Sciencemadness Discussion Board

Stoichiometry of soap

weeksie98 - 12-4-2014 at 17:37

I'll get this out there: I hate following recipes, I love the independence chemistry allows. You can understand my annoyance, then, when I find that the oil I am about to use to produce soap is a mixture of various fats, proportions of which are unspecified, and it is nearly impossible to calculate its relative mass. Does anyone have any ideas on how to stoichiometrically calculate the NaOH/oil ratios for scaling up and so my bar isn't painfully caustic?

Thanks!

Etaoin Shrdlu - 12-4-2014 at 17:41

If you have a recipe you're working from, scaling up just means making a bigger batch with all the ingredients in the same ratio, no stoichiometry required. What's the oil? What are you trying to calculate?

ldanielrosa - 12-4-2014 at 22:38

I don't like working from a recipe either. It makes me captive to someone else's errors. Also, I'm not out to make the prettiest or best stinking soap- I just want castile without perfume or dye.

Vegetable fats (and animal fats for that matter) are generally reliable within a type or species, eg. olive oil from Spain will be chemically about the same as olive oil from Italy. That said, you can use a soap calculator of which there are many online.

weeksie98 - 13-4-2014 at 02:30

I know, and soap calculators are very useful, but, for someone who is interested in the chemistry involved, following a recipe or using a calculator is too much like cooking, and prevents the understanding of the maths involved. Also, it kind of prevents me inventing my own formula when I am more experienced, as soap making could become kind of a sub-hobby for me.

Oh well, I'd better stick to the calculator. :(

weeksie98 - 13-4-2014 at 02:32

On reflection, I wonder if, just for the interest, one could isolate one particular molecule from oil, or purchase it separately, and make decent soap stoichiometrically from that... :o

CHRIS25 - 13-4-2014 at 03:34

Well as I understand things so far, even if you make all your own stuff you will not be able to diverge away from one calculation and that is the saponification values, I am open to correction, but this is one piece of "Stoichiometry":( that can not be avoided http://en.wikipedia.org/wiki/Saponification_value

unionised - 13-4-2014 at 04:15

Why worry?
Use a slight excess of NaOH and then wash away the spare caustic with brine.
Alternatively, use a slight deficit of alkali and make a soap which still has a little oil in it.

If you want to be really clever, use an excess of caustic, wash it out with brine and titrate it to find out how much on an excess you used.
Then you can calculate how much to use to get a stoichiometric soap next time.

blogfast25 - 13-4-2014 at 05:49

Quote: Originally posted by weeksie98  
I'll get this out there: I hate following recipes, I love the independence chemistry allows. You can understand my annoyance, then, when I find that the oil I am about to use to produce soap is a mixture of various fats, proportions of which are unspecified, and it is nearly impossible to calculate its relative mass. Does anyone have any ideas on how to stoichiometrically calculate the NaOH/oil ratios for scaling up and so my bar isn't painfully caustic?


‘the independence chemistry allows’??? Meaningless weasel words.

If you know the fatty acid composition of a vegetable oil, then calculating the average molar mass of the triglyceride and the saponification stoichiometry becomes a doddle.

If that composition isn’t known, maybe you could show some ‘independence’ by determining it yourself? Or at a very minimum determine its saponification value? If you don’t know how to do that, chances are you know little about soap, ‘independence’ or not…
Quote: Originally posted by ldanielrosa  
I don't like working from a recipe either. It makes me captive to someone else's errors. Also, I'm not out to make the prettiest or best stinking soap- I just want castile without perfume or dye.


It also makes you ‘captive’ to someone else’s expertise. Or do you think each time someone wants to prepare a known chemical they should reinvent the wheel and ignore any peer reviewed methods out there?

weeksie98 - 13-4-2014 at 05:56

Quote: Originally posted by blogfast25  
Quote: Originally posted by weeksie98  
I'll get this out there: I hate following recipes, I love the independence chemistry allows. You can understand my annoyance, then, when I find that the oil I am about to use to produce soap is a mixture of various fats, proportions of which are unspecified, and it is nearly impossible to calculate its relative mass. Does anyone have any ideas on how to stoichiometrically calculate the NaOH/oil ratios for scaling up and so my bar isn't painfully caustic?


‘the independence chemistry allows’??? Meaningless weasel words.

If you know the fatty acid composition of a vegetable oil, then calculating the average molar mass of the triglyceride and the saponification stoichiometry becomes a doddle.

If that composition isn’t known, maybe you could show some ‘independence’ by determining it yourself? Or at a very minimum determine its saponification value? If you don’t know how to do that, chances are you know little about soap, ‘independence’ or not…
Quote: Originally posted by ldanielrosa  
I don't like working from a recipe either. It makes me captive to someone else's errors. Also, I'm not out to make the prettiest or best stinking soap- I just want castile without perfume or dye.


It also makes you ‘captive’ to someone else’s expertise. Or do you think each time someone wants to prepare a known chemical they should reinvent the wheel and ignore any peer reviewed methods out there?


It represents a desire to put the principles of chemistry into practise, rather than blindly following a recipe. That is cooking. My 'meaningless weasel words' we're meant to illustrate that I enjoy working things out for myself. And yes, I do know what the saponification value is, I was merely looking at it from a different viewpoint.

I think I will determine the sap of different oils, seems the easiest way to go.

blogfast25 - 13-4-2014 at 10:36

Quote: Originally posted by weeksie98  
It represents a desire to put the principles of chemistry into practise, rather than blindly following a recipe. That is cooking.

[snip]

I think I will determine the sap of different oils, seems the easiest way to go.


'Blindly following a recipe'... why do you think there are tomes and tomes (some excellent ones in the library of this website) on preparative chemistry? You're creating a false dichotomy. Preparing things according to a recipe is just as much chemistry.

At least the last bit of your post is correct.

But if you really wanted to so some more original work on soaps, there's plenty choice. Transesterification of oils allows separation of the fatty acids for instance (not easy). Or how about ammonium based soaps?

[Edited on 13-4-2014 by blogfast25]

12AX7 - 13-4-2014 at 14:09

You can average the atomic weights of the typical composition and get an equivalent, or you can titrate it and measure it directly.

BTW... is it reasonable to melt down caustic soap and add oil, or vice versa? The good old empirical route!

Tim

[Edited on 4-13-2014 by 12AX7]

ElizabethGreene - 13-4-2014 at 14:19

Hi. Welcome to the random and unpredictable nature of biology.

The $0.64 question is, how much NaOH? Biodiesel processors regularly face a similar problem when transestrification-ing an unpredictable feedstock oil. This requires an unknown quantity of Sodium Methoxide to turn it into biodiesel. The solution is Titration. Take a representative sample, add sodium methoxide (for biodiesel, not soap!) until you get the target pH desired.

You should be able to do the same thing for soap.
The internet (http://amrita.vlab.co.in/?sub=3&brch=63&sim=688&...) says that this is the right track. That website lists a procedure, has a simulator, and references.

HTH,
Greene