Solubility of thiourea in water

LarsAC · Oct 23, 2015

Hello,

1) would like to mix my own toner on thiourea basis. I found a recipe here which commands to dissolve 200g of thiourea (thiocarbamide) in 1000ml water. On wikipedia, I found a solubility of 14.2g in 100ml water at 25C. Sounds to me, I will not succeed in dissolving the above mentioned 200g. Did I overlook something ?

2) Instead of dissolving 100g of sodium hydroxide in 1000ml water, is it correct to simply dilute 20% NaOH with water 1:1 ?

Thanks for ideas / help.

Regards,
Lars

PinRegistered · Oct 23, 2015

If you heat the water, you would be able to dissolve more crystals. Keeping the molecules in suspension will be a matter of storage conditions. Crystals will form if a saturated solution is disturbed.

I would dilute 1 part 20% NaOH with 2 parts H2O to get the equivalent concentration of 100mL to 1000mL

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ME Super · Oct 23, 2015

Solubility also may depend on what other chemicals are or are not present. IIRC, certain developers won't dissolve in water very well if sulfite is present, so when mixing the developer you put in the developing agent first, and stir until it dissolves before you add the sulfite. IDK if this is the case for thiourea or not.

pentaxuser · Oct 23, 2015

In his book Tim Rudman's thiourea ratio seems to be 100g to 1000ml so 200g of thiourea seems very high. I hope in saying this I haven't infringed any copyright. For chemists here to help you it might be advisable if you give all the ingredients

pentaxuser

LarsAC · Oct 24, 2015

Thanks for the answers. I will crosscheck some other recipes to verify the numbers.

PinRegistered: how did you determine that 1:2 ratio?

Lars

pentaxpete · Oct 24, 2015

I use Thiourea Sepia Toner and no longer weigh out the chemicals-- 1 litre is usually too much -- you don't need so much unless you are doing 'dozens' of prints !! I get a pot of warm water about 300mls and put in half a teaspoon of the Thiourea ( be CAREFUL not to SNIFF up any of the powder !) and stir then one tablet of Sodium Hydroxide and stir and it dissolves OK .. For the re-halogensing Bleach I take half a teaspoon of Potassium Ferricyanide and half a teaspoon of Potassium Bromide and dissolve in warm water. about 300 mls in usually enough-- if too strong a bleach you can always add more water into the dish.

Doremus Scudder · Oct 24, 2015

LarsAC said:
Thanks for the answers. I will crosscheck some other recipes to verify the numbers.

PinRegistered: how did you determine that 1:2 ratio?

Lars

The confusion here could stem from the different ways chemists (and photographers) indicate ratios. The pesky colon ":" in ratios is used two ways. That's why I prefer the plus sign "+" instead. 1:2 taken strictly means one part of original solution in two parts total of final solution. I'd indicate this as 1+1 (one part + one part = two parts).

1+2 is wrong, however.

Solutions in percentages by weight are figured like this: x grams in 100ml = an x% solution.

100ml of 20% solution contains 20g (of whatever, by weight). To make that a 10% solution, add 100ml of water to make 200ml total, again with 20g of whatever. That's now a 10% solution.

Now on to NaOH; you need 100g. 1000ml of 20% NaOH has 200g of NaOH in it. You need half that, so simply take 500ml of your 20% solution (this has 100g NaOH in it) and add water to make 1000ml. So - 1+1 it is!

A 1+2 solution would be: 100ml water containing 20g whatever expanded to 300ml containing 20g whatever. That's a 6.666...% solution and 1000ml of it would contain 66.66...g, not 100.

Best,

Doremus

Prof_Pixel · Oct 24, 2015

Doremus Scudder said:
The confusion here could stem from the different ways chemists (and photographers) indicate ratios. The pesky colon ":" in ratios is used two ways. That's why I prefer the plus sign "+" instead. 1:2 taken strictly means one part of original solution in two parts total of final solution. I'd indicate this as 1+1 (one part + one part = two parts).

1+2 is wrong, however.

Solutions in percentages by weight are figured like this: x grams in 100ml = an x% solution.

100ml of 20% solution contains 20g (of whatever, by weight). To make that a 10% solution, add 100ml of water to make 200ml total, again with 20g of whatever. That's now a 10% solution.

Now on to NaOH; you need 100g. 1000ml of 20% NaOH has 200g of NaOH in it. You need half that, so simply take 500ml of your 20% solution (this has 100g NaOH in it) and add water to make 1000ml. So - 1+1 it is!

A 1+2 solution would be: 100ml water containing 20g whatever expanded to 300ml containing 20g whatever. That's a 6.666...% solution and 1000ml of it would contain 66.66...g, not 100.

Note PinRegistered said: "I would dilute 1 part 20% NaOH with 2 parts H2O to get the equivalent concentration of 100mL to 1000mL" which is wrong.

LarsAC · Oct 24, 2015

Thanks, so I got my chemistry / maths correct. Will of course wear protective gear (eyes / nose / hands).

I figured the recipe in Rudman's book (it is also in "The Photographers Master Printing Course" which I own). It is not completely clear, whether this is a concentrate or working solution strength, however.

Lars

PinRegistered · Oct 24, 2015

Thanks for the correction.

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dburian · Oct 26, 2015

Just to keep things confused, Anchell copies the Photographers Formulary formulae for stock solutions: 5% thiourea (B); 10% NaOH (A).
For brown tones the mix is 14 mL A, 14 mL B, water to make 250 mL total.
D

Solubility of thiourea in water

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