pH of ready soap

Different oils will give different pH when used for soap making, that is sure.

However as measuring exact pH is difficult without lab equipment, I wish someone (DeAnna or anyone else with real chemistry knowledge) to tell me if I am right with the hypothesis that higher SAP value will produce a soap with higher pH. Should it not cause more free OH- to be available?

"...higher SAP value will produce a soap with higher pH..."

Nope, it turns out this is not correct. Stearic and oleic acids (olive oil, lard, palm, tallow) have the highest pH and pKa values of the fatty acids commonly found in soap. The pH and pKa are lower for the shorter chain saturated fatty acids (myristic, lauric from coconut oil, PKO) and for the longer chain unsaturated fatty acids (linoleic, linolenic from sunflower, safflower, etc.). Source: Scientific Soapmaking, Kevin Dunn.

"...Should [high sap value] not cause more free OH- to be available? ..."

It sounds like you may be confusing the chemistry of saponification with the chemistry of ionic dissociation.

The sap value (grams of lye per grams of fat) is a measure of how much lye is required to exactly react with certain fat molecules on a weight basis. The sap value is based on weight for our convenience, since it's hard for us to count molecules, but you really have to look at the molecules when talking about the chemistry of saponification.

If the fat molecules are smaller, more of them will fit into each gram (or per ounce) than large fat molecules. That means the sap value for short-chain fats (those with lots of myristic and lauric acid) will be higher than the sap value for long-chain fats.

Looking at saponification on a molecular basis, 3 molecules of lye are required to saponify 1 molecule of fat, regardless of the length of the fatty acids that make up the fat. Out of that reaction you get 3 molecules of soap and 1 molecule of glycerin. That is the basis on which the sap value is determined -- that the lye must exactly equal that needed to saponify the fat. There is no OH- left over from this reaction to affect the pH.

It's like setting a table for a holiday meal -- you count the number of people to feed and set the table accordingly. It doesn't matter how big, tall, short, or small each person is. Every person requires a place setting, regardless of their size.

If you deliberately add more lye than the amount needed for saponification, then, yes, the pH would be higher. But that's a whole 'nother story.

edit: In the table below, "Laurie" should be "Lauric". Oops.

Dee Anna the science guru to the rescue again......you rock!

Awesome, interesting idea and interesting answer. Love it!

Thanks DeeAnna! I always learn something useful when you explain! I am collecting all your nuggets of knowledge into a file!

You're welcome, everyone. I'm glad I could help. Now to see what Engblom thinks!

When I do explanations like this, I tend to simplify somewhat, and there's always the possibility of simplifying too far -- but I'd rather start simple than go waaaay over people's heads. So ask again if something's not clear or you're not getting the detail you need.

Thank you!

Pka Value for Castor Oil?

I was wondering if anyone knew the pKa value for castor oil, I get the impression since it is composed primarily of ricinoleic acid, that the pH and pKa values may be lower, making it more suitable for a low pH formulation such as for shampoo.

I have ordered Kevin Dunn's book on Scientific Soapmaking and I can't wait to read it!

Thanks for any help!

Lisa

Non-soap shampoos generally have a pH between 5 and 7. At that range of pH, a true soap will break down into fatty acids.

Only when the pH is alkaline can a soap actually be a soap -- meaning a salt of sodium (or potassium) alkali and fatty acids. A pH below about 8 is not feasible for a soap product, so it cannot meet the criterion of having an acidic pH.

The use of castor oil in lye soaps used as shampoos, IMO, is to get a soap that rinses off cleanly.