Glycerin as by-product of soap making

As a percentage of the total batch (approximately), does anyone know how much glycerin is generated during CP saponification? I read somewhere that hand-made soaps contain 25% glycerin but quickly want to check with y'all if it's true.

It is kind of correct, because when saponification is completed you end up with 1 molecule of glycerin for each 3 molecules of salt (soap). So roughly 25% of the molecules are indeed glycerin (if you ignore water and other inert additives, such as clay).

You won't, however, get 25% glycerin by weight.

A rough way to calculate the amount of glycerin in a batch of CP soap is:

amount of NaOH used, divided by 40g (approximate molecular weight of NaOH), divided by 3, times 92g (approximate molecular weight of glycerin)

So if you use 150g of NaOH in your soap, you will have approximately 115g of glycerin.

Thanks for that explanation about glycerin content. I did not know that. Always fun to start off the day learning something new.

I want to thank you as well. I've always known (since starting to make soap anyway) that real soap retains it's glycerine, but I have no way of letting people know just about how much is actually in the soap. This is great information for both customer and producer.

You are both very welcome. Glad I could be of some help. I am kind of a geek when it comes to making soap - don't ask me about swirls and imbeds and colors, but I love the math! :twisted:

Seifenblasen said:

You are both very welcome. Glad I could be of some help. I am kind of a geek when it comes to making soap - don't ask me about swirls and imbeds and colors, but I love the math! :twisted:

Click to expand...

I love your posts because of your "geeky" answers I hate math (Algebra? Bah Humbug!), but if its "soap math", I love it :wink:

Yo, Seifenblasen, are you German?

Nein, aber mein Mann ist!

Achso, daher die "Seifenblasen", was?

Actually the percentage of solids which is glycerin is closer to 10%, not 25%. You have to go into the chemistry a little bit.

the chemical reaction is (in words)

triglyceride + 3 molecules of (NaOH) ------> (saponification) ------> glycerin + 3 molecules (soap)

This is for a pure soap, lets say sodium stearate as an example. What we normally get is a combination of soaps from the oils we combine. Even for a single oil, you get a combination of soaps since no oil is made of a single fatty acid, they are all a combination of several fatty acids, or more precisely triglycerides. For sodium stearate for example the formulas are like this:

(CH2)16CH3 OC lNaOOH H2COH2C +3NaOH -----> CHOH +3COHC O H2CC(CH2)16CH3H2CH2C H2C H2C H2C H2C H2C H2C H2C H3CCH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2OHOCOO (CH2)16CH3

Glycerin has this formula: C3H8O3

It is easier to visualize with a 3D representation of the reaction, where you see the triglyceride 'head', which is the glycerol molecule, react with the OH from the Na Oh to become glycerine, while the Na gets attached to each of the three fatty acids in the triglyceride, and you get the 3 soap molecules, which are polar, one side has a Na atom (which likes water) the other side is oil (repels water but likes oil).

To get an estimate, average all your different soap molecules to one average carbon chain length, determine the molecular weight and compare. You can even look it up, goggle for example molecular weight of glycerine, and molecular weight of a pure soap (sodium stearate is an easy one to find). Just remember that you get 3 molecules of soap and only one molecule of glycerine. This will give you an estimate. Add molecular weight of glycerin plus three times the molecular weight of your chosen soap. Now take the molecular weight of glycerin and divide by that total number (glycerin plus 3 soap molecules). This is your glycerin fraction.

You get a greater proportion of glycerin per total soap when you use the shorter chain fatty acids, like lauric acid for example. 12 carbons instead of let's say 18, makes the glycerol portion a greater part of the original triglyceride molecule, so the weight proportion of glycerin that you get from let's say a pure coconut soap will be more than from a pure olive oil soap (18 carbon chains for oleic acid).

I did a couple of numerical examples because I too was curious about what the fraction was, and I figured this would be clearer.

molecular weight of glycerin: 92.09 (all in grams per mole)

molecular weight of sodium stearate 306.45

molecular weight of sodium olivate 304.44

molecular weight of sodium laurate 222.3

So % glycerin in solid part of soap for a pure sodium stearate soap = 92.02/(92.02 + 3(306.45)) = 9.1% of the solid part of your resulting cold process soap is glycerin. This is if you were able to make a pure sodium stearate soap.

Looking at the smaller sodium laurate molecule and doing the same thing gives us 12.13% glycerin as part of the total soap solids. See? I told you that using all coconut would give us a soap with a greater portion of glycerin, just because they are shorter fatty acid chains, lower molecular weight. Remember that part of the weight of your soap is water, and this varies as the soap cures. This calculation pertains only to the solid (non water) part of your soap.

So here you go, your answer is about 10%. It depends on what fatty acid composition you have, but it will range from 9 to 12%. The only common triglyceride chain even shorter than lauric acid is capric acid, (found in goat's milk).

I don't like to argue, but if you go back and re-read my very short post, I maintain that my calculations are correct.

I must also add, my explanation is also more concise and to the point without going into all the chemistry. While I enjoy it and have worked as a research scientist for an explosives manufacturer (and unlike making soap, a little error in calculation can literally, be fatal), not everyone is into it.

This is a soap making forum, and I thought some folks would find the chemistry interesting. I do, anyway.

I did not mean any insult, and I apologize if I have ofended you. I just wanted to show people how one would come up with this answer. I calculated this when I formulated a shave soap, so just shared the calculations with the forum when the question came up.

http://www.ijens.org/Vol_12_I_02/122402 ... -IJENS.pdf

In the paper linked above, it is stated in the introduction that cold processed soap contains about 10% glycerin. They talk about recovering (extracting) the glycerin from cold processed soap, they manage to recover only about half that amount. Interesting read.

1. You started out stating that the percentage of glycerin is closer to 10%, not 25%. The question is, percentage of WHAT and in what UNITS? Hausfrau007 was correct that the glycerin MOLECULES compose of 25% of the molecules resulted from saponification. (And since you want to "go into the chemistry", you should know that in chemistry we often deal with the number of molecules instead of weight).

2. I specifically stated that one would not get 25% of glycerin by weight.

3. In chemistry, we strive for the simple, elegant solutions. In normal soap making, we want all the lye to be used up in the reaction, therefore the amount of lye is the limiting factor, and therefore it is actually more accurate to calculate the weight of glycerin based on the amount of lye. The molecular weight of different oils are irrelevant.

To calculate the weight of glycerin, one only needs to know the among of lye used (and I hope all of us know how much lye we use!) to come up with the weight of the glycerin. As to percentage, it is as simple as divide the weight of the glycerin by the weight of the entire block of soap. One simple calculation, throw the soap on the scale, and voila, percentage of glycerin! It is something everyone can understand and explain to a customer.

The thing is, if I made a mistake and someone were to call it out, I would gladly accept the correction. No issues with that what so ever. But when someone starts the first sentence of a post saying that I am wrong and need to "go into the chemistry" a bit without even reading a few very short paragraphs first, it can be rather annoying.

I will stop now. Apologies to everyone if I sound irritated. Because I am, kind of.

Seifenblasen said:

It is kind of correct, because when saponification is completed you end up with 1 molecule of glycerin for each 3 molecules of salt (soap). So roughly 25% of the molecules are indeed glycerin (if you ignore water and other inert additives, such as clay).

You won't, however, get 25% glycerin by weight.

A rough way to calculate the amount of glycerin in a batch of CP soap is:

amount of NaOH used, divided by 40g (approximate molecular weight of NaOH), divided by 3, times 92g (approximate molecular weight of glycerin)

So if you use 150g of NaOH in your soap, you will have approximately 115g of glycerin.

Click to expand...

Thank you for this - it is very informative and at a level I can understand without my brain swimming and my eyes going backward into my skull. I do enjoy knowing some of the science behind what I do I am not a scientist nor a chemist, so I like the KISS method.

Green Soap most of us here are not scientists nor do we wish to become ones. Your reply was antagonistic and unnecessary, not even sure why you made it. This is a great conversation and let's keep it friendly. Competition like that does not make for a polite thread and drama is so not appreciated....

Cheers

This is the label I have found of a soaper that sells soap...

"Ingredients : Saponified extra virgin oil (sodioum olivate), Saponified Coconut Oil (Sodium Cocoate 5%), extract and grated peel of lemon, Natural Glycerin , Aqua."

does the natural glycerin that it is written actually mean the glycerin that is produced when saponification takes place or you believe they have added it as an extra compound in the soap making process?

This thread is over two years old, you may not get an answer from some of the people.

My uneducated opinion is that it could be either. It would not be correct if they did not actually add glycerin, but when has that stopped anyone? OTOH, they could have used some glycerin to dissolve the NaOH.

Just guessing but that looks like a "what comes out" label, with saponified oils of, not a "what went in" label as they do not list lye. So that is probably the glycerin created, not added.

I believe in Canada (and possibly the EU?) they list what is actually in the already made soap, not what ingredients they used to get there, but as I am in US I am not sure.

I'm with N12S - it's a label with what is in there now rather than what went in.

Of course, it's a very badly done list as it stands, but hey........