help with sodium thiosulphate pentahydrate vs sodium thiosulphate anhydrous

rbrigham · Apr 24, 2020

Hi

I'm trying to make a developer which needs 2g of sodium thiosulphate pentahydrate but I have the anhydrous version

how much anhydrous is equivalent the pentahydrate version ?

thanks

robin

iakustov · Apr 24, 2020

I think you need 1.57 times less for anhydrous, so 1.27 g.

Donald Qualls · Apr 24, 2020

Just to check (since the anhydrous is relatively rare): does your thiosulfate have crystals like angular grains of rice, slippery to the touch, or is it a powder roughly like talc with no crystalline character to the eye? If the latter, it's anhydrous, but if it's crystals it's pentahydrate already.

The developer formula you're working to calls for the pentahydrate because the anhydrous is relatively hard to find.

Ian Grant · Apr 24, 2020

I replied on FADU Mr Bell The conversion factor is 1.274156

Ian

fgorga · Apr 24, 2020

iakustov said:
I think you need 1.57 times less for anhydrous, so 1.27 g.

This is correct.

In the interest of education, I am a retired chemistry professor. ;-)

Here is how you figure this out:

First a couple of definitions... "anhydrous" means "without water" and "pentahydrate" means "with five waters".

The formula weight of sodium thiosulfate (Na2S2O2) is 158.11 g/mol.

The formula weight of water (H2O) is 18.02 g / mol. Therefore, five waters are 90.1.

Thus the formula weight of sodium thiosulfate pentahydrate is 158.11 + 90.1 = 248.21 g/mol

Therefore the "conversion factor" between the two is 158.11 / 248.21 = 0.637. (The value cited by iakustov is the reciprocal of this value.)

In other words for every gram of the pentahydrate your recipe calls for use 0.64 grams of the anhydrous form.

Regards,

--- Frank

rbrigham · Apr 25, 2020

Thanks for all the replies everybody

Donald thanks for explaining the difference as I'm definitely not a chemist
I bought it from silverprint 10 years ago and it says anhydrous on the label, but good to know that it matches your description
I actually need 0.2g not 2 g per 1000ml so I'm presently trying to work out how to weigh such a small amount

best

robin

Martin Rickards · Apr 25, 2020

fgorga said:
This is correct.

In the interest of education, I am a retired chemistry professor. ;-)

Here is how you figure this out:

First a couple of definitions... "anhydrous" means "without water" and "pentahydrate" means "with five waters".

The formula weight of sodium thiosulfate (Na2S2O2) is 158.11 g/mol.

The formula weight of water (H2O) is 18.02 g / mol. Therefore, five waters are 90.1.

Thus the formula weight of sodium thiosulfate pentahydrate is 158.11 + 90.1 = 248.21 g/mol

Therefore the "conversion factor" between the two is 158.11 / 248.21 = 0.637. (The value cited by iakustov is the reciprocal of this value.)

In other words for every gram of the pentahydrate your recipe calls for use 0.64 grams of the anhydrous form.

Regards,

--- Frank

The mlecular weight's right but you've managed to lose an oxygen from the formula Na2S2O3

Ian Grant · Apr 25, 2020

rbrigham said:
Thanks for all the replies everybody

Donald thanks for explaining the difference as I'm definitely not a chemist
I bought it from silverprint 10 years ago and it says anhydrous on the label, but good to know that it matches your description
I actually need 0.2g not 2 g per 1000ml so I'm presently trying to work out how to weigh such a small amount

best

robin

I've just posted how to mix a 10% or 1% solution on the other Forum

Ian

fgorga · Apr 25, 2020

Martin Rickards said:
The mlecular weight's right but you've managed to lose an oxygen from the formula Na2S2O3

Oops... sorry about that.

I blame all of those html tags for the subscripts... they make proofreading difficult!

fgorga · Apr 25, 2020

rbrigham said:
Thanks for all the replies everybody

I actually need 0.2g not 2 g per 1000ml so I'm presently trying to work out how to weigh such a small amount

/QUOTE]

Don't even try to weigh 0.2g. Rather, make a stock solution and dilute it out.

Something like this... weigh out 2 grams and dissolve in 100 mL then add 10 mL of this to your final solution.

Save the rest of stock solution for the next batch.

Raghu Kuvempunagar · Apr 25, 2020

Which is this developer that requires 0.2g of Thiosulphate and what purpose does it serve in the developer?

Ian Grant · Apr 25, 2020

fgorga said:
Don't even try to weigh 0.2g. Rather, make a stock solution and dilute it out.

Something like this... weigh out 2 grams and dissolve in 100 mL then add 10 mL of this to your final solution.

Save the rest of stock solution for the next batch.

The 0.2g per litre is using Sodium Thiosulphate Pentahydrate, so a 10% solution would need 63.7g per litre of Sodium Thiosulphate anhydrous to be equivalent and you'd add 2ml. 10% Sodium Thiosulphate is always useful for Farmers Reducer.

Ian

Donald Qualls · Apr 25, 2020

rbrigham said:
I actually need 0.2g not 2 g per 1000ml so I'm presently trying to work out how to weigh such a small amount

I've got an old CCI reloading scale that I've had since 1981 (RCBS, Lee Precision, and several other companies make effectively identical units). This is an analog beam scale that will measure against a preset mass to a precision of 0.1 grain. Since a gram is 15.4 grains, you can easily measure down to about 6 milligrams with one of these -- and they're still only about $40 new. Despite the low price, these have adjustable zero, so you can compensate (for instance) for using a weighing paper on the pan, and a leveling foot to compensate if your darkroom counter isn't quite level.

You can also buy small electronic scales that will weigh to milligram accuracy, but they require a battery and i doubt they'll last forty years without maintenance...

help with sodium thiosulphate pentahydrate vs sodium thiosulphate anhydrous

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