perhaps more importantly it would appear from a 2005 thread in Photrio that some suppliers sell stuff they call anhydrous erroneously when it is actually penta. As this was the U.S. I cannot say that non U.S. suppliers make the same mistake as only the U.S. was mentioned but this would at least explain the "puzzle" of why stuff that looks to be crystalline and not powder was labelled wrongly as anhydrous when it isn't

Mike
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Old 30th May 2021, 08:40 PM
Nat Polton Nat Polton is offline
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Thanks Mike.
Seems like I am not the first to be in this dilemma.


I have thought of buying both types from a specialist lab supplies outlet, then comparing them, but the cost of chemicals from these places is through the ceiling.

Hopefully some one will offer a simple, cheap test.

Cheers.
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Old 30th May 2021, 09:56 PM
Mike O'Pray Mike O'Pray is offline
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I hope you find a reasonably priced supplier. For what it is worth and in the U.K. probably not much it would seem that there is stuff at your local Pool Supplies retailer that is penta and much cheaper. Unfortunately local is in the U.S . where in some states the climate supports pools which I assume to swimming pools. However home pools are not entirely unknown in the U.K. so I wonder who are the suppliers here?

Mike
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Old 31st May 2021, 09:50 AM
John King John King is offline
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Are these crystals on the longer sides roughly hexagonal in shape, if so that is the crystalline form, Anhydrous is usually a coarse powder about the same as caster sugar
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Old 31st May 2021, 10:43 AM
Lostlabours Lostlabours is offline
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It sounds like Sodium Thiosulphate Pentahydrate that's the right crystal size. The anhydrous form absorbs water so is generally only sold as an analytical reagent.

Photographic grade Sodium thiosulphate is always the Pentahydrate. It's sold by swimming pool suppliers to neutralise the Sodium Hypochlorite used to treat pool water (usually overnight). So a 25kg bag is relatively inexpensive.

Ian
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Old 31st May 2021, 11:14 AM
Nat Polton Nat Polton is offline
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Thank you John and Ian.

A couple of snaps. One of the crystals stood upright on the blue tack roughly displays a hexagonal shape, but the rest, although they have a longitudinal grain are roughly shaped.


http://www.film-and-darkroom-user.or...1&d=1622459404

http://www.film-and-darkroom-user.or...1&d=1622459404


Thanks again gents.
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Old 31st May 2021, 02:39 PM
Lostlabours Lostlabours is offline
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That's definitely the Pentahydrate.

Ian
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Old 31st May 2021, 06:31 PM
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Uwe Pilz Uwe Pilz is offline
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Pentahydrate solves in its own crystal water if heated. Very impressive.
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Old 1st June 2021, 07:42 AM
Nat Polton Nat Polton is offline
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Some extracts from The Manual of Photography, Focal Press, under the sub title of "Processing Following Development, Making up fixing baths."


Talking about Sodium thiosulphate,

+++++++++++++++++++++++++++++++++
"This is available in two forms: crystalline (sodium thiosulphate pentahydrate) and anhydrous. The crystals are fairly clear and hexagonal about the size of a pea. Anhydrous sodium thiosulphate is a white powder. Crystals of sodium thiosulphate contain 36% of water and only 64% of salt, whereas anhydrous hypo contains practically 100% of the salt."

"The making up of the baths is effected most rapidly if the sodium thiosulphate is first dissolved in hot water. This is especially advantageous when crystalline sodium thiosulphate is employed, since solution of this form is accompanied by a fall in temperature. (Anhydrous sodium thiosulphate gets slightly warmer on dissolving.)

+++++++++++++++++++++++

After trawling the internet for a couple of days I have found makers and suppliers advertising anhydrous as being available in crystal or powder form, very confusing.

As mine is crystalline and cools when water is added I have the pentahydrate.

Well worth testing to see what you have before working out your quantities to use, as mine appears to be wrongly labelled anhydrous .

Just take an egg cup size glass and stir in a teaspoon of water.
If the temperature drops you have pentahydrate.
If the temperature rises you have anhydrous.

A very big thankyou to all who have helped me with this puzzle.

Cheers.
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