Chemical Forums
Specialty Chemistry Forums => Citizen Chemist => Topic started by: soap_newbie on January 10, 2012, 03:47:10 PM
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Hi everyone,
I've been making soap as a hobby for a few months and I have noticed that the resulting pH is relatively high (i.e. usually 8-9) even after curing. From what I have read this is because during saponification a mild acid (oil) reacts with a strong base (NaOH), so the resulting salt has to be alkaline. Of course I make sure I use more oils than NaOH can saponify, so that there is no lye left in the bar.
So far so good, but my skin is extremely sensitive and I prefer neutral or slightly acidic pH. While all my friends love my soap, I find it very drying for my hands. Even professional organic soap bars irritate my skin. But liquid cleansers are usually mild enough, which makes me think that the pH of liquids is more easily manipulated than that of bars.
My question is:
If I stick to making natural soap with vegetable oil and NaOH and still want it in a soap bar, is there a way to lower the pH?
Some thoughts:
1) If I melt the soap after it has cured and add some acid (e.g. citric acid) to it, would that help? Would the acid react with the soap? Would it result in sludge?
2) If I add an acid to the oils before saponification, what should I expect?
Thank you in advance!
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I'd only read about making homemade soaps in books, so while I do understand what I suspect you're doing, I can't really help. I also made soap in basic chemistry class, so I have a suggestion based on what manufacturers do. First be certain you're following a good recipe for soap. And be sure you know the exact concentration and amount of your sodium hydroxide, and proper good quality fat to make your soap. I'd suggest a plant oil like olive oil, its likely purer than rendered animal fat. What you can also do is salt out the product with sodium chloride. After precipitating out the soap with salt, and rinsing the salt free of the curdled soap, much of sodium hydroxide should be left behind.
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I made my own soap a few years ago from beef tallow and it was so caustic I would get chemical burns every time I had a shower lol. Its pretty simple to lower the pH. Just melt the soap, add an acid, stir vigorously and let it resolidify. You could use lemon or lime juice which would also give the soap a citrus odor.
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When you saponify fats with NaOH, you are making the sodium salts of the fatty acids. That is why they are still basic. You can protonate the fatty acids by lowering the pH of the solution, but the protonated forms don't have as high a melting point as the sodium salts - odds are that you would end up with a liquid rather than a solid soap.
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Thank you all for your answers!
You can protonate the fatty acids by lowering the pH of the solution, but the protonated forms don't have as high a melting point as the sodium salts - odds are that you would end up with a liquid rather than a solid soap.
Do you think it will make a difference if I protonate saturated fats? Is this going to increase the melting point?
Just melt the soap, add an acid, stir vigorously and let it resolidify. You could use lemon or lime juice which would also give the soap a citrus odor.
Sounds very interesting! Does this affect the quality of the soap (e.g. lather)?
What you can also do is salt out the product with sodium chloride. After precipitating out the soap with salt, and rinsing the salt free of the curdled soap, much of sodium hydroxide should be left behind.
Does anyone have instructions for salting out soap? I'm guessing this refers to hot process soap making; just after the cooking? Do I need to use more water in order to collect the curds? Because the amount of water I use is 1/4 of the oils by weight and the texture of the soap after cooking is like mashed potatoes.
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Yes. The hot liquid saponified mixture is mixed with a saturated sodium chloride solution. I don't know the amounts, maybe a home soap making source has a recipe for you. The curdled soap will float, you'll strain it out, rinse it with some water to get rid of excess salt (losing some soap in the process), then, I guess you'll press the curds into blocks and let it dry. If you're sensitive to the high pH of your homemade soap, you'll want to switch to a salted out recipe.
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Many thanks.
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I just made my first bar of soap from amaranth oil and I found the pH to be 13 after 50 min at 80C. I heated for another hour and it dropped to pH 12. I then added about 4-6 g of citric acid in 1 lb batch and it dropped to a more skin friendly pH of 6.5. It foams lovely on the hands. You can measure the pH by purchasing Panpeha pH indicator strips range 0-14 from Sigma Aldrich (no affiliation), just smear the soap over the strip if it is too solid, and read from the scale. I consider pH meters to be an expensive hassle. I used sap number of 137.8 mg/g NaOH.
Doc Ray Schep.