NaOH vs. KOH in Soap Making: What Are the Differences?

NaOH and KOH: what is the difference between these two bases in soap making?

When making natural soaps, you quickly come across two key ingredients: sodium hydroxide (NaOH) and potassium hydroxide (KOH). They are used to transform oils into soap. However, they do not produce exactly the same type of soap, nor are they used in the same quantities. So, what is the difference between NaOH and KOH? Which one should you choose for making solid or liquid soap? And can one be substituted for the other in a recipe? To understand this, let’s first look at their role in saponification.

NaOH and KOH are two alkaline compounds that enable the transformation of fats into soap through a chemical reaction known as saponification. This reaction occurs when fats, composed primarily of triglycerides, react with a strong base. It produces fatty acid salts (soap) as well as glycerin.

The main difference between these two bases is the metal element they contain:

• NaOH: sodium hydroxide (caustic soda)

• KOH: potassium hydroxide (caustic potash)

These two substances serve similar functions, but their chemical properties yield different soaps.

NaOH: The base for solid soaps

NaOH is used to make solid soaps, particularly artisanal soaps produced via the cold-process method. When it reacts with oils and butters, it forms sodium salts of fatty acids. These salts tend to yield solid soaps. This is why NaOH is the preferred choice for making:

• Body soaps.

• Hand soaps.

• Superfatted soaps.

However, the final hardness of the soap depends on other factors as well: the composition of the oils, the proportion of each fat, the amount of water used, and the curing time. In other words, using NaOH does not guarantee a solid soap; one must also learn how to balance the recipe.

KOH: The base used for liquid soaps

KOH is used to make liquid soaps or soap pastes intended for dilution. When it reacts with fats, it forms potassium salts of fatty acids. These are more water-soluble than sodium salts, which facilitates the preparation of liquid soaps, though the formula and manufacturing method remain crucial for achieving a good result.

Furthermore, KOH does not directly yield a ready-to-use liquid soap; the resulting paste must be diluted, and its clarity, viscosity, and stability depend on the formulation.

Why can’t NaOH be replaced by KOH in a recipe?

This is a crucial point when learning to formulate your own soap recipes. NaOH and KOH do not have the same molar mass:

• NaOH: 40.00 g/mol.

• KOH: 56.11 g/mol.

This difference means you must adjust the amount of base used to saponify a given fat. For instance, if a recipe calls for a specific amount of NaOH to saponify an oil and you want to create an equivalent recipe using KOH, you cannot simply substitute KOH for NaOH on a gram-for-gram basis. This is because KOH (potash) is heavier than NaOH (caustic soda). That is why soap calculators provide separate SAP values ​​for NaOH and KOH; these values ​​allow you to determine the amount of base required for a specific weight of oil.

You must always use the SAP value that corresponds to the base you have chosen. A calculation error can result in soap that is too alkaline or contains an excess of unsaponified fat.

What are the NaOH and KOH SAP values?

The saponification value, or SAP value, indicates the amount of base required to saponify a given weight of fat. In soapmaking, two values ​​are encountered:

• NaOH SAP: the amount of NaOH needed to saponify a given quantity of oil.

• KOH SAP: the amount of KOH needed to saponify that same quantity of oil.

Coconut oil has different NaOH and KOH SAP values. This does not mean the oil has changed; it simply means the base used is different. Understanding this distinction is important for learning how to formulate recipes.

Can both bases be used in the same recipe?

Yes, you can use both NaOH and KOH in the same formulation; this is known as a mixed-base recipe. It allows you to combine the properties of sodium and potassium salts. For instance, some formulas use both bases to create a soap that retains a certain firmness while being easier to dissolve or work with, depending on your needs.

If you are just starting out, it is easier to work with one base at a time. This helps you better understand the oils, lye calculations, and the effect of superfatting before attempting complex formulas.

If you want to make a solid handmade soap for the body or hands, NaOH is the best choice. If you want to make liquid soap or a soap paste for dilution, KOH is perfect. In any case, remember that the choice of oils, the superfatting level, the manufacturing method, and the quality control of the finished product all play a vital role in making good soap.