Classic Bells > Soapy stuff > Dual lye recipe

Calculating a dual lye recipe

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. Soap maker and 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.

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. Two suitable soap calculators are the Summer Bee Meadow Advanced calculator and the Soapee calculator.

If you have not used either calc, I strongly recommend Soapee, since it is easier to learn and use.

To start a dual-lye recipe in Soapee, 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 below as fats are added.

Calculating 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

Making a lye solution 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.

If you use a masterbatched 50% NaOH solution, here is how I usually proceed: Weigh the correct amount of the 50% NaOH 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 weight of water-based liquid used to make the lye solution(s) must be at least equal to the total weight of alkali (NaOH + KOH). You can use more water, but you cannot use less. If you try to use less, the lye concentration will be over 50% and the alkali may not completely dissolve.

Disclaimer: I take no credit for being the inventor of this idea; I am passing the idea on.

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Extra Credit: Geeky Soaper methods to hand calculate the weights of KOH, NaOH, and water

Geeky Soaper Method 1. If you are not familiar with SBM Advanced or Soapee, 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 --

Enter your complete recipe into the recipe calculator as usual with all the oil weights, superfat percentage, water settings, etc.

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 "100% NaOH weight" from this calculation.

The second time choose KOH as the lye. Record the "100% KOH weight" for this version.

TIP: If using SoapCalc, be sure to also put a check mark in the box for 90% KOH purity if you are not sure about your KOH purity -- most KOH is about 90% pure.

For the first calculation, the recipe calculator will tell you how much NaOH and water you would need if you wanted NaOH as 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 was ALL of the lye.

Alkali weights -- Multiply the weight of each alkali by the desired percentage desired in the recipe. For our example, if you want a mixture of 95% NaOH molecules and 5% KOH molecules in the soap, you would do these calculations to find the weight of each alkali:

NaOH weight for recipe at 95% = (100% NaOH weight) X 95 / 100

KOH weight for recipe at 5% = (100% KOH weight) X 5 / 100

If your NaOH:KOH percentages are different, replace the "95" in the first calculation by the percentage of NaOH molecules that you want. Replace the "5" in the second with the desired percentage of KOH molecules.

Water weight -- If your recipe uses "water as % of oils" to calculate the water weight, then simply use the water weight from the NaOH version of your recipe -- the water weight if calculated as a % of oils will not change whether the alkali is NaOH or KOH.

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)

OR

Total water weight = (Total alkali weight) X (Water:Lye ratio)

Example: A recipe calls for 35 grams of NaOH and 80 grams of KOH. The lye concentration is 33.33%. This concentration is the same as a water:lye ratio of 2 (in other words 2 parts water for 1 part alkali). What is the total water needed for the recipe?

Total alkali weight = 35 + 80 = 115 grams

Total water weight = (115) / (33.33) X 100 - 115 = 230 grams

OR

Total water weight = (115) X (2) = 230 grams

Geeky Soaper Method 2. Here is a slightly different version that allows you to enter your own number for the KOH purity.

Enter your complete recipe into your favorite soap recipe calculator with all the oil weights, superfat percentage, water settings, etc. Calculate the recipe using only NaOH as the lye. Record the "100% NaOH weight" from this calculation.

Alkali weights -- Use the "100% NaOH weight" to calculate the correct NaOH and KOH weights for your dual-lye recipe. Assuming you want 40% NaOH molecules and 60% KOH molecules, here is how to calculate the correct NaOH and KOH weights --

NaOH weight for recipe at 40% = (100% NaOH weight) X 40 / 100

KOH weight for recipe at 60% = 1.403 X (100% NaOH weight) X 100 / (KOH purity %) X 60 / 100

If your KOH purity is 85%, then you would put the number 85 where you see "KOH purity %"

If your NaOH and KOH percentages are different, replace the "40" in the first calculation by the percentage of NaOH molecules that you want. Replace the "60" in the second with the desired percentage of KOH molecules. Do not change or erase the 1.403 factor in the KOH formula.

Water weight -- If your recipe uses "water as % of oils" to calculate the water weight, then simply use the water weight from the NaOH version of your recipe -- the water weight if calculated as a % of oils will not change whether the alkali is NaOH or KOH.

If your recipe is based on lye concentration or water:lye ratio to calculate the water weight, then do the same calculations as for Method 2 --

Total alkali weight = NaOH weight + KOH weight

Total water weight = (Total alkali weight) / (% Lye concentration) X 100 - (Total alkali weight)

OR

Total water weight = (Total alkali weight) X (Water:Lye ratio)

Example: A recipe calls for 15 grams of NaOH and 90 grams of KOH. The lye concentration is 40%. This concentration is the same as a water:lye ratio of 1.5 (in other words 1.5 parts water for 1 part alkali). What is the total water needed for the recipe?

Total alkali weight = 15 + 90 = 105 grams

Total water weight = (105) / (40) X 100 - 105 = 157.5 grams

OR

Total water weight = (105) X (1.5) = 157.5 grams