Saponification value

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Saponification value (or "saponification number", also referred to as "sap" in short) represents the number of milligrams of potassium hydroxide or sodium hydroxide required to saponify 1g of fat under the conditions specified. It is a measure of the average molecular weight (or chain length) of all the fatty acids present. As most of the mass of a fat/triester is in the 3 fatty acids, it allows for comparison of the average fatty acid chain length. If there are more moles of potassium hydroxide (KOH) used for 1 gram of fat then there are more moles in the fat. Therefore the chain lengths are smaller, due to the equation: Number of moles = mass of oil/relative atomic mass. The relative atomic mass would be smaller with smaller fatty acid chains, meaning more moles. The higher the saponification value the smaller the chain lengths.

Handmade Soapmakers who aim for bar soap use NaOH sap values, which are derived from the saponification value calculated by laboratories (KOH sap value). To convert one to the other, multiply or divide by 56.11/40 (the ratio of the molecular weights).

Standard methods for analysis are for example: ASTM D 94 (for petroleum) and DIN 51559.

The calculated saponification value is not applicable to fats and oils containing high amounts of unsaponifiable material, free fatty acids (>0.1%), or mono- and diacylglycerols (>0.1%).