Some soap makers use two alkalis -- sodium hydroxide (NaOH) and potassium hydroxide (KOH) -- to make some types of soap. In the hand crafted soaping world, these dual-lye recipes are often used for specialty soaps such as cream soap, shave soap, and liquid soap.
Dual-lye recipes are much less common for bar soap. In her book Castile Soapmaking, author Anne Watson recommends a blend of 5% KOH and 95% NaOH for making classic castile (100% olive oil) soap. She says the KOH reduces the stringy, gelatinous goo that castile (or any soap high in oleic acid) is infamous for making. A bit of KOH in a soap high in tallow, lard, or palm oil (in other words, soap high in stearic and palmitic acids) can also increase the solubility and lather of this type of soap.
The first thought many people have when designing a dual-lye recipe is to just calculate the alkali weight as if the recipe was a single-lye recipe and divide this total weight in proportion to the percentages of KOH and NaOH. If you do that, however, it will not work. Your soap will either be soft and greasy from not nearly enough alkali or brittle and dangerously alkaline from too much alkali.
The key to remember is each batch of soap requires a specific number of alkali molecules, whether they be KOH, NaOH, or some combination of both. Because KOH molecules weigh 1.403 times more than NaOH molecules, a soaper must allow for that weight difference so the batch gets the correct number of alkali molecules to make good soap.
How to calculate the weights of KOH, NaOH, and water for making soap?
Let's say you want make a dual-lye soap using 95% NaOH molecules and 5% KOH molecules. How can you calculate the correct weights for KOH and NaOH?
Geeky Soapers may want to do the calculations by hand. The easiest way, however, is to let a dual-lye recipe calculator do the work. Three suitable soap calculators are Soapee, SoapmakingFriend, and Summer Bee Meadow Advanced.
If you have not used either calc, I strongly recommend Soapee or SoapmakingFriend. I think they are easier to learn and use.
To start a dual-lye recipe in Soapee (Soapmaking Friend is similar), click the button next to the "Hybrid Soap" option in Section 1 --
Click next to the "% KOH" option and type the percentage of KOH you want in your recipe. Soapee will calculate the percentage of NaOH for you. Or vice versa -- either way works.
Enter the KOH purity. If you don't know the purity, check with your supplier. If the supplier will not provide that information, I suggest using 90% KOH purity, since KOH is often about that pure.
Continue entering the information for your recipe -- units of measure, water, superfat, and fragrance. To enter the fats, double click on the name of your first fat. New windows will appear. One will show the properties of the fat and another will allow you to enter the percentage or weight of that fat in your recipe. The finished recipe will appear as fats are added.
How to calculate the weight of extra alkali if you also add an acid to dual-lye soap?
Some people add acids, such as citric acid (citrus juices), acetic acid (vinegar), ascorbic acid (vitamin C), or lactic acid (yogurt), to their soap. Depending on the type of acid chosen and the amount added, this can make soap batter more fluid, reduce soap scum in the shower or sink, increase shelf life of the finished soap, and/or increase hardness of bar soap.
Any time you add an acid to soap, the acid will consume some of the alkali. If you do not add extra alkali to compensate for what the acid will use up, there will not be enough alkali left over to fully saponify the fats according to your recipe. This means the soap batch will have a higher superfat than you may want.
The solution to this problem is to add the extra alkali that the acid needs. For a dual-lye recipe, here is a simple way to calculate this extra alkali --
Decide how much acid you want to use in your batch. Calculate the NaOH (sodium hydroxide) needed to react with this acid. Click on a link (above) for the acid you want to use to get more help.
Add this extra NaOH weight to the NaOH weight needed for the soap. The answer is the total weight of NaOH needed for the recipe --
Total NaOH wt = NaOH for acid + NaOH for saponification
The KOH weight will not change --
Total KOH wt = KOH for saponification
How to make a dual lye blend with NaOH and KOH?
Wear your usual safety gear for working with lye. At a minimum, please use protective gloves to protect your hands and either chemical splash goggles or a face shield to protect your eyes. Make sure your work area has good ventilation or work outdoors. Review the first aid for lye....
If you are using solid NaOH and solid KOH, weigh out each alkali. Add one alkali to the water-based liquid in your recipe, and mix until that alkali is dissolved. Add the second alkali and mix until it too is dissolved. (It doesn't matter which one you use first.) Add the lye solution to your fats and make soap as usual.
How to use a masterbatched 50% NaOH solution along with KOH?
Weigh the correct amount of the 50% NaOH masterbatch solution to get the proper amount of NaOH for the recipe. Set this container aside.
Weigh the additional water-based liquid needed for the recipe in another container. Weigh out the solid KOH. Add the KOH to the container of water-based liquid and stir until the KOH is dissolved.
Pour the containers of 50% NaOH solution and weak KOH solution into the fats and make soap as usual.
Caution -- The total weight of water-based liquid in the recipe must be equal to or more than the total weight of alkali (NaOH + KOH). If you try to use less water, the lye concentration will be over 50% and the alkali may not completely dissolve.
More discussion about bar soap made with mostly NaOH and a little KOH
"Dual lye castile" by DeeAnna: https://www.soapmakingforum.com/threads/dual-lye-castile.59747/
"Castile 95% NaOH 5% KOH calculations help" by Earlene: http://www.soapmakingforum.com/showthread.php?t=60350
"Dual Alkali Soaps (NaOH & KOH)" by Nikos (ngian): http://www.soapmakingforum.com/showthread.php?t=61561
"Dual lye soap? What is this amazing concept?" by Mikvanrose: http://www.soapmakingforum.com/showthread.php?p=625451
I take no credit for being the inventor of the dual-lye idea. In addition to Anne Watson, I know soap makers Evik (Curious Soapmaker, http://curious-soapmaker.com/), and Sistrum (https://www.soapmakingforum.com/members/sistrum.2742/) have shared this idea as well.
Extra Credit: How to hand calculate the weights of KOH, NaOH, and water
If you are not familiar with SBM Advanced, Soapee, or Soapmaking Friend, you can trick your favorite single-lye soap calculator, such as the well known SoapCalc calculator, into doing some of the work, but you will have to break out your trusty calculator to finish up. Here's how --
1. Enter your complete recipe into the recipe calculator as usual with all the oil weights, superfat percentage, water settings, etc.
If the calculator allows you to adjust the KOH purity, leave the purity set at 100%. You will correct for that later on in this procedure.
2. Calculate the exact same soap recipe twice. Leave EVERYTHING the same for each calculation EXCEPT the choice of lye.
The first time, choose NaOH as the lye. Record the "All-NaOH weight" from this recipe calculation.
The second time choose KOH as the lye. Do not change anything else. Record the "All-KOH weight" for this version of your recipe.
For the first calculation, the recipe calculator will tell you how much NaOH and water you would need if NaOH is ALL of the lye in the recipe.
In the second version, the calculator will tell you how much KOH and water you would need if KOH is ALL of the lye.
3. Decide what percentage of NaOH molecules you want in your recipe.
4. Multiply the weight of each alkali by the desired percentage desired in the recipe. This step is also where you will adjust for the KOH purity. if you are not sure about your KOH purity, a good guess is to assume it is around 90% pure. --
NaOH weight for recipe = (All-NaOH weight) X (% NaOH molecules) / 100
KOH weight for recipe = (All-KOH weight) X (100 - % NaOH molecules) / (KOH purity %)
Example 1: The recipe calculator says 213 grams NaOH is needed to make my soap recipe if I use all NaOH. For the same recipe made with all KOH, the KOH weight is 299 grams. I want to use 95% NaOH molecules and 5% KOH molecules in the soap to make a 95:5 dual lye batch. The purity of my KOH is 92%. What is the weight of each alkali to make a 95% NaOH, 5% KOH version of my recipe?
NaOH weight for recipe = 213 X 95 / 100 = 202.4 grams
KOH weight for recipe = 299 X (100 - 95) / 92 = 299 X 5 / 92 = 16.3 grams
5a. If your recipe uses "water as % of oils" to calculate the water weight, then use the water weight from either version of your recipe. "Water as % of oils" will stay the same as long as the oil weight stays the same.
5b. If your recipe is based on lye concentration or water:lye ratio to calculate the water weight, then do these calculations:
Total alkali weight = NaOH weight + KOH weight
Total water weight = (Total alkali weight) / (% Lye concentration) X 100 - (Total alkali weight)
Total water weight = (Total alkali weight) X (Water:Lye ratio)
Example 2: Based on the numbers from Example 1, I want to use 202.4 grams NaOH and 16.3 grams KOH for a 95:5 dual lye recipe. I want to use a lye concentration is 33.33%. This concentration is the same as a water:lye ratio of 2. What is the total water needed for the recipe?
Total alkali weight = 202.4 + 16.3 = 218.7 grams
Total water weight = 218.7 / 33.33 X 100 - 218.7 = 656.8 - 218.7 = 438.1 grams
Total water weight = 218.7 X 2 = 437.4 grams
The small difference in these answers is due to rounding error.
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