How to test KOH and NaOH for activity

[TallTayl]

Saving this handy gem here for the future, and to share with others who may have experienced problems with older lye or potassium hydroxide.

 

I’m making liquid soap for a project and it just would not get “clear”. The paste and dilution were milky. I had noticed my KOH didn’t get super hot when creating the solution, and didn’t hear the familiar snap crackle and pop. 
 

adding more KOH helped to get to a clear state eventually, but I knew there has to be a better way.  Thanks to the magic of the internet, and Dr. Kevin Dunn, we can test the activity of the alkali pretty simply.  Links to the PDF written set and video below. I will add a summary of the printed page in the next post in case the link breaks over time.

 

once my new phenolphthalein and KOH arrive I will document how the tests worked for my old and new KOH.

 

here’s a link to a super PDF instruction to print for your library:

 

https://classicbells.com/soap/NaOH_KOH_Purity_Check.pdf

 

and the video:

 

 

[TallTayl]

I formatted Pasted PDF text in case the link ever breaks.

 

NaOH or KOH Purity Check for Soap Makers
Soap makers often assume the purity of the alkali in their soaping stash is whatever the distributor says it is. Even if the purity is high when the alkali (NaOH or KOH) is newly purchased, the purity can drop, sometimes by quite a bit, as time passes. Alkali becomes less pure by absorbing water vapor and by reacting with carbon dioxide gas in the air. Based on a method from Kevin Dunn, here is a method to quickly and easily check the purity of your NaOH or KOH --
Ingredients
Citric acid powder (canning supplies at a grocery or hardware store, online suppliers) Distilled water -- do not use tap water! (grocery store)
NaOH or KOH for soap making (hardware store, online)
Phenolphthalein indicator solution (beer brewing suppliers, online)
Equipment
Clear glass container that holds about 2 cups (1 pint, 0.5 liter) of liquid. A pint canning jar works well if you have one.
Scale (also called a balance) that can weigh to 0.1 gram or better
Safety glasses, rubber gloves, and other safety gear for working with NaOH or KOH Plastic or stainless steel spoons for measuring citric acid and alkali
Oven-safe glass or ceramic dish
Paper, pencil or pen, and calculator
Make anhydrous citric acid
This is optional if you know your citric acid is already in anhydrous form. If you do not know your citric acid is anhydrous (water free), please do this step. It is not hard to do and it will ensure you get the most accurate results.
Heat your oven to about 200 degrees F (95 C).
Put at least 100 grams of citric acid powder in a thin layer in the bottom of an oven-safe ceramic or glass dish. Weigh the entire dish and citric acid. Record this starting weight.
Put the dish into the oven. Heat it for 1/2 to 1 hour. Reweigh the entire dish and citric acid and record this weight.
If the last weight is less than the previous weight, put the dish back in the oven and heat for another 15 minutes. Reweigh and record the total weight. Compare this weight to the previous weight. If the weight continues to drop, repeat the heating and reweighing steps until the last weight is the same as the previous weight.
When the weight stays the same from one reading to the next, the citric acid is anhydrous and is ready for use.
Store anhydrous citric acid in an air-tight container. About 100 grams of anhydrous citric acid will do 9 or 10 tests of alkali purity.
Questions or comments? Contact DeeAnna Weed at coyote@acrec.com Page 1 of 4

NaOH purity check
Put the glass container on the scale. Tare (zero) the scale. Pour 100 to 105 grams of distilled water into the container. Do not use tap water.
Add 3 or 4 drops of phenolphthalein solution to the water. Swirl to mix. This solution should stay water clear. If it turns pink, discard the solution, carefully clean the container with soap and water, rinse thoroughly with distilled water, and try again.
Tare the scale. Weigh 10.0 to 10.5 grams of anhydrous citric acid into the water. Swirl to mix until the citric acid powder dissolves -- about 30 seconds. The solution will be a cloudy white color.
Record the actual weight of citric acid added to the water: ________________ g citric acid
With the container still on the scale, tare the scale. Weigh 6.0 to 6.2 grams of NaOH into the solution. Swirl to dissolve the NaOH and mix the solution well. The solution will heat up, but it should remain cloudy white. If the solution turns pink, you have added too much NaOH. Discard this solution and start over.
Carefully add about 0.1 gram of solid NaOH to the solution. Swirl to dissolve the NaOH and mix the solution well.
Check the color of the solution by holding the container in front of a white surface (sheet of paper or paper towel works well). Compare the color of the solution against the white paper.
If the solution remains cloudy white, add another 0.1 gram of NaOH, mix, and compare. Repeat the color check until the solution turns the barest shade of faint pink. Stop adding NaOH.
Put the container back on the scale.
Record the total weight of NaOH added to the solution: ________________ g NaOH Calculate the NaOH purity:
NaOH purity % = 62.46 X (grams citric acid) / (grams NaOH)
NaOH purity % = 62.46 X ________________ g citric acid / ________________ g NaOH = ________________ %
Repeat this test at least two more times for the best accuracy. Find the average of the three answers, and that is your result.
   Grams citric acid
Grams NaOH
Purity %
    Test 1 Test 2 Test 3 Average
              Questions or comments? Contact DeeAnna Weed at coyote@acrec.com
Page 2 of 4

KOH purity check
Repeat the NaOH purity test except use KOH instead of NaOH.
Weigh 8.5 to 8.7 grams of solid KOH into the citric acid solution at first. Swirl to dissolve the KOH and mix well.
Add 0.1 gram of KOH, swirl to dissolve the KOH and mix well, and check the color. Repeat until the solution turns from cloudy white to the barest faintest pink.
Stop adding KOH at that point and record the total weight of KOH added to the mixture. Calculate the KOH purity:
KOH purity % = 87.62 X (grams citric acid) / (grams KOH)
Repeat this test at least two more times for the best accuracy. Find the average of the three answers, and that is your result.
   Grams citric acid
Grams KOH
Purity %
    Test 1 Test 2 Test 3 Average
              Adjusting the weight of alkali (NaOH or KOH) for making soap
All of the soap recipe calculators I have checked as of the time of this writing assume NaOH is 100% pure.
Many calcs also assume KOH is 100% pure. There are a few exceptions --
Soapcalc allows you to choose either 100% or 90% KOH purity Summerbeemeadow and Brambleberry calcs assume KOH is 95% pure Soapee.com calc allows you to enter the KOH purity
What if your alkali (NaOH or KOH) purity is something other than what your soap recipe calculator assumes? You can hand-correct the alkali weight produced by the soap calculator if you know the actual purity.
Here's how --
Corrected alkali weight = (Alkali weight from the soap recipe calculator) X (Soap recipe calculator purity %) / (Actual alkali purity %)
This works for NaOH or for KOH. Use this "corrected alkali weight" when measuring your NaOH or KOH for soap.
Questions or comments? Contact DeeAnna Weed at coyote@acrec.com Page 3 of 4

The Math -- in fine print, of course!
Background chemistry info
Molecular Weight (MW) of H3Cit (anhydrous citric acid) = 192.12 grams/mole MW NaOH = 39.997 grams/mole
MW KOH = 56.1056 grams/mole
Stoichiometric ratio NaOH:Citric acid = (3 mol NaOH / 1 mol H3Cit)
Stoichiometric ratio KOH:Citric acid = (3 mol KOH / 1 mol H3Cit)
The stoichiometric ratio means "How many molecules of NaOH (or KOH) will neutralize 1 molecule of citric acid?
Part 1. How did Dunn create his formula for calculating NaOH purity? (See Reference 1)
He gave this formula in his presentation: NaOH purity % = 62.46 X (grams H3Cit) / (grams NaOH). (See Reference 1 below for the source.) How did he get this?
Equation 1a -- How many grams of 100% pure NaOH are required to neutralize 100% pure H3Cit ?
The stoichiometric ratio and the molecular weights of citric acid and NaOH come into play here
grams pure NaOH = (grams H3Cit) / (192.12 g/mol H3Cit) X (3 mol NaOH / 1 mol H3Cit) X (39.997 g/mol NaOH)
Simplify this to --
grams pure NaOH = 0.6246 X (grams H3Cit)
Equation 2a -- What is a general equation to calculate the purity of real life NaOH?
NaOH purity % = (grams pure NaOH) / (grams real NaOH) X 100
Equation 3a -- Replace "grams pure NaOH" in Equation 2a with the stuff on the right side in Equation 1a.
NaOH purity % = 0.6246 X (grams H3Cit) / (grams real NaOH) X 100
Simplify this to --
NaOH purity % = 62.46 X (grams H3Cit) / (grams real NaOH) Eq 3a
Part 2. Can a similar formula be created to calculate KOH purity?
Kevin Dunn did not provide a formula for calculating KOH purity, but it can be done by following the method given in Part 1 and substituting the chemistry info for KOH.
Equation 1b -- How many grams of 100% pure KOH are required to neutralize 100% pure H3Cit ?
grams pure KOH = (grams H3Cit) / (192.12 g/mol H3Cit) X (3 mol KOH / 1 mol H3Cit) X (56.1056 g/mol KOH)
Simplify this to --
grams pure KOH = 0.8762 X (grams H3Cit)
Equation 2b -- What is a general equation to calculate the purity of real life KOH?
KOH purity % = (grams pure KOH) / (grams real KOH) X 100
Equation 3b -- Replace "grams pure KOH" in Equation 2b with the stuff on the right side in Equation 1b.
KOH purity % = 0.8762 X (grams H3Cit) / (grams real KOH) X 100
Simplify this to --
KOH purity % = 87.62 X (grams H3Cit) / (grams real KOH) Eq 3b
Accuracy of this method
Dunn's method requires simple equipment, modest chemistry skill, and easy-to-find ingredients. With a decent scale and careful technique, it is plenty accurate enough for soap making; I estimate the accuracy is plus or minus 1% to 2%. This is not how alkali purity is measured in the real world, however. A chemist would use a more detailed and rigorous method to accurately measure the purity of an alkali.
References
(1) Dunn, Kevin. Video lecture at http://fyi101.com/the-balancing-act-part-ii-presented-by-dr-kevin-dunn/ Questions or comments? Contact DeeAnna Weed at coyote@acrec.com Page 4 of 4
Eq 1a
Eq 2a
Eq 1b
Eq 2b
 

[Candybee]

Wow! That's an interesting read and test to try out on my lye. I usually don't worry much about the purity when I am doing most bar soap unless the lye is pretty old. I only tend to worry about the purity or think about it when I am making something like liquid soap or shaving soap. It's nice to know there is a way to test it.

 

I am not clear on anhydrous citric acid though. Do you generally buy it that way or make it? Or is it simply water free citric acid. Told you I was unclear on that.

[TallTayl]

1 hour ago, Candybee said:

Wow! That's an interesting read and test to try out on my lye. I usually don't worry much about the purity when I am doing most bar soap unless the lye is pretty old. I only tend to worry about the purity or think about it when I am making something like liquid soap or shaving soap. It's nice to know there is a way to test it.

 

I am not clear on anhydrous citric acid though. Do you generally buy it that way or make it? Or is it simply water free citric acid. Told you I was unclear on that.

I think it is just that citric acid can absorb water from humidity. It minimizes a variable. I'll try anything once.

 

I didnt think about purity either, until my liquid soap flopped. I JUST made shaving soap with dual lye.  Now I know I have to some how amend it as I used the same KOH. Oh well....

[TallTayl]

My phenolphthalein arrived earlier than expected, so I gave it a go yesterday.  The better your measurements, the better the outcome. The first test was a little fast and loose, but I learned a lot over the 5 iterations.

 

first lesson: use a large enough jar.  I started in a smallish glass measuring pitcher. Swirling lost a little liquid each time, throwing off the measurements just enough to be unreliable.

 

second lesson - the big one - shake up your bottle of KOH well.  The tests from flakes at the top came out to be about 75% active.  Lower down in the bottle after shaking the results were in the low 80’s.
 

Once I got to the fifth test the KOH in water finally started to heat up and sizzle/pop as usual.

 

third little lesson - the fun one - you can keep adding citric and KOH back and forth to dial in the result. One flake of KOH would make my solution go from clear to deep pink. A few grains of citric rolled back the reaction.

 

fourth and probably best lesson for me - I now get why so many liquid soap formulas intentionally use too much lye, then neutralize later.  It is easy! Older formulas would use borax, which is not terribly popular as an ingredient in body products for many reasons.  I noticed Stephenson and others use simple citric acid. The reaction between citric acid and KOH produces potassium citrate (a gentle chelation) in addition to using up the free lye in the batch.  If your liquid soap tests pink after cooking and diluting, a few drops of citric acid should solve the issue.

 

editing to add... since the “strength” of KOH in the bottle can vary so much depending on the level, it makes sense to over lye a bit, considering that lye might be far less than 90% pure to begin with.