Soap with no excess alkali?

[NewbieSoapMaker]

Hey ya'll, long time lurker, first time poster . Let's get to business!

When performing titration/stoichiometry for total alkali content of soap, does the end point; I.e. the exact moment where the indicator solution (ethanol+phenolphtalein+soap) turns from pink to colorless, signify that all the excess alkali is indeed neutralized, or does it simply indicate a certain pH value of the solution? Namely pH 8.2, as per info found here: https://en.wikipedia.org/wiki/Phenolphthalein

Having trouble wrapping my head around the fact that it somehow becomes colorless at a pH higher than 8.2 (the soap solution is likely pH 9-11ish, as is normal for soap).

Enlightenment please ?

 

[Susie]

It simply measures the pH. Not whether there is excess alkali. The zap test is fast, free, and will tell you without a doubt if there is excess lye.

BTW, welcome to the forum.

 

[NewbieSoapMaker]

For clarification; the procedure in question is found @ pages 245-249 of Scientific Soapmaking. Chapter 15.1, "Total Alkali".

Quote:
"As shown in table 13-1 (page 229), the pH of a soap solution may vary over a considerable range. Consequently, pH alone will not tell us whether soap contains excess alkali. For analytical work, we need a method for precisely measuring the amount of excess alkali in soap. The TA test expresses the alkali in ppt of NaOH, which is sufficiently precise to answer the kinds of qestions we wish to ask. When an excessively alkaline soap is dissolved in ethanol containing phenolphtalein, the solution turns pink. From this point forward, that color change will be the deviding line between "excessively alkaline" (pink) and "tongue neutral" (colorless). The total alkali may be measured by adding acid until the pink color disappears. The more acid required, the more excess alkali is present. The total alkali may then be calculated from the weight of acid required to reach the phenolphtalein endpoint."

It's weird to me too (and that's why I'm here asking this exact question ), but the author seems quite certain that we are not measuring pH here, but rather that we are measuring precise amounts (traces even!) of excess (unreacted) alkali.

 

[Susie]

Dr. Dunn is a professor at a college. Scientific Soapmaking (if my memory is correct) is a textbook for a college course. It is not really a "how-to" manual on making soap. The "tongue" test he is referring to to determine excess alkali is the zap test. He is trying to teach those students how to accurately measure the amount of excess alkalinity. Not how to see if a soap is safe. The zap test tells you if a soap is safe. pH does not answer whether a soap is safe, or indeed, how much excess alkali exists in it.

If you are asking a different question, please forgive me. I have to work today, but perhaps DeeAnna, or one of the other sciencey types will be along soon. I will check back when I am back home to see what the question really was, though, and will look up my copy of Scientific Soapmaking to see what is being discussed.

 

[The Efficacious Gentleman]

I've not read that much in my copy, but from the two posts, it seems that you are correct - tongue neutral (no zap) would be clear (no colour) in that test. I do not imagine that it would be the same pH at that point for ALL soaps (8.2 as your example), as the pH of soaps with no excess lye would vary depending on the recipe itself.

 

[DeeAnna]

The key thing about Dunn's method is the soap solution you make for this test is an ALCOHOLIC solution, not a water solution. The use of phenolphthalein and the careful addition of an acid to an alcoholic soap solution, will tell you how much free alkali -- actually free OH- ions (hydroxide or hydroxyl ions) -- is present in the solution.

If you use water to make the soap solution rather than alcohol, the soap itself will dissociate (break apart) due to the presence of water. That breakdown will provide OH- ions as well. If there is any soda ash, that too will dissociate in water, but not in alcohol. You don't want to measure OH- ions that come from soap or soda ash. You only want to measure the free OH- ions in the soap. That is what you will measure by using alcohol that has been de-acidified so the alcohol is "neutral" to phenolphthalein. You could also use a pH meter and titrate an 8.2 pH endpoint, but phenolphthalein is easier and cheaper.

I want to stress this titration test is NOT measuring pH as a stand-alone piece of information as soapers want to measure pH. This test does NOT tell you the pH of the soap. It gives you information about a chemical reaction -- the addition of acid to the alcoholic soap solution. The addition of acid continues until the addition of acid causes the pH to drop to an industry-standard ending point (aka colorless to phenolphthalein). The results of this test per Dunn's method will give an answer as the parts per thousand (ppt in Dunn's book) of NaOH in the soap. A result of 1 ppt or less is acceptable.

For the chemists in the group, the "ppt" in Dunn's book to mean parts per thousand is unusual. The shorthand ppt usually means "parts per trillion" not "parts per thousand." So don't get thrown by this.

 

[Sapo]

Watching diligently

 

[NewbieSoapMaker]

THANK YOU!, you literally filled in the missing puzzle pieces.

But you know how it works; if you get an answer for one question; two more questions appear (or more..)!

1. How does the water inherently present in the soap fit into all this? Does a water content of 5% in a bar mean that some of the soap is disassociated already? If so, what does that mean for this test?

2.Would the test produce less reliable results with a bar that has only been curing for a month (higher water content) vs. with a bar that has been allowed to dry for longer or in more proper conditions? In short, like stated at the beginning, how does the water already present in soap fit into all this?

3. If one were to use this test for liquid soap as well as bar soap, given this new knowledge of the importance of absence of water - it is probably a good idea to test the PASTE as opposed to testing the already dilluted LS? Is a dilluted LS disassociated; or does that only start happening once you add more water than the actually required minimum to dilute the LS?

Basically asking how to prevent said disassociation of soap in all it's forms to prevent the presence of soapy OH- ions, to ensure reliable test results.

 

[DeeAnna]

1. How does the water inherently present in the soap fit into all this? Does a water content of 5% in a bar mean that some of the soap is disassociated already? If so, what does that mean for this test?

As in all things, consistency, consistency, consistency. There is probably some dissociation, yes, but if you're consistent in how you set up and do the test, you should be able to get reasonably repeatable results.

2.Would the test produce less reliable results with a bar that has only been curing for a month (higher water content) vs. with a bar that has been allowed to dry for longer or in more proper conditions? In short, like stated at the beginning, how does the water already present in soap fit into all this?

Well, reliability is more about YOUR testing method than it is about the soap. If you're consistent, your results are likely to be reliable.

A younger soap is likely to have more free alkali than older soap for maybe a week or three and the free alkali content can vary depending on where you test on the soap bar. That's not an issue of reliability; it's related to the nature of the soap.

Older soap should, if properly made, test low for free alkali. Unless you plan to do experimental work or if you are the kind of person who wants to make sure just to make sure, I doubt you're going to get a lot of exciting information from testing a properly cured soap.

3. If one were to use this test for liquid soap as well as bar soap, given this new knowledge of the importance of absence of water - it is probably a good idea to test the PASTE as opposed to testing the already dilluted LS? Is a dilluted LS disassociated; or does that only start happening once you add more water than the actually required minimum to dilute the LS?

You have to make an alcoholic dilution. So just sit for a bit and THINK about this question -- does it make sense to make an alcoholic dilution using liquid soap that's over 50% water?

Basically asking how to prevent said disassociation of soap in all it's forms to prevent the presence of soapy OH- ions, to ensure reliable test results.

Fair warning --you're going at this as if you're going to argue the soap into absolutely following your will, and that's unrealistic. Chemical analysis is about minimizing error and ensuring consistency by using proper procedure. Given Dunn's test methods, the inherent error is likely to make a trained analytical chemist unhappy, but if you can accept reasonable kitchen chemistry results, you should do fine.

 

[NewbieSoapMaker]

Interesting, interesting.

Evaporating moisture via oven heating springs to mind, too, for minimizing error. Kinda like when making DIY anhydrous citric acid.

Thanks for the good information!

 

[DeeAnna]

Don't make this harder or more complicated than it needs to be. Just be consistent.

 

[NewbieSoapMaker]

I don't understand :\.

If, for example, I formulate LS with an intentional lye excess, and afterwards I want to ensure that all of it is neutralized, but not OVER neutralized, I need to know the actual alkali content of each batch of paste. How does being consistent help with that, if for example the relative uncertainty was 2% (due to said disassociation of soap) with every batch?

Wouldn't I be consistently 2% off the mark?

 

[Susie]

Why would you want to make LS with an intentional lye excess? You don't have to, and it creates extra steps. I make my soap with 3% superfat, and it comes out perfect every time.

 

[DeeAnna]

No chemical test is 100% accurate nor precise. In other words, you cannot prevent error, you can only try to make the unavoidable errors as small and consistent as possible.

You've made a big assumption that you have to dry the soap to improve accuracy of the excess alkali test. That assumption is based on very few concrete facts to support it, unless you are more of a soap chemist than I suspect you are. All the extra work might make you feel good, but it won't necessarily make your results any better and it may introduce different kinds of error without your knowledge.

Just use the method per Dunn's book without embellishment. His method is an adaptation of a standardized ASTM test that is used all the time in the soap making industry. Trained and capable soap chemists accept the reliability and accuracy of this analytical procedure, and they have decades of experience with it.

Hone your experimental technique -- poor bench skills will introduce far more error than most people realize, unless you're a former lab tech and then you'll know what I'm talking about. Double check your results by doing at least three analyses of each sample, eliminating outliers, and finding the average of the rest. Follow up with a zap test as a secondary confirmation of the results. And leave it at that.

As far as making LS with an intentional lye excess and then neutralizing, as Susie said, it's truly not necessary. But if you want to take that tack, you have the analytical method to do so without having to reinvent the wheel.

I don't understand :\.

If, for example, I formulate LS with an intentional lye excess, and afterwards I want to ensure that all of it is neutralized, but not OVER neutralized, I need to know the actual alkali content of each batch of paste. How does being consistent help with that, if for example the relative uncertainty was 2% (due to said disassociation of soap) with every batch?

Wouldn't I be consistently 2% off the mark?

 

[Sapo]

If I'm not mistaken the worst thing that can happen with using Dunn's method to the T is...

...you end up with a slight superfat (because you overneutralized by a small bit, due to the OH- ions present in the solution from the soap disassociation from the inherently+unavoidably present water).