Saponification Value Formula Derivation

What is Saponification?

Saponification is a chemical reaction in which aqueous alkali converts fat, oil, or lipid into soap and alcohol (e.g. NaOH). Soaps are carboxylic acids with long carbon chains, which are salts of fatty acids. Sodium oleate is a standard soap.

Saponified materials include vegetable oils and animal fats. These oily compounds, known as triglycerides, are made up of a variety of fatty acids. Triglycerides can be made into soap in one or two steps. The triglyceride is processed with a strong base (e.g. lye) in the classic one-step procedure, which cleaves the ester link, producing fatty acid salts (soaps) and glycerol.

Table of Contents

• Saponification Value or Saponification Number
• Saponification Number of Vegetable Oils
• Determining Saponification Value (Formula)
• Relationship with Fats and Oils’ Average Molecular Weight
• Frequently Asked Questions – FAQs

Saponification Value or Saponification Number

The number of milligrams of potassium hydroxide (KOH) or sodium hydroxide (NaOH) necessary to saponify one gramme of fat under the stated conditions is referred to as the saponification value or saponification number (SV or SN).

It is a measurement of the average molecular weight (or chain length) of all fatty acids present as triglycerides in the sample. The smaller the average length of fatty acids, the lower the mean molecular weight of triglycerides, and vice versa, the higher the saponification value. In practise, fats or oils having a high saponification value (such as coconut and palm oil) are better for manufacturing soap.

Saponification Number of Vegetable Oils

Determining Saponification Value (Formula)

To quantify SV, the sample is saponified entirely with an excess of alkali, and the excess is measured by titration (in mg KOH/g). The molecular weight and percentage concentration of fatty acid components present in FAMEs of oil determine the saponification number. The SV is a useful tool for calculating the average relative molecular mass of oils and fats.

Glycerides, such as triglycerides, diglycerides, and monoglycerides, but also free fatty acids and other ester-like components such as lactones, consume alkali. The remaining alkali is titrated against a standard solution of hydrochloric acid at the end of the reaction (HCl). As a result, the SV (milligrams of potassium hydroxide per gram of sample) is calculated as follows:

where,

(B – S) is indicated as the difference between the volume (in mL) of HCl solution used for the blank run and for the tested sample;

M is defined as the molarity of HCl solution, in mol·L⁻¹;

56.1 is indicated as the molecular weight of KOH , in g·mol⁻¹; and

W is indicated as the weight of sample, in g.

The SV can also be estimated using gas chromatography data on fatty acid composition.

Relationship with Fats and Oils’ Average Molecular Weight

A pure triglyceride molecule’s theoretical SV can be computed using the following equation (where MW is its molecular weight):

Molecular Weight (oil/fat) = (3 * 1000 * {56.1}/SV) + 38.049

Where, 3 is the fatty acids residues per triglyceride, 1000 is a conversion factor (mg/g) and 56.1 is the molecular weight of KOH.

Three oleic acid residues are esterified to a molecule of glycerol with a total MW of 885.4 (g·mol^-1) in triolein, a triglyceride found in many fats and oils. As a result, its SV is 190 (mg KOH·g^-1). Trilaurin has a MW of 639 and an SV of 263, while lauric acid contains three shorter fatty acid residues.

The SV of a fat is inversely proportional to its molecular weight, as shown by the formula above. Because fats and oils contain a variety of triglyceride species, the average MW can be computed using the following formula:

This means that coconut oil, which is high in medium-chain fatty acids (mostly lauric), contains more fatty acids per unit of weight than olive oil, for example (mainly oleic). As a result, there were more ester saponifiable functions per gramme of coconut oil, implying that more KOH is needed to saponify the same amount of matter, resulting in a higher SV.

Fats and oils with substantial levels of unsaponifiable material, free fatty acids (> 0.1%), or mono- and diacylglycerols (> 0.1%) are not eligible for the computed molecular weight.