Saponification value or saponification number (SV or SN) represents the number of milligrams of potassium hydroxide (KOH) required to saponify one gram of fat under the conditions specified. It is a measure of the average molecular weight (or chain length) of all the fatty acids present in the sample as triglycerides. The higher the saponification value, the lower the fatty acids average length, the lighter the mean molecular weight of triglycerides and vice-versa. Practically, fats or oils with high saponification value (such as coconut and palm oil) are more suitable for soap making.
To determine saponification value, the sample is hot-saponified with an excess of alkali (usually potassium hydroxide dissolved in ethanol), in standard conditions, generally for half an hour under reflux. Alkali is mainly consumed by glycerides : triglycerides, diglycerides, monoglycerides but also by free fatty acids, as well as by other ester-like components such as lactones. At the end of the reaction the remaining quantity of alkali is titrated against standard solution of hydrochloric acid (HCl). Therefore, the SV (mg KOH/ g of sample) is calculated as following:
- Where :
- (B - S) is the difference between the volume of HCl solution used for the blank run and for the tested sample, in mL ;
- M is the molarity of HCl solution, in mol · L−1 ;
- 56.1 is the molecular weight of KOH , in g · mol−1;
- W is the weight weight of sample, in g.
Handmade soap makers who aim for bar soap use sodium hydroxide (NaOH), commonly known as lye, rather than KOH (caustic potash) which produces soft paste, gel or liquid soaps. In order to calculate the lye amount needed to make bar soap, KOH values of SV can be converted to NaOH values by dividing KOH values by the ratio of the molecular weights of KOH and NaOH (1.403).
Where : 3 is the number of fatty acids residues per triglyceride, 1000 is a conversion factor (mg/ g) and 56.1 is the MW of KOH.
For instance, triolein, a triglyceride occurring in many fats and oils, has three oleic acid residues esterified to a molecule of glycerol with a total MW of 885.4 (g · mol−1). Therefore, its SV equals 190 (mg KOH · g−1). In comparison, trilaurin with three shorter fatty acid residues (lauric acid) has a MW of 639 and an SV of 263.
As it can be seen from the above formula (2) the SV of a given fat is inversely proportional to its molecular weight. Actually, as fats and oils contain a mix of different triglycerides species, the average MW can be calculated according to the following relation :
This means that coconut oil with an abundance of medium chain fatty acids (mainly lauric) contain more fatty acids per unit of weight than, for example, olive oil (mainly oleic). Consequently, more ester saponifiable functions were present per g of coconut oil, which means more KOH is required to saponify the same amount of matter, and thus a higher SV. The calculated molecular weight (Eq. 3) is not applicable to fats and oils containing high amounts of unsaponifiable material, free fatty acids (> 0.1%), or mono- and diacylglycerols (> 0.1%).
Unsaponifiables are components of an fatty substance (oil, fat, wax) that fail to form soaps when treated with alkali and remain insoluble in water but soluble in organic solvents. For instance, typical soybean oil contain, by weight, 1.5 – 2.5% of unsaponifiable matter. Unsaponifiables include nonvolatile components : alkanes, sterols, triterpenes, fatty alcohols, tocopherols and carotenoids as well as those that mainly result from the saponification of fatty esters (sterols esters, wax esters, tocopherols esters, ...). This fraction may also contain environmental contaminants and residues of plasticizers, pesticides, mineral oil hydrocarbons and aromatics.
Unsaponifiable constituents are an important consideration when selecting oil mixtures for the manufacture of soaps. Unsaponifiables can be beneficial to a soap formula because they may have properties such as moisturization, conditioning, antioxidant, texturing etc. On the other hand, when proportion of unsaponifiables is too high (> 3%), or the specific unsaponifiables present do not provide significant benefits, a defective or inferior soap product can result. For example, shark oil is not suitable for soap making as it may contain more than 10% of unsaponifiable matter.
Determination of unsaponifiables involves a saponification step of the sample followed by extraction of the unsaponifiable using an organic solvent (i.e. diethyl ether). Official methods for animal and vegetable fats and oils are described by ASTM D1065 - 18, ISO 3596: 2000 or 18609: 2000, AOCS method Ca 6a-40.
|Fat||Saponification value (mg KOH/ g) ||Unsaponifiable matter (%) |
|Beeswax||60 – 102||> 52|
|Canola oil||182 – 193||< 0.2|
|Cocoa butter||192 – 200||0.2 – 1|
|Coconut oil||248 – 265||0.1 – 1.4|
|Corn oil||187 – 195||1 – 3|
|Cottonseed oil||189 – 207||< 2|
|Fish oil ||179 – 200||0.6 – 3|
|Lanolin ||80 – 127||40 – 50|
|Lard ||192 – 203||< 10|
|Linseed oil||188 – 196||0.1 – 2|
|Olive oil||184 – 196||0.4 – 1.1|
|Palm kernel oil||230 – 254||< 1|
|Palm oil||190 – 209||< 1.4|
|Peanut oil||187 – 196||0.2 – 4.4|
|Rapeseed oil||168 – 181||0.7 – 1.1|
|Safflower oil||188 – 194||< 1.6|
|Shea butter||170 – 190||6 – 17|
|Soybean oil||187 – 195||1.5 – 2.5|
|Sunflower oil||189 – 195||0.3 – 1.2|
|Whale oil ||185 – 202||< 2|
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