Why is soap alkaline?

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engblom

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Even if you superfat a soap, it will be to some degree alkaline. Why is this? Lye + excess of acids should not end up being alkaline.

Is it because the NaOH solution gets too weak for breaking the fatty acids away from the oils and the whole process halts?

Could this explain why even fully gelled soaps get milder by age? Is it because of a natural breaking down into fatty acids take place, which is neutralizing the soap when it gets older?

Could this be an additional reason why washed and salted out soap is very mild on the skin? In a washed and salted out soap, any lye residues are washed away.
 
sounds like you need a chemist to answer this..... let me take a stab with the knowledge that i have...

i think with your statement that base + acid = neutral doesn't apply to soap making. oils are not acid, nor are they basic. they're a fat. so adding lye to oils/fats won't give you the same reaction as mixing baking soda with vinegar. here's wiki's explanation of saponification: http://en.wikipedia.org/wiki/Saponification

a gelled soap is soap that went through saponification quicker than non gelled soap. non gelled soap takes a few more days for the whole process to complete.

when a soap is cured, the water evaporates out of the soap. i would also think that any leftover minute amount of lye is still working eating away at any un-saponified oils left in the bar. so over time, the soap gets hard and mild. all soaps, HP, CP, M&P, would get milder over time. (think almost like fine wine - takes a long time to get really really good)

i'm not sure what washed soap is, but with salt bars, i've never personally used them, but from reading the forum, some people love it some don't, works on some skin and not on others. also, salt bars take a LONG time to cure. the long curing time could account for the fact that it becomes so mild.

i hope that helps some, and that someone else can chime in with some more details.....
 
Salt bars don't really take that long of a cure, 8 weeks is good for most people including me who has a sensitivity to coconut oil soaps.

Also, I didn't find washed and salted soap to mild, in fact it dried my skin so bad that I had to toss it out and I'll never salt out soap again. To answer your question though, yes, any excess lye will get washed out.
 
huh...i thought i read somewhere salt bars take a while to cure? guess i read wrong or am remembering wrong (i vote remembering wrong....). thanks for the clarification :D
 
edit: Um, Neeners, I regret to differ with you. The quick chemistry answer is this: The fats break down into fatty acids and glycerin. The fatty acids react with the lye (a base) to form soap molecules. But read on for the long answer........

Yes, soap will always be alkaline, even with superfat. The natural alkalinity of soap is determined by the reaction kinetics of the fatty acids and the base (the lye). It's hard to explain it fully without getting into a chemistry course, but here's the best brief explanation I can offer:

Fatty acids are acidic, yes, but they are ~weak~ acids. If you put, say, stearic acid on your skin, it is a weak enough acid that it can't damage your skin. A strong acid is something like hydrochloric acid. If you put hydrochloric acid on your skin, it is a strong enough acid that it will definitely cause a chemical burn. The difference is how "willing" each acid is to break apart and release its hydrogen ion(s) (H+) when mixed in water. A strong acid ionizes almost completely. A weak acid does not ionize as easily -- it will only "give up" some of its hydrogen ions when made into a water solution. The natural pH of a solution of water and a strong acid will be lower (closer to 1) than a weak acid solution for this reason.

There are strong bases and weak bases as well. A weak base is bicarbonate of soda (baking soda). If you put it on your skin, baking soda won't damage your skin. A strong base is the lyes we use to make soap -- sodium hydroxide and potassium hydroxide. If you put either of these on your skin, it will certainly cause a chemical burn. The idea is the same as weak and strong acids, except bases release hydroxide ions (OH-) rather than hydrogen ions. The pH of a solution of water and a strong base will be higher (closer to 14, the max pH possible) than the pH of a water solution of a weak base.

So, if you mix acids and bases together -- what happens?

If you mix chemically equal parts of a strong acid and strong base, the resulting pH may indeed be close to a neutral (7) pH. What a neutral (7) pH means is there are exactly the same number of OH- ions in the water solution as there are H+ ions -- they cancel each other out, in effect. But that's not necessarily true when you mix, say, a strong base with a weak acid.

The pH of such a mixture, even if you carefully measure out equal parts of each chemical so they should precisely cancel each other out, will still not be neutral. The reason for that, as I've explained, is that the weak acid does not easily give up the H+ ions needed to cancel out the OH- ions from the strong base. The weak acid gives up ~some~ H+ ions and thus lowers the pH of the water solution, but it won't give them ALL up. Vice versa is true in a water solution of a weak base and strong acid.

When we make soap, we're combining a weak acid with a strong base just as I've described. The resulting bar of soap is not a boringly static, unchanging block of stuff. Soap is a "colloid", meaning it's a complex structure of solid soap crystals surrounded by a film of water-based liquid. The chemistry going on in this colloid is constantly changing -- it's like a huge party where all kinds of microscopically tiny people are circulating around. Some people "hook up" with each other and some just cruise. Some may hook up, but later unhook, cruise for awhile, and hook up with someone different.

In chemistry terms, all this partying and circulating is called a "dynamic equilibrium". Obviously, many soap particles are created when the fatty acids and the sodium ions "hook up" more or less permanently to form the chemically neutral molecules we call soap. But there are also the chemical loners who circulate in the watery liquid inside the soap structure -- these include some sodium ions (Na+), some fatty acid ions (Ste- and others), and the H+ and OH- ions released by the lye and the fatty acids.

In soap, the pH of this dynamic equilibrium is naturally alkaline, meaning there are more OH- ions from the lye wandering around than there are H+ ions liberated from the fatty acids. This will be true EVEN IF there are exactly as many fatty acid ions in the soap as there are molecules of lye -- or even if there are more fatty acids and fats than lye molecules. It's just the nature of the beast when you're dealing with the dynamic equilibrium between a strong base and a weak acid.

Whew! That's enough for now -- I'm going to let others chime in on your other questions ... gotta get back to work!
 
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When you salt out a soap you are removing the glycerin, which is the liquid you see released. Depending on how much salting & washing down you do will determine how much glycerin you are going to lose. It is similar to the process used by commercial soap companies to harvest the glycerin.

Most people are not going to find a salted out soap to be mild. There are always exceptions but that is the general rule.
 
huh...i thought i read somewhere salt bars take a while to cure? guess i read wrong or am remembering wrong (i vote remembering wrong....). thanks for the clarification :D

I don't think you are wrong at all. If I remember right, there is one member on here who has to cure salt bars for 4-6 months but thats not the norm.

Another wonderful post by Deeanna, thank you.
 
I had been wondering this exact question the other day. Great timing.

Thanks for the great explanation, DeeAnna!
 
DeeAnna comes to the rescue of us mere mortals once again. Speaking as a person who dropped out of college chemistry you make things so easy to understand.
 
Well, it helps to have taught science and math for 10 years at the local community college. I had lots of math- and science-phobic students, so I tried my best to translate these subjects to "mere mortal" level. Teaching like that was more fun for me too.

One time I had to teach the rules of correct mathematical rounding to a bunch of guys who had more cocky attitude than common sense. Their collective attitude was "H*ll, just round UP. Who cares about this crap!?!?" complete with bored yawns and rolling eyes.

Then I explained why they might care a LOT by using an example of a guy getting stopped on a Saturday night after having a beer or three and having to blow in the "machine" to measure his blood alcohol.

If the officer always rounded the machine's results UP, I showed how the driver was going to end up in jail. If the officer rounded the results properly, the driver would be okay.

Interesting how these yahoos ended up thinking maybe it might be good to have a clue about rounding properly.

That was fun! :)
 
Ah! Your explanation makes perfect sense, DeeAnna. Thanks for filtering it for us mere mortals :)

So, in your opinion, is there any possible way to bring soap's pH down or is it inherent in the nature of soap? I am thinking of this shampoo bar recipe from Soap Queen. Is adding the dissolved citric acid at a thin trace simply reacting/neutralizing the lye (resulting in additional superfatting rather than simply lowering the overall pH)?

Thanks for any insight you'd be willing to share~
 
"...So, in your opinion, is there any possible way to bring soap's pH down..."

It's a black and white answer, not an opinion. The chemistry of lye soap says absolutely no. You can lower the pH to the 8 or 9 range by choosing appropriate fatty acids.

But a lye soap being neutral (7) or even acidic? Not a chance. If that's the direction a person is determined to go, he/she needs to look at non-soap detergents or a blend of soap and non-soap detergents.

"...Is adding the dissolved citric acid at a thin trace simply reacting/neutralizing the lye..."

Yep that's basically what happens. If you add enough citric (or other) acid, you'll end up with a soft mush of fatty acids, unreacted fat, and sodium citrate. And the pH will remain alkaline until you really go overboard with the citric acid.
 
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Many thanks, DeeAnna. It was what I suspected but I am an utter novice, so thank you for letting me pick your brain :)
 
"
Yep that's basically what happens. If you add enough citric (or other) acid, you'll end up with a soft mush of fatty acids, unreacted fat, and sodium
citrate.
And what happens if you have ready reacted liquid soap wanting to make it neutral? The same, as the ions from the salt are separated when it is disolved in water?
 
I wondered that same thing, engblom, regarding already saponified soap. I thought one day I would try experimenting to see what happens...
 
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