Sodium or Potassium Hydroxide: How Each Names Your Soap

Ask a room of new soap makers what the difference between bar soap and liquid soap comes down to, and most will say consistency, texture, maybe the mold. The real answer is one letter in the chemistry, and it changes how you name your finished product on paper.

Two alkalis, two different salts

Saponification is the reaction between a fat or oil and an alkali that produces soap. Bar soap almost always uses sodium hydroxide, commonly called lye. Liquid soap almost always uses potassium hydroxide instead. Neither hydroxide survives the reaction as itself, assuming your batch is properly formulated and fully saponified. What remains in the finished bar or liquid is the salt formed between the fatty acid and whichever alkali metal you used.

That distinction is not cosmetic trivia, it directly determines your INCI declaration. Soap made with sodium hydroxide produces sodium salts of the fatty acids present, things like Sodium Tallowate, Sodium Cocoate, or Sodium Palmate, depending on which oils went into the pot. Soap made with potassium hydroxide produces the potassium equivalents, Potassium Cocoate, Potassium Olivate, and so on. Swap the alkali, swap the name on your ingredient list, even if the fat blend going in was identical.

Why the finished-product naming matters more than the raw alkali

A common point of confusion is whether "sodium hydroxide" itself should appear on the ingredient list of a finished, fully cured bar. In a properly saponified soap, free lye should be at or near zero, meaning the hydroxide has done its job and converted into the fatty acid salt. INCI declarations for soap typically name the resulting salts, the sodium or potassium tallowates and cocoates, rather than declaring residual sodium hydroxide as if it were a leftover ingredient in the final product. If your process leaves meaningful unreacted alkali behind, that is a formulation and safety issue well before it is a labeling one.

A side-by-side view

Where makers actually get this wrong

The most common slip is copying a bar soap recipe's ingredient declaration onto a liquid soap product because the oil blend looks the same on the label, without updating sodium to potassium in the salt names. It looks like a small wording difference, but Health Canada and any other regulator reading the declaration is checking whether it accurately reflects the actual chemistry of what you made. Sodium Cocoate and Potassium Cocoate are not interchangeable labels for the same thing, they describe genuinely different salts with somewhat different properties, and using the wrong one misrepresents the product.

The second common slip is treating a soap that blends both alkalis, some makers do use small combinations to get particular textures, as if only one alkali needs declaring. If your process genuinely produces a mix of sodium and potassium salts, both families need to show up correctly in your ingredient list, proportioned to what is actually present.

Where this connects back to notification

Whichever alkali you use, the resulting soap likely needs a Cosmetic Notification Form in Canada the same as any other cosmetic-claiming product, since true-soap treatment generally does not apply once you are making cosmetic claims like moisturizing or gentle-for-sensitive-skin. When you get to that filing, the INCI names you declare need to reflect the actual salts your process produced, not a copy-pasted list from a different soap type. If you are working through this for the first time and are not sure whether your specific oil blend maps to Sodium Tallowate versus Sodium Palmate versus something else, that ingredient-to-INCI mapping is exactly the kind of check Cosmetic Comply automates, carrying your real formula through to the correct declared names before anything gets filed.