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Old May 20th 2019, 02:40 PM   #1
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Soap Saponification Question

Hi, I'm trying to understand what chemically happens when soap is made with rancid oils (fatty acids). No one ever seems able to answer it, so I'm hoping someone here can help (I'm not studying chemistry, I make soap).

In a typical batch of soap, you add lye solution (NaOH+H2O) to a fatty acid solution (not a single oil, but multiple oil blends). You mix it all up, add heat, and you get soap + glycerin+ water which evaporates out while drying.

Using rancid oils however also produces soap. The 'word on the street' is that if you use rancid oils to make your soap, your soap will get rancid. It does NOT get rancid immediately, it makes regular soap, not rancid soap.

Where do the rancid aldehydes and ketones go from the smelly oils if it's making regular soap?

For illustrative purposes, here are the formulas for 2 common soaping oils:
Peanut Oil: C30H45N9O5
Coconut Oil: C4H8NNaO2

Lye Solution: NaOH + H2O

Of course, more oils are used than the above, but to keep it as simple as possible, lets say there's a 50/50 of Peanut Oil and Coconut Oil blended. This is mixed with the lye.

If you were to use rancid oils, that would be CHO for the aldehydes, and RC (R being a carbon group) for the ketones....

How does saponification differ after adding the rancidity factor (all fatty acids used are rancid)?

I would be REALLY grateful for an answer, this question has puzzled me a long long time. Thank you.

Last edited by glassgal; May 20th 2019 at 02:42 PM.
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