What’s the Difference Between Sodium Hydroxide and Potassium Hydroxide for Soapmaking?


When it comes to soapmaking, there’s some confusion about the difference between sodium hydroxide and potassium hydroxide. Sodium hydroxide (NaOH) is commonly known as lye, or caustic soda. Potassium hydroxide (KOH) is also called potash. While they are both key ingredients in soapmaking, there are key differences, and the two cannot be used interchangeably. Soaps made using sodium hydroxide are harder (often bar soaps), whereas potassium hydroxide based soaps are more water-soluble, and the results are often soft soaps, or liquid/gel soaps.

The confusion stems from the term lye as it’s applied to both sodium hydroxide and potassium hydroxide. 

Can you make soft liquid soap with sodium hydroxide, or hard bar soaps with potassium hydroxide? In a nutshell yes, but it would take some effort. You would work against the chemical compounds’ natural properties (sodium hydroxide likes to harden, potassium hydroxide does not). You would be better off selecting the appropriate ingredient from the start, and save yourself time and effort.

Let’s dive into the similarities and difference between sodium hydroxide and potassium hydroxide as it relates to soapmaking.

Similarities of sodium and potassium hydroxide

If you’re looking into making your own soaps at home, you’ve likely stumbled across products labelled as lye with slightly different scientific names: sodium hydroxide, or potassium hydroxide.

Let’s try to clear things up.

As both chemicals are similar at the molecular level, they have many similar reactions and applications in various manufacturing processes.

Both sodium and potassium hydroxide are caustic alkali metal compounds which are used to make lathering soaps. When either NaOH or KOH are combined with fat/oil and water, saponification occurs.

While both sodium and potassium hydroxide are poisonous and corrosive chemicals on their own, once saponification occurs, they are rendered harmless. That lovely bar of soap you made is now completely safe to use on your skin.

You need either sodium hydroxide OR potassium hydroxide in order to make soap. These chemicals kickstart saponification and encourage fats and water to mix. Without one of these two chemicals, you would just be left with a gooey mess.

Here are some key similarities that sodium hydroxide and potassium hydroxide share:

  • Appearance: They look similar in solid form (white flakes, pellets, or coarse powder).
  • Low flammability: In both solid and liquid form, NaOH and KOH have low flammability properties.
  • Corrosive: Both are very corrosive and can cause damage to eyes, skin, or the respiratory system. When handling, it’s vital to wear the appropriate protective equipment.
  • Hygroscopic and deliquescent: Both NaOH and KOH are highly reactive with water and moisture. They require storage in sealed, air-tight containers. They absorb water (hygroscopic), and also dissolve in water (deliquescent).
  • Exothermic water reaction: When added to water, both NaOH and KOH produce a lot of heat.
  • FDA-approved for use in food: Both chemicals are commonly used in various food processing applications as a thickener and stabilizer. NaOH helps to remove skins from fruits and vegetables during canning and create a golden-brown color on pretzels.
  • Commercial applications: Both chemicals play a key role in the production of various goods including paper and petrochemical products, and various cleaning products (such as drain cleaners) and detergents.
Credit: Freestocks / Unsplash

Difference between sodium hydroxide and potassium hydroxide

As it pertains to soapmaking, you’ll find a few key differences between NaOH and KOH. 

On a molecular level, potassium hydroxide is slightly smaller. This means that it’s better able to cut through grease and oil. KOH is also more water-soluble and able to rinse away grease, especially with hot water.

However, if you’re looking to make your own homemade soaps, most DIYers prefer to use sodium hydroxide for its hardening properties. It’s also slightly less expensive.

If you’re looking to buy a lot of potassium hydroxide, it’ll cost you more—about three times more per ton. However, if you’re just looking to buy a small amount for home-use, the price difference is pretty negligible.

It costs more to produce the compound potassium chloride. On the other hand, it’s relatively cheaper to make sodium chloride which is just regular old table salt.

This price difference is a large reason why most manufacturers choose sodium hydroxide for the majority of soapmaking processes today.

Table: What’s the difference between sodium hydroxide and potassium hydroxide for soapmaking?

Sodium hydroxide (NaOH)Potassium hydroxide (KOH)
Creates opaque bar soapsSoaps are typically clear (although certain added elements can make it cloudy)
Soaps are harder (the types of bar soap most of us are accustomed to)Soaps are softer, often used for liquid soaps
Water-solubleGreater water solubility, able to rinse away oil
Typically, less expensiveMore expensive (about 3 times more expensive per ton)
Stronger exothermic reaction with water (gives off more heat)Strong exothermic reaction with water
Slightly larger on a molecular levelSmaller on a molecular level: able to better penetrate oil and rinse away
Sodium hydroxide solutions leave a yellowish tracePotassium hydroxide solutions leave a whitish trace

What the old-timers used to make soap

The art of soapmaking is older than the hills. One of the earliest accounts of soap was in 3000 B.C. by the Sumerians who made a slurry of water and ashes to clean grease from raw wool to prepare fabric to be dyed.

How did settlers and old-timers make their own soap? 

They created their own form of potash lye by mixing hardwood ashes and water. They collected fat and drippings from farm animals, combined this with their homemade potash, and boiled it to make their own soft soaps.

The takeaway

While sodium hydroxide and potassium hydroxide are similar in many ways, they can’t be used interchangeably in soapmaking. When deciding which chemical compound to use, it mainly depends on what type of soap you’d like to make. Sodium hydroxide is commonly used today to make soap as it’s less expensive and it creates hard bars of soap we’ve become accustomed to.

When making your own soap at home, ensure you purchase pure, high-quality lye from a reputable supplier for best results. And don’t let your sodium hydroxide sit too long before using it up—it has a shelf life

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  1. eClean Magazine, Sodium Hydroxide vs. Potassium Hydroxide: What is the difference?, https://www.ecleanmag.com/sodium-hydroxide-vs-potassium-hydroxide-what-is-the-difference/. Accessed December 2020.
  2. NOAH Technologies, Side by Side Comparison: Potassium Hydroxide and Sodium Hydroxide – Similarities, Differences and Use Cases, https://info.noahtech.com/blog/sicomparison-potassium-hydroxide-and-sodium-hydroxide. Accessed December 2020.
  3. Britannica, Early synthetic detergents, https://www.britannica.com/science/soap/Early-synthetic-detergents. Accessed December 2020.
  4. OpenLearn, The history of soapmaking, https://www.open.edu/openlearn/history-the-arts/history/history-science-technology-and-medicine/history-science/the-history-soapmaking. Accessed December 2020.

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