Glycerin/Water percentages with Potassium Hydroxide

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Greenly

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Can anyone tell me what the formula is for working out what percentage yields there are for water and glycerin when you use Potassium Hydroxide to make soap rather that Sodium Hydroxide? Is it the same? I need to know so that I can put ingredients on my labels in the right order.
Thanks in advance for any help,
Lindsay
 

Susie

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If you are talking about how much water and glycerin is needed, it is different for each batch, but I can help.

If you are asking how much is created by the chemical reaction, you need DeeAnna.
 

DeeAnna

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Hey, I heard my name called! Hi, Susie and Greenly!

If you want to know the amount of glycerin made by saponification, I think I have what you need on my website. See: https://classicbells.com/soap/glycerin.html I updated this article today to include calculations for NaOH-only soap, KOH-only soap, and dual lye (NaOH + KOH) soap.

The water content in a bar soap is whatever water you used for the recipe less the water lost by evaporation during cure. I can't give you a formula to predict that-- the weight loss varies somewhat depending on the recipe and on the environment in which the soap cures. A rough estimate is that the soap will lose 8% to 10% of its total weight, but if you need accurate numbers, you need to measure the weight loss of your soap during cure. Essentially all of that weight loss will be water. You know the water in your newly made soap, you know the percentage of water loss, so you can back calculate the water content after cure.

Of course, if you're making diluted liquid soap, the whole problem is quite different. You will be adding water for dilution in addition to the water in the soap paste when it is fully saponified. You'll just need to keep careful records of the paste weight after saponification and the water added for dilution.
 
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Dahila

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Naoh x 0.77 = glycerin
Koh x 0.44 = glycerin
water will be around 12% and glycerin just after aqua :)
 

DeeAnna

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Dahlia -- the OP is making KOH soap. I'm not sure if she's making liquid soap or a shave croap or what, but regardless, the water content is going to be higher than 12%.

You also say "Koh x 0.44 = glycerin" I'm getting a different number -- KOH x 0.55 = glycerin. I'm not saying I know I'm right -- but what I do know is we both need to agree on a single formula. Otherwise, people are going to get confused. :) Could you double check your math and set me straight about why 0.44 is the right number?
 

Dahila

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Yes she needs to add the one she used in dilution but if she used only water KOH times 0.44 will give her the amount of glycerin, So summing up the water it should be on first place on the top?
I did Cosmetic Notification form with KoH LS and somehow it came ok........ I do not remeber exactly how I counted it, my SM3 is very helpful in situation like that
 

DeeAnna

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But what I"m trying to say is I'm coming up with 0.55 NOT 0.44. And I'm trying to figure out why your number is different than mine. Here's how I calculated this --

Molecular weight --
39.997 g/mol NaOH
56.1056 g/mol KOH

Stoichiometric ratio of KOH to NaOH = 56.1056 / 39.997 = 1.403

The stoichometric ratio tells a person how much weight of one chemical is chemically equivalent to another. So if X grams of NaOH will do a given job, you'd need 1.403 times more KOH to do the same job because a molecule of KOH weighs 1.403 times more than a molecule of NaOH. This formula shows this relationship --

Formula 1 --
wt KOH = 1.403 X (wt NaOH)

Using some algebra to rearrange this formula, you can figure the chemically equivalent weight of NaOH from a given weight of KOH like this --

Formula 2 --
wt NaOH = wt KOH / 1.403

So if we agree the calculation for the glycerin made by NaOH is this --

Formula 3 --
glycerin wt = 0.77 X (wt of NaOH)

... and we want to convert this formula to the equivalent version for KOH, we would have to substitute the stuff on the right side of formula #2 whereever you see "wt of NaOH" in Formula #3. Like this --

Formula 4 --
glycerin wt = 0.77 X wt KOH / 1.403

Dividing 0.77 / 1.403 = 0.54, so tidying up Formula 4 gives this result --

Formula 5 --
glycerin wt = 0.55 X wt KOH

So either your number is incorrect or mine is ... and I can't see where I've gone astray. So I'm counting on you to set me straight where I went wrong with my chain of thought. Or are you just accepting an answer from the soapmaker program? Either way, I'm confused. Help?
 

Greenly

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Thank you ! I have been trying to find the answer to this for weeks and it took you guys no-time at all to answer.
I am making hot process soap paste with the KOH
For cp soap I have been using (NaOH/40)/3*92 which works out to lye x .77 (which calculation I got off the internet but it may have been from here)
so for this soap paste I can just KOH x .55? or .44?
So say 100g of KOH x .55 =55 glycerin? or .44 grams of glycerin?
All this makes me wish I had paid more attention in Chem.
 

DeeAnna

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"...So say 100g of KOH x .55 =55 glycerin? or .44 grams of glycerin?..."

Unless Dahlia can confirm the reasoning behind her use of 0.44, I'm going on the record to say the correct number is 0.55.

And I want to add this warning about the limitations of this method --

This formula is based on the assumption that ALL of the lye reacts with triglyceride fats to make soap, so it will not be accurate if your recipe includes one or more of these characteristics --

Unusually lye heavy. The recipe can have a zero % superfat, a slight negative superfat % (down to about -3%), or any positive superfat %.
Fatty acids (example: stearic acid)
Ingredients that consume lye, but do not make soap (examples: rosin, pine tar)

Any soap that is very lye-heavy or made with fatty acids or other unusual ingredients will contain less glycerin than you will calculate from this formula.
 
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Greenly

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Thanks Deeanna,
I will use factor .55, I was pretty sure that it would be different from the NaOH number, and I appreciate the reasoning behind your answer and since none of the caveats apply, should be accurate enough.
I am now working through your soapy tutorials .... very informative!
 

cmzaha

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This is why I like to label what goes in the pot not what comes out of the pot. :) I need Simple!! But that is acceptable for US labeling not sure for others
 

Dahila

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I got this formula from our guild but will look who gave it. It was a chemist
oops is 0.55 not 0.44 for KOH I am sooo sorry DeeAnna like always is right ,
Glycerin calulations:

NaOH it is lye amount x 0.77 = glycerin e.g. 136g x 0.77 = 104.72 g of glycerin

KOH it is lye x 0.55 = glycerin amount

Here is the calculation in use and why from Scott:
1g NaOH gives 0.77g glycerine
1.4g KOH gives 0.77g glycerine
so 1g KOH gives you 0.55g glycerine


Quick explanation - it's the OH part of the alkali (lye) that "makes" glycerine in the saponification reaction - K atom is heavier than Na atom so you need more KOH (weight) than NaOH to provide the same amount of OH

When using NaOH, 1g of oil/fat gives 1 g soap salts
When using KOH, 1g of oil /fat gives 1.4g soap salts

Scott owns http://www.cosmeticsafetyassessment.com/

There is link to follow. ........
 

DeeAnna

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Thank you, Dahlia. I'm not always right ... my DH tells me so! ;)

Seriously, thought, I know I'm not always right and I do worry about that. Sometimes I make errors and sometimes my understanding of a topic isn't as accurate and complete as I'd like. I always reserve the right to make corrections as I learn more.
 

Dahila

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Your DH know nothing, he is like mine, I found your sharing of knowledge fantastic, I feels so good, to read your posts
 

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