Liquid soap Chemistry - Neutralizing excess KOH with Citric Acid - - is it a 3:1 ratio even if citric acid is a weak acid?

[LazyUmbrella]

Hello!

I am following Catherine Failor's process for making liquid soap, where you mix your oils with excess KOH to ensure that all the fatty acids are saponified, and then during the dilution phase, you add citric acid (or borax, or boric acid) to 'neutralize' the excess KOH. I am using citric acid (CA) and I am trying to calculate how much I need to add to my soap solution.

I have about 11.41 g of excess KOH, and I calculate that I need about 13.022 g CA
I won't bore you with the calculations (unless you want to see them), but this result is based on my understanding that CA is triprotic (can dissociate three hydrogen ions), and therefore one mole of CA can neutralize 3 moles of KOH.

My question is the following - my calculation above assumes that all of the CA reacts with the KOH. However CA is a weak acid which, if I remember my college chemistry, not all of the CA dissociates into the H+ and citrate salt. Is this correct?

And if it is correct, doesn't that mean that I actually need more than the 13.022 g CA, which I originally calculated?

Am I overthinking this? I just want to make sure that I don't have any excess KOH left in my soap solution.


thank you!
LU

 

[Zany_in_CO]

I am following Catherine Failor's process for making liquid soap,

Failor's process for making LS is passé. To combine FAs (Fatty Acids) and lye solution you can HP or CP -- whichever process you are most familiar with in making hard bars. Most people HP in a crockpot so there is no need to watch temps.
If you CP,
Combine when oils are 160°F (71°C) and lye solution is 140°F (60°C).
Stir by hand, then SB (on and off) to trace 10 -15 minutes, over low to medium-low heat. Maintain temp at 160°F (71°C) until trace occurs.
Let sit 5 minutes off heat to make sure it doesn’t separate.
If it starts to puff up, that’s a good sign! Stir it down.
Cover and CURE 1-2 WEEKS.

NOTE: The soap base is opaque at first but becomes translucent and soft,
similar in color and texture to vaseline, one week later. I then put it in a
ZipLoc and store it in the fridge until I’m ready to dilute.
Soap base may be stored in fridge for up to 12 weeks.

 

[Zany_in_CO]

Neutralizing excess KOH with Citric Acid ​

News Flash: There's no need to use a neutralizer unless there is excess KOH.
Failor's recipes factor in 10% excess KOH.
Today's calculators do not factor in the 10% excess.
If you want to, you can do the calculation to find the amount of KOH needed to saponify the batch then add 10% more KOH to that number.

Read more here:
Alaiyna B's Blogspot - Basic Beginner Liquid Soap

QUOTE: Citric acid is a pH adjuster that will neutralize the excess lye AND lower your pH. If you use citric acid, you must be very careful not to use too much as you can lower your pH to the “breaking point” of your soap and your soap will start to separate out into fatty acids, water and potassium citrate and will no longer be soap.

 

[Zany_in_CO]

Here's how to set up SoapCalc to make liquid soap:

 

[DeeAnna]

We're not dealing with a dilute mixture of an acid and base, which is when the dissociation constants of the acid and the base would be useful for calculating the concentrations of ions in the solution.

We're making a highly concentrated solution of potassium hydroxide, which is a strong base that dissociates almost completely in water. Even when most of the KOH has been consumed by saponification, the resulting water-based solution is still highly alkaline. To further complicate matters, the mixture also contains soap, which is the salt of an even weaker acid than citric acid.

We're adding only a small amount of citric acid to this complicated solution of alkali plus soap. In this circumstance, the citric acid is going to dissociate all but completely, even though it is considered to be a "weak" acid.

https://www.thoughtco.com/ph-pka-ka-pkb-and-kb-explained-4027791
edit: What you're going to find when making lye-heavy soap and then neutralizing is that it takes only relatively small amounts of citric acid to turn the soap from lye heavy and not safe for the skin ... to almost balanced (only slightly fat heavy) ... to overly fat heavy with major separation of fatty acids from the soap.

Calculations aren't a lot of help here because you're not working with chemistry lab reagents. The numbers you'll come up with will be rough estimates at best because you don't absolutely know the purity of the KOH, nor the exact saponification values for the fats, nor the purity of the citric acid.

You're better off treating this as a titration with sample of unknown alkalinity (the soap) and an acid solution of unknown acidity (the citric acid).

 

[LazyUmbrella]

Calculations aren't a lot of help here because you're not working with chemistry lab reagents. The numbers you'll come up with will be rough estimates at best because you don't absolutely know the purity of the KOH, nor the exact saponification values for the fats, nor the purity of the citric acid.

You're better off treating this as a titration with sample of unknown alkalinity (the soap) and an acid solution of unknown acidity (the citric acid).

Such excellent points - thank you!
Not only did I overthink the process, but I oversimplified the chemistry! Time to pull out those chemistry books from college...

I like the idea of titrating - i think this batch will be my 'experimental batch' - except instead of phenolphthalein, i'll use an "indicator" of broken down soap, or something.
And going forward I'll probably keep the soapmaking process simple and not add excess KOH.

Thanks again!

 

[LazyUmbrella]

News Flash: There's no need to use a neutralizer unless there is excess KOH.
Failor's recipes factor in 10% excess KOH.
Today's calculators do not factor in the 10% excess.
If you want to, you can do the calculation to find the amount of KOH needed to saponify the batch then add 10% more KOH to that number.

Read more here:
Alaiyna B's Blogspot - Basic Beginner Liquid Soap

QUOTE: Citric acid is a pH adjuster that will neutralize the excess lye AND lower your pH. If you use citric acid, you must be very careful not to use too much as you can lower your pH to the “breaking point” of your soap and your soap will start to separate out into fatty acids, water and potassium citrate and will no longer be soap.

That's interesting about the new calculators vs. Failor's rationale for using XS KOH. In my head I thought that it was imperative not to have any excess oils/fatty acids in the liquid soap mixture, but upon reflection, i can see that it's not necessarily the case.

I'll create a new batch using the approach that you proposed. that will probably be less of a headache. Thank you for your advice!

 

[DeeAnna]

Titrating a soap solution to a phenolpthalein (phph) endpoint is not helpful for this specific situation. The goal is to add enough acid to consume all free alkalinity and stop at that point. A pH of 8.2 is too acidic for that end point.

Phph changes color at pH 8.2, which is too low for soap to remain functional soap. The "titration endpoint" instead should be approximately where the soap begins to "break" (decompose) into fatty acids. That can be anywhere from pH 9.5 to pH 11.5 depending on the fatty acids in the soap.

Assuming the initial lye-heavy soap is a clear mixture, then you will see increasing cloudiness in the soap as the soap starts to decompose. Add a bit more acid past that point and the soap will increasingly separate into a floating layer of free fatty acids with cloudy soap below. Add a bit of alkali, and the fatty acid layer will gradually disappear and the soap will become less cloudy.

Remember too that a soap solution is a buffer, so the change in the pH of a soap solution isn't linearly related to the change in alkalinity.

***

edit -- Making liquid soap with no superfat is a valid goal to have if you want crystal clear liquid soap (LS). IIRC, Failor said that was really important to her, so I can appreciate why she developed the method she used. With a carefully designed recipe, this method does make crystal clear LS.

That said, it can be pretty tedious to accurately neutralize soap with citric acid or borax or other acid. Failor tried to explain the process as if it is a cut-and-dried procedure, and in my experience neutralizing LS is anything but cut-and-dried.

In addition, Failor's method doesn't guarantee every batch of LS soap will be crystal clear -- some additives (fragrances especially) will cloud LS and some fats do the same -- for example, a LS with a high % of lard or tallow is probably not going to be clear. Some people use NaOH in addition to KOH to make their LS or they try to thicken their LS with table salt (NaCl) and the addition of sodium can cloud LS as well.

Many of us aren't all that focused on making water-clear LS, so it makes more sense to produce LS with a slight superfat to ensure a skin safe soap. That eliminates the need to neutralize so we can (usually) avoid that fiddly bit.

 

[LazyUmbrella]

Titrating a soap solution to a phenolpthalein (phph) endpoint is not helpful for this specific situation. The goal is to add enough acid to consume all free alkalinity and stop at that point. A pH of 8.2 is too acidic for that end point.

Phph changes color at pH 8.2, which is too low for soap to remain functional soap. The "titration endpoint" instead should be approximately where the soap begins to "break" (decompose) into fatty acids. That can be anywhere from pH 9.5 to pH 11.5 depending on the fatty acids in the soap.

Assuming the initial lye-heavy soap is a clear mixture, then you will see increasing cloudiness in the soap as the soap starts to decompose. Add a bit more acid past that point and the soap will increasingly separate into a floating layer of free fatty acids with cloudy soap below. Add a bit of alkali, and the fatty acid layer will gradually disappear and the soap will become less cloudy.

Remember too that a soap solution is a buffer, so the change in the pH of a soap solution isn't linearly related to the change in alkalinity.

***

edit -- Making liquid soap with no superfat is a valid goal to have if you want crystal clear liquid soap (LS). IIRC, Failor said that was really important to her, so I can appreciate why she developed the method she used. With a carefully designed recipe, this method does make crystal clear LS.

That said, it can be pretty tedious to accurately neutralize soap with citric acid or borax or other acid. Failor tried to explain the process as if it is a cut-and-dried procedure, and in my experience neutralizing LS is anything but cut-and-dried.

In addition, Failor's method doesn't guarantee every batch of LS soap will be crystal clear -- some additives (fragrances especially) will cloud LS and some fats do the same -- for example, a LS with a high % of lard or tallow is probably not going to be clear. Some people use NaOH in addition to KOH to make their LS or they try to thicken their LS with table salt (NaCl) and the addition of sodium can cloud LS as well.

Many of us aren't all that focused on making water-clear LS, so it makes more sense to produce LS with a slight superfat to ensure a skin safe soap. That eliminates the need to neutralize so we can (usually) avoid that fiddly bit.

thank you so much! i really appreciate the time you took to completely answer my question!

 

[Zany_in_CO]

thank you so much! i really appreciate the time you took to completely answer my question!

Not @DeeAnna here, but I just want to say how much we appreciate you for not only understanding the time it takes to write detailed responses but also expressing gratitude.
Well done.

 

[janesathome]

@DeeAnna @Zany_in_CO (and I must include @AliOop) - just piping in here to say how much I appreciate you. I just answered my front door to accept delivery of my first supplies to make Liquid soap - and a few basic lotions and creams. WOOT!
I’ve had a year of learning CP soapmaking (I figure I’ve moved all the way from preschool to about grade 3 level) and am keen to try a few new things. Reading your posts about cold process soapmaking has helped me so much in the last year. (DeeAnna, I think I have the Classic Bells site on the internet equivalent of speed dial!)
I know I will continue to rely on you for more sound info and advice - and sometimes even humour! THANK YOU!

 

[AliOop]

Very excited for you, @janesathome, and looking forward to seeing what you create.

 

[Zany_in_CO]

THANK YOU!

YOU'RE WELCOME!