What Is Saponification? The Chemistry of Every Soap Bar

Saponification is the chemical reaction between a fat and a strong alkali that produces soap and glycerine. The word comes from the Latin sapo, meaning soap, and the US regulator defines soap as a product whose bulk is an alkali salt of fatty acids. In a clay pot or a steel tank, the reaction is the same.

What chemical reaction is saponification?

Saponification is the reaction in which hydroxide ions cut a triglyceride into three fatty acid salts and one glycerol molecule. A triglyceride is a glycerol backbone carrying three fatty acid chains. As Kevin Dunn writes, each oil molecule can need three sodium hydroxide molecules.

triglyceride + 3 NaOH  →  3 sodium salts of fatty acids  +  glycerol
      (fat)     (lye)              (SOAP)                  (glycerine)

Those fatty acid salts are the soap. The glycerol is the by-product.

What does saponification make besides soap?

Saponification also makes glycerol, usually called glycerine, a humectant that draws water toward the skin. Handmade cold process soap keeps its glycerine. In modern industrial soapmaking the glycerin is usually separated and resold, so a handmade bar and a factory bar are different products even though the reaction is identical.

Is there lye in finished soap?

Finished soap contains no lye when the recipe is calculated correctly. Lye is a reactant, consumed by the reaction in the way raw egg is consumed when a cake bakes. Makers also deliberately use a small deficit of lye, so the alkali runs out first. Superfat and lye discount explains that margin.

The Indian toilet soap standard, IS 2888:2004, takes the same view: it caps free caustic alkali, expressed as sodium hydroxide, instead of allowing any.

How much lye does each oil need?

Each oil needs its own amount of lye, set by its saponification value, or SAP: the alkali needed to saponify one gram. Coconut oil needs about 0.183 g of sodium hydroxide per gram, and olive oil about 0.135 g, according to SoapCalc’s oil list. Shorter fatty acid chains mean a higher value.

Real oils vary from these averages, which is why no recipe uses the full value.

How long does saponification take?

Cold process saponification takes about 18 to 24 hours, followed by 3 to 4 weeks of cure, according to a 2018 study of cold process soaps. Dunn’s test bars agree: most saponification happened in the first 24 hours, while moisture kept falling for 60 days. The cure itself is a separate process.

Does saponification need heat?

Saponification in cold process needs no outside heat, because the reaction warms the batch itself. The Handcrafted Soap & Cosmetic Guild notes that the heat is internally generated. Hot process adds a heat source to finish the reaction faster. The methods compared trade speed against control.

What is trace?

Trace is the point where oil and lye solution have emulsified into a stable batter. The Guild describes it as ridges of liquid visible on the surface when the stick blender is lifted out. Trace shows the mixture will not separate again. It does not show the reaction is finished, which takes hours more.

Which fats and oils can be saponified?

Any triglyceride can be saponified, which is why vegetable oils and animal fats are both used. According to Wikipedia, toilet soaps are commonly made from coconut, olive or palm oil, or from tallow. The choice changes the bar’s hardness, lather and mildness, not whether it counts as soap. See the full oil comparison below.

How was soap made before commercial lye?

Before commercial sodium hydroxide existed, soapmakers used a boiling solution of potassium carbonate leached from wood ashes, according to a university chemistry text. The soft potassium soaps that resulted were then converted to harder sodium soaps by washing with salt solution. The reaction was the same one.

Can soap turn back into fat?

Soap can turn back into insoluble fatty acids if its pH falls far enough. LibreTexts notes that acidic contaminants make insoluble fatty acids precipitate as scum. That is the chemistry behind why real soap is alkaline and why a bar cannot be pushed down to skin pH.

Is cold process soap ever fully saponified?

Cold process soap is never 100% saponified, and it is not meant to be. The 2018 study found that 86 to 99% of the polyunsaturated fatty acids in its soaps stayed unsaponified. That leftover oil is the superfat doing its job. It is also the part that can oxidise and turn rancid with time.

Why does the oil blend change how soap behaves?

The oil blend changes a bar because each fatty acid becomes a different soap molecule. SoapCalc’s chart credits lauric, myristic, palmitic and stearic acids with hardness, and oleic, linoleic and linolenic acids with conditioning. Every oil carries several acids at once.

Shares are the figures Wikipedia tabulates; they vary between samples. A pure olive bar is mild but wants a long cure.

Do sodium hydroxide and potassium hydroxide make the same soap?

Sodium hydroxide and potassium hydroxide run the same reaction but make different soaps. According to Wikipedia’s soap article, sodium soaps are firm, while potassium soaps are softer or often liquid. Bar soap uses sodium hydroxide. Liquid soap uses potassium hydroxide.

What counts as soap in law and standards?

Soap, in the US regulator’s sense, is a product whose bulk is an alkali salt of fatty acids, whose detergent action comes from those salts, and which is sold only as soap. 21 CFR 701.20 states this. A bar that fails the test is legally a cosmetic or a drug, so reading the label is worth the minute.