Digressions around the 100% olive oil soap

[DeeAnna]

26% salt Saturated solution 26g/100g...

Please keep in mind that is the solubility of salt in a mixture of salt and plain water. Not the solubility of salt in a sodium hydroxide solution.

"...150g lye in 150g water total needed 250g to hold 26g salt and NaOH in solution...."

It doesn't quite work that way. If you make a 26% solution of salt and water (26 g sodium chloride + sufficient water to make 100 g total), and then you add sufficient NaOH, you'll find the solution will turn milky. That is salt precipitating out of the NaOH solution -- meaning some of the dissolved salt has returned to its solid form.

The problem being addressed was that a small amount of salt was not dissolving easily when it was added to the lye solution. From a practical sense, reversing the add order (salt first, lye second) solves the problem (the salt gets fully dissolved first).

I realize what the question that was asked. I gave a valid answer.

If you want a solution of NaOH, salt (sodium chloride), and water that is saturated with salt, the order of mixing doesn't matter. Add the salt first, add it second -- it really doesn't matter. You will end up with a solution of NaOH and water that is as saturated with sodium chloride as is possible for the given NaOH solution concentration.

What is affected by the order of mixing is what happens to the excess of salt in the mixture. If you add the salt first to the water to get a saturated solution of salt in water and then add the NaOH, not all of the salt can remain in solution. Excess salt will precipitate out as fine crystals.

If you do the reverse -- NaOH first and salt second -- enough salt will dissolve to make a saturated solution, but excess salt cannot dissolve and will remain as larger crystals.

In both cases, you still have a saturated salt solution with any excess salt in the form of solid crystals, it's just the size of the salt crystals that's different.

I realize what I'm saying might sound like I'm picking nits, but if we're going to be talking about the science of how this works, I think it's good to be accurate about the science. Otherwise, let's all just keep it real simple by saying "add the salt to the water and then add the NaOH" and leave it at that.

 

[penelopejane]

Please keep in mind that is the solubility of salt in a mixture of salt and plain water. Not the solubility of salt in a sodium hydroxide solution.

"...150g lye in 150g water total needed 250g to hold 26g salt and NaOH in solution...."

It doesn't quite work that way. If you make a 26% solution of salt and water (26 g sodium chloride + sufficient water to make 100 g total), and then you add sufficient NaOH, you'll find the solution will turn milky. That is salt precipitating out of the NaOH solution -- meaning some of the dissolved salt has returned to its solid form.

I realize what the question that was asked. I gave a valid answer.

If you want a solution of NaOH, table salt, and water that is saturated with salt, the order of mixing doesn't matter. Add the salt first, add it second -- it really doesn't matter. You will end up with a solution of NaOH and water that is as saturated with sodium chloride as is possible for the given NaOH solution concentration.

What is affected by the order of mixing is what happens to the excess of salt in the mixture. If you add the salt first to the water to get a saturated solution of salt in water and then add the NaOH, not all of the salt can remain in solution. Excess salt will precipitate out as fine crystals.

If you do the reverse -- NaOH first and salt second -- enough salt will dissolve to make a saturated solution, but excess salt cannot dissolve and will remain as larger crystals.

In both cases, you still have a saturated salt solution with any excess salt in the form of solid crystals, it's just the size of the salt crystals that's different.

I realize what I'm saying might sound like I'm picking nits, but if we're going to be talking about the science of how this works, I think it's good to be accurate about the science. Otherwise, let's all just keep it real simple by saying "add the salt to the water and then add the NaOH" and leave it at that.

I did understand that (salt in plain water). I was just trying to work out how much water CA need to make a saturated solution. nO the internet it says 450g CA dissolves in 940ml water. Which is about 48% not sure if that is the saturation level or not.

I do understand what you are saying about the salt being fine crystals if it precipitates out of a NaOH solution. However, I have found, for me anyway and it appears others as well, that salt dissolves much more readily in water than it does in a NaOH solution. Even if there is only a tiny amount of salt and lots of water which theoretically should be far below saturation level, it just doesn't dissolve readily. It is a really good idea (no matter what the saturation) to add the salt to the water and completely dissolve it before you add the NaOH. Your smaller crystal explanation makes it even more important.

 

[Dawni]

What I'm getting from this, as someone who sucked at science lol.... Is that it might not necessarily be undissolved (is that a correct word?) salt crystals... Just that since they're bigger when NaOH is added after the salt, they "feel and look" bigger, compared to smaller crystals that might not feel anything against the spoon or container. Maybe?

 

[DeeAnna]

...since they're bigger when NaOH is added after the salt, they "feel and look" bigger, compared to smaller crystals that might not feel anything against the spoon or container....

Yes -- you got it, Dawni!

 

[Dawni]

Woo-hoo! Not so science-challenged at all then... Just slightly lols

Thanks all, very informative thread here

 

[kayak1987]

First of all, what you mean with "CA"?

reading this I started to doubt about what I'm thinking so I decided to take paper pen and calculator to clarify to me some points
I take this starting case: a 400ml of water (135 oz) with 134g (4.72oz) of Lye dissolved,
how much salt (sodium chloride) can it hold before getting saturated?

The answer is not behind the corner, to get this you need some algebra and to solve a second degree equation (that I will skip you)
The problem is that you are putting in the same solution two salts that have a common Ion (Na+) so the sodium from the Lye affects the solubility of the Sodium chloride.
for now it doesn't matter from where we start, but just where we arrive.

and the result is: 76.5 g of salt

why is this result interesting? what does it means?

It's interesting because of this "common ion effect" that means that lye affects solubility of salt (KOH no for example)
so that you can dissolve in those 400mL "only" 76 g instead of the expected 143 g of salt
the other interesting thing is that if you mix two separate unsaturated solutions, i.e. our salt solution and lye solution, you can get a saturated solution of salt.

So that unlike what I was thinking at first, yes, the order you add the ingredientts makes the difference.
Not in the final result, but on how easily you can dissolve them.
considering the lye is much more soluble than salt
my conclusion is:
- by theory the best should be dissolving in two different solution an then mix but it have several problems (ie overheating) and is useless.
- pratically probably the best is to dissolve salt first, then lye, this have 3 major pros, easyer to dissolve the salt, slows down a little the lye reaction with water, and the eventual excess of salt will precipitate under fine cristals instead of having big pieces undissolved
- adding the salt to the lye solution have anyway the pro that you can easily separate the excess of salt from the lye solution that remain on the bottom in big crystals and you probably will never have a milky solution (if it is a problem?).

And yes, you already knows and said this but I need to understand the theory behind

 

[DeeAnna]

"...The answer is not behind the corner, to get this you need some algebra..."

Actually, you need experimental data to confirm the actual solubility, not just algebra. Mathematical models are only guesstimates -- they aren't necessarily accurate unless you can validate them by testing. In this particular situation, your second-order model is quite inaccurate.

 

[SaltedFig]

First of all, what you mean with "CA"?

reading this I started to doubt about what I'm thinking so I decided to take paper pen and calculator to clarify to me some points
I take this starting case: a 400ml of water (135 oz) with 134g (4.72oz) of Lye dissolved,
how much salt (sodium chloride) can it hold before getting saturated?

The answer is not behind the corner, to get this you need some algebra and to solve a second degree equation (that I will skip you)
The problem is that you are putting in the same solution two salts that have a common Ion (Na+) so the sodium from the Lye affects the solubility of the Sodium chloride.
for now it doesn't matter from where we start, but just where we arrive.

and the result is: 76.5 g of salt

why is this result interesting? what does it means?

It's interesting because of this "common ion effect" that means that lye affects solubility of salt (KOH no for example)
so that you can dissolve in those 400mL "only" 76 g instead of the expected 143 g of salt
the other interesting thing is that if you mix two separate unsaturated solutions, i.e. our salt solution and lye solution, you can get a saturated solution of salt.

So that unlike what I was thinking at first, yes, the order you add the ingredientts makes the difference.
Not in the final result, but on how easily you can dissolve them.
considering the lye is much more soluble than salt
my conclusion is:
- by theory the best should be dissolving in two different solution an then mix but it have several problems (ie overheating) and is useless.
- pratically probably the best is to dissolve salt first, then lye, this have 3 major pros, easyer to dissolve the salt, slows down a little the lye reaction with water, and the eventual excess of salt will precipitate under fine cristals instead of having big pieces undissolved
- adding the salt to the lye solution have anyway the pro that you can easily separate the excess of salt from the lye solution that remain on the bottom in big crystals and you probably will never have a milky solution (if it is a problem?).

And yes, you already knows and said this but I need to understand the theory behind

"First of all, what you mean with "CA"?"

CA is an abbreviation for Citric Acid. This thread has a useful list of abbreviations that are used here:
https://www.soapmakingforum.com/threads/the-acronym-and-abbreviation-definition-thread.51841/

"So that unlike what I was thinking at first, yes, the order you add the ingredientts makes the difference.
Not in the final result, but on how easily you can dissolve them."

One of my favourite soaps is made with a supersaturated brine solution as the "water" to make the lye solution - the resulting soap is very fine on the skin. This would not be possible if the solutions were made the other way around

 

[kayak1987]

"...The answer is not behind the corner, to get this you need some algebra..."

Actually, you need experimental data to confirm the actual solubility, not just algebra. Mathematical models are only guesstimates -- they aren't necessarily accurate unless you can validate them by testing. In this particular situation, your second-order model is quite inaccurate.

I can't agree or disagree on what you are saying, it depends on WHY you say this and whithin what limits is inaccurate..
Lye ansd salt solutions are not that complicated models that Chemistry (math) cant describe, but....

About CA, Thank you for the explaination, books says can be dissolved in water in an amount between 750 and 1450 grams per liter at 20° (depending if the anhydrous form or monohydrate) that is really a lot!