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

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

Oil

SN

Babassu

241–253

Camelina

185–194

Canola

170–190

Cardoon

194

Cashew nut

168

Castor

176–187

Coconut

242–263

Corn

187–196

Cottonseed

190–207

Indian mustard

171

Jatropha

188–198

Jojoba

92–95

Karanja

189

Linseed

180–196

Mahua

187–197

Microalgal

189

Mustard

170–178

Nahor

191

Olive

187–196

Palm

200–205

Palm kernel

240–257

Peanut

184–196

Poppy seed

189–197

Rapeseed

166–198

Rubber seed

186–198

Safflower

186–203

Sesame

188–193

Soybean

189–195

Sulfur olive

193

Sunflower

186–194

Tigernut

190–194

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.

Frequently Asked Questions on Saponification Value Formula Derivation

Q1

What do you mean by Unsaponifiables?

Unsaponifiables are fatty substance components (oil, fat, wax) that do not produce soap when exposed to alkali and remain insoluble in water but soluble in organic solvents. Nonvolatile components such as alkanes, sterols, triterpenes, fatty alcohols, tocopherols, and carotenoids, as well as those that primarily emerge from the saponification of fatty esters, are classified as unsaponifiables (sterols esters, wax esters, etc.).

Q2

What is the saponification equation?

General saponification equation is fat + chemical salt + water → glycerol + fatty acid salt (soap). Fat and chemical salts are the reactants, while glycerol and soap are the products.

Q3

Why do we determine saponification value?

The saponification number is the amount of potassium hydroxide required to saponify one gram of fat. This data can be used to compute the number of acids (esters and free acids) in a fat or oil. The more saponification, the more short- and medium-chain fatty acids are present in the fat.

Q4

What are the factors that affect saponification?

The effect of three parameters, such as ethanolic KOH content, reaction temperature, and reaction time, which were explored for the optimum saponification, were evaluated using a D-optimal design.

Q5

Does the saponification value of soap impact its quality?

The saponification value is sometimes used to check for adulteration. The higher the saponification number, the more capable the oil is in making soap. Higher triglyceride saponification values suggest more medium chain fatty acids.

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